Programmable Logic Control(PLC)
A Programmable Logic Controller, PLC, or Programmable Controller or Logic Box is an electronic device used for automation of industrial processes, such as control of machinery on factory assembly lines. Unlike general-purpose computers, the PLC is designed for multiple inputs and output arrangements, extended temperature ranges, immunity to electrical noise, and resistance to vibration and impact. Programs to control machine operation are typically stored in battery-backed or non-volatile memory. A PLC is an example of a real time system since output results must be produced in response to input conditions within a bounded time, otherwise unintended operation will result.

Contents
1 Features
2 PLC compared with other control systems
3 Digital and analog signals
3.1 Example
4 System scale
5 Programming
6 User interface
7 Communications
8 History
9 See also
10 External links
1.Features
Control panel with PLC
The main difference from other computers is that PLC are armored for severe condition (dust, moisture, heat, cold, etc) and have the facility for extensive input/output (I/O) arrangements. These connect the PLC to sensors and actuators. PLCs read limit switches, analog process variables (such as temperature and pressure), and the positions of complex positioning systems. Some even use machine vision. On the actuator side, PLCs operate electric motors, pneumatic or hydraulic cylinders, magnetic relays or solenoids, or analog outputs. The input/output arrangements may be built into a simple PLC, or the PLC may have external I/O modules attached to a computer network that plugs into the PLC.
PLCs were invented as replacements for automated systems that would use hundreds or thousands of relays, cam timers, and drum sequencers. Often, a single PLC can be programmed to replace thousands of relays. Programmable controllers were initially adopted by the automotive manufacturing industry, where software revision replaced the re-wiring of hard-wired control panels when production models changed.
Many of the earliest PLCs expressed all decision making logic in simple ladder logic which appeared similar to electrical schematic diagrams. The electricians were quite able to trace out circuit problems with schematic diagrams using ladder logic. This program notation was chosen to reduce training demands for the existing technicians. Other early PLCs used a form of instruction list programming, based on a stack-based logic solver.
The functionality of the PLC has evolved over the years to include sequential relay control, motion control, process control, distributed control systems and networking. The data handling, storage, processing power and communication capabilities of some modern PLCs are approximately equivalent to desktop computers. PLC-like programming combined with remote I/O hardware, allow a general-purpose desktop computer to overlap some PLCs in certain applications.
Under the IEC 61131-3 standard, PLCs can be programmed using standards-based programming languages. A graphical programming notation called Sequential Function Charts is available on certain programmable controllers.

2.PLC compared with other control systems
PLCs are well-adapted to a certain range of automation tasks. These are typically industrial processes in manufacturing where the cost of developing and maintaining the automation system is high relative to the total cost of the automation, and where changes to the system would be expected during its operational life. PLCs contain input and output devices compatible with industrial pilot devices and controls; little electrical design is required, and the design problem centers on expressing the desired sequence of operations in ladder logic (or function chart) notation. PLC applications are typically highly customized systems so the cost of a packaged PLC is low compared to the cost of a specific custom-built controller design. On the other hand, in the case of mass-produced goods, customized control systems are economic due to the lower cost of the components, which can be optimally chosen instead of a "generic" solution, and where the non-recurring engineering charges are spread over thousands of sales.
For high volume or very simple fixed automation tasks, different techniques are used. For example, a consumer dishwasher would be controlled by an electromechanical cam timer costing only a few dollars in production quantities.
A microcontroller-based design would be appropriate where hundreds or thousands of units will be produced and so the development cost (design of power supplies and input/output hardware) can be spread over many sales, and where the end-user would not need to alter the control. Automotive applications are an example; millions of units are built each year, and very few end-users alter the programming of these controllers. However, some specialty vehicles such as transit busses economically use PLCs instead of custom-designed controls, because the volumes are low and the development cost would be uneconomic.
Very complex process control, such as used in the chemical industry, may require algorithms and performance beyond the capability of even high-performance PLCs. Very high-speed or precision controls may also require customised solutions; for example, aircraft flight controls.
PLCs may include logic for single-variable feedback analog control loop, a "proportional, integral, derivative" or "PID controller." A PID loop could be used to control the temperature of a manufacturing process, for example. Historically PLCs were usually configured with only a few analog control loops; where processes required hundreds or thousands of loops, a distributed control system (DCS) would instead be used. However, as PLCs have become more powerful, the boundary between DCS and PLC applications has become less clear-cut.

3.Digital and analog signals
Digital or discrete signals behave as binary switches, yielding simply an On or Off signal (1 or 0, True or False, respectively). Pushbuttons, limit switches, and photoelectric sensors are examples of devices providing a discrete signal. Discrete signals are sent using either voltage or current, where a specific range is designated as On and another as Off. A PLC might use 24 V DC I/O, with values above 22 V DC representing On and values below 2VDC representing Off. Initially, PLCs had only discrete I/O.
Analog signals are like volume controls, with a range of values between zero and full-scale. These are typically interpreted as integer values (counts) by the PLC, with various ranges of accuracy depending on the device and the number of bits available to store the data. Pressure, temperature, flow, and weight are often represented by analog signals. Analog signals can use voltage or current with a magnitude proportional to the value of the process signal. For example, an analog 4-20 mA or 0 - 10 V input would be converted into an integer value of 0 - 32767.
Current inputs are less sensitive to electrical noise (i.e. from welders or electric motor starts) than voltage inputs.

4.Example
As an example, say the facility needs to store water in a tank. The water is drawn from the tank by another system, as needed and our example system must manage the water level in the tank.
Using only digital signals, the PLC has two digital inputs from float switches (tank empty and tank full). The PLC uses a digital output to open and close the inlet valve into the tank.
If both float switches are off (down) or only the 'tank empty' switch is on, the PLC will open the valve to let more water in. If only the 'tank full' switch is on, the valve turns off. Both switches being on would signal that something is wrong with one of the switches, as the tank cannot be both full and empty at the same time. Two float switches are used to prevent a 'flutter' condition where any water usage activates the pump for a very short time causing the system to wear out faster.
An analog system might use a load cell (scale) that weighs the tank, and an adjustable (throttling) valve. The PLC could use a PID feedback loop to control the valve opening. The load cell is connected to an analog input and the valve is connected to an analog output. This system fills the tank faster when there is less water in the tank. If the water level drops rapidly, the valve can be opened wide. If water is only dripping out of the tank, the valve adjusts to slowly drip water back into the tank.
In this system, to avoid 'flutter' adjustments that can wear out the valve, many PLCs incorporate "hysteresis" which essentially creates a "deadband" of activity. A technician adjusts this deadband so the valve moves only for a significant change in rate. This will in turn minimize the motion of the valve, and reduce its wear.
A real system might combine both approaches, using float switches and simple valves to prevent spills, and a rate sensor and rate valve to optimize refill rates. Backup and maintenance methods can make a real system very complicated.

5.System scale
A small PLC will have a fixed number of connections built in for inputs and outputs. Typically, expansions are available if the base model does not have enough I/O.
Modular PLCs have a chassis (also called a rack) into which is placed modules with different functions. The processor and selection of I/O modules is customised for the particular application. Several racks can be administered by a single processor, and may have thousands of inputs and outputs. A special high speed serial I/O link is used so that racks can be distributed away from the processor, reducing the wiring costs for large plants.
PLCs used in larger I/O systems may have peer-to-peer (P2P) communication between processors. This allows separate parts of a complex process to have individual control while allowing the subsystems to co-ordinate over the communication link. These communication links are also often used for HMI (Human-Machine Interface) devices such as keypads or PC-type workstations. Some of today's PLCs can communicate over a wide range of media including RS-485, Coaxial, and even Ethernet for I/O control at network speeds up to 100 Mbit/s.

6.Programming
Early PLCs, up to the mid-1980s, were programmed using proprietary programming panels or special-purpose programming terminals, which often had dedicated function keys representing the various logical elements of PLC programs. Programs were stored on cassette tape cartridges. Facilities for printing and documentation were very minimal due to lack of memory capacity. More recently, PLC programs are typically written in a special application on a personal computer, then downloaded by a direct-connection cable or over a network to the PLC. The very oldest PLCs used non-volatile magnetic core memory but now the program is stored in the PLC either in battery-backed-up RAM or some other non-volatile flash memory.
Early PLCs were designed to be used by electricians who would learn PLC programming on the job. These PLCs were programmed in "ladder logic", which strongly resembles a schematic diagram of relay logic. Modern PLCs can be programmed in a variety of ways, from ladder logic to more traditional programming languages such as BASIC and C. Another method is State Logic, a Very High Level Programming Language designed to program PLCs based on State Transition Diagrams.
Recently, the International standard IEC 61131-3 has become popular. IEC 61131-3 currently defines five programming languages for programmable control systems: FBD (Function block diagram), LD (Ladder diagram), ST (Structured text, similar to the Pascal programming language), IL (Instruction list, similar to assembly language) and SFC (Sequential function chart). These techniques emphasize logical organization of operations.
While the fundamental concepts of PLC programming are common to all manufacturers, differences in I/O addressing, memory organization and instruction sets mean that PLC programs are never perfectly interchangeable between different makers. Even within the same product line of a single manufacturer, different models may not be directly compatible.

7.User interface
PLCs may need to interact with people for the purpose of configuration, alarm reporting or everyday control. A Human-Machine Interface (HMI) is employed for this purpose.
A simple system may use buttons and lights to interact with the user. Text displays are available as well as graphical touch screens. Most modern PLCs can communicate over a network to some other system, such as a computer running a SCADA (Supervisory Control And Data Acquisition) system or web browser.

8.Communications
PLCs usually have built in communications ports for at least RS232, and optionally for RS485 and ethernet. Modbus is the lowest common denominator communications protocol. Others are various fieldbuses such as Profibus. Other communications protocols that may be used are listed in the List of automation protocols.

9.History
The PLC was invented in response to the needs of the American automotive industry. Before the PLC, control, sequencing, and safety interlock logic for manufacturing automobiles was accomplished using relays, timers and dedicated closed-loop controllers. The process for updating such facilities for the yearly model change-over was very time consuming and expensive, as the relay systems needed to be rewired by skilled electricians. In 1968 GM Hydramatic (the automatic transmission division of General Motors) issued a request for proposal for an electronic replacement for hard-wired relay systems.
The winning proposal came from Bedford Associates of Bedford, Massachusetts. The first PLC, designated the 084 because it was Bedford Associates eighty-fourth project, was the result. Bedford Associates started a new company dedicated to developing, manufacturing, selling, and servicing this new product: Modicon, which stood for MOdular DIgital CONtroller. One of the people who worked on that project was Dick Morley, who is considered to be the "father" of the PLC. The Modicon brand was sold in 1977 to Gould Electronics, and later acquired by German Company AEG and then by Schneider Electric, the current owner.
One of the very first 084 models built is now on display at Modicon's headquarters in North Andover, Massachusetts. It was presented to Modicon by GM, when the unit was retired after nearly twenty years of uninterrupted service.
The automotive industry is still one of the largest users of PLCs, and Modicon still numbers some of its controller models such that they end with eighty-four. PLCs are used in many different industries and machines such as packaging and semiconductor machines. Well known PLC brands are ABB Ltd., Koyo, Honeywell, Siemens, Modicon, Omron, Allen-Bradley, General Electric, [Panasonic]] (Matsushita), and Mitsubishi.
In a similar way that Linux has changed personal and business computing there is an effort to bring Linux to the PLC world. With many challenges to overcome the Linux open source PLC project is being worked on around the world.

PROFIBUS
Overview
PROFIBUS is the most universal fieldbus for plantwide use accross all sectors of the manufacturing and process industries. It is the fieldbus having world wide best economic success. Independent market studies confirm market leadership for PROFIBUS today and high growth rates for the future. Using PROFIBUS means to have great cost advantages and improved flexibility.
First of all there are major cost savings in hardware and assembly. Here most important is you need less hardware components as I/O, terminal blocks, and barriers. The installation becomes easier, quicker and cheaper.
Very impressive are the cost savings in engineering and documentation, which makes the configuration much easier (only one tool for all devices), which improves the asset management of an automation system and which facilitates the documentation of the of the production procedure(easy and up-to-date documentation)
An important feature of PROFIBUS based automation is the greater manufacturing flexibility. This feature covers a lot aspects, of which the most important are: improved functionality increases plant productivity, improved availability and reduced down time and optimized use of limited resources and raw materials

One of the most important advantage of using fieldbusses in the automation is reduced installation efforts. E.g., the cost savings estimated in process automation if PROFIBUS is used instead of the conventional 4..20mA technology are sumed up to more than 40%.

PROFIBUS Basics
Fieldbuses are industrial communication systems that use a range of media such as copper cable, fiber optics or wireless, with bit-serial transmission for coupling distributed field devices (sensors, actuators, drives, transducers, analyzers etc.) to a central control or management system. Fieldbus technology was developed in the 80s with the aim of replacing the commonly used central parallel wiring and prevailing analog signal transmission (4-20 mA- or +/- 10V interface) with digital technology. Due, in parts, to the different industry-specific demands and preferred proprietary solutions of large manufacturers, several bus systems with varying properties were established in the market. The key technologies are now included in the standards IEC 61158 and IEC 61784. PROFIBUS is an integral part of these standards. Fieldbuses increase the productivity and flexibility of automated processes compared to conventional technology and they create the basic prerequisite for the configuration of distributed automation systems. PROFIBUS is an open, digital communication system with a wide range of applications, particularly in the fields of factory and process automation. PROFIBUS is suitable for both fast, time-critical applications and complex communication tasks. The application and engineering aspects are specified in the generally available technical documents of PROFIBUS International. This fulfills user demand for manufacturer independence and openness and ensures communication between devices of various manufacturers. Building on a very efficient and extensible communications protocol, coupled with the development of numerous application-oriented profiles and a fast growing number of devices, PROFIBUS began its advance, initially in factory automation and, since 1995, in process automation. Today, PROFIBUS is the fieldbus world market leader.

PROFIBUS has a modular design (PROFIBUS Tool Box) and offers a range of transmission and communication technologies, numerous application and system profiles, as well as device management and integration tools. Thus PROFIBUS covers the diverse and application-specific demands from the field of factory to process automation, from simple to complex applications, by selecting the adequate set of components out of the tool box.

Transmission Technologies


PROFIBUS provides five different transmission technologies, all of which are based on international standards and are assigned to PROFIBUS in both IEC 61158 and IEC 61784: RS485 and MBP, RS485-IS and MBP-IS (both providing intrinsic safety protection), and Fiber Optics. RS485 transmission technology is simple and cost-effective and primarily used for tasks that require high transmission rates. Shielded, twisted pair copper cable with one conductor pair is used. No expert knowledge is required for installation of the cable. The bus structure allows addition or removal of stations or the step-by-step commissioning of the system without influencing other stations. Subsequent expansions (within defined limits) have no effect on stations already in operation. Various transmission rates can be selected between 9.6 Kbit/s and 12 Mbit/s. One uniform speed is selected for all devices on the bus when commissioning the system. Up to 32 stations (master or slaves) can be connected in a single segment. For connecting more than 32 stations repeaters may be used. The maximum permissible line length depends on the transmission rate. Different cable types (type designation A - D) for different applications are available on the market for connecting devices either to each other or to network elements (segment couplers, links and repeaters). When using RS485 transmission technology, PI recommends the use of cable type A. RS485-IS transmission technology responds to an increasing market demand to support the use of RS485 with its fast transmission rates in intrinsically safe areas. A PROFIBUS document is available for the configuration of intrinsically safe RS485 solutions with simple device interchangeability. The interface specification details the levels for current and voltage that must be adhered to by all stations in order to ensure safe functioning during interconnection. An electric circuit permits maximum currents at a specified voltage level. When connecting active sources, the sum of the currents of all stations must not exceed the maximum permissible current. In contrast to the FISCO model (see below), all stations represent active sources. Up to 32 stations are expected to be connected to the intrinsically safe bus circuit. MBP type transmission technology ("Manchester Coding" and "Bus Powered") is a new term that replaces the previously common terms for intrinsically safe transmission such as "Physics in accordance with IEC 61158-2", "1158-2", etc. The reason for this change is that, in its definitive version, the IEC 61158-2 (physical layer) describes several different connection technologies, MBP technology being just one of them. MBP is synchronous transmission with a defined transmission rate of 31.25 Kbit/s and Manchester coding and frequently used in process automation as it satisfies the key demands of the chemical and petrochemical industries for intrinsic safety and bus power using two-wire technology. MBP transmission technology is usually limited to a specific segment (field devices in hazardous areas) of a plant, which are then linked to a RS485 segment via a segment coupler or links (Fig. ). Segment couplers are signal converters that modulate the RS485 signals to the MBP signal level and vice versa. They are transparent from the bus protocol standpoint. In contrast, links have their own intrinsic intelligence. They map all the field devices connected to the MBP segment as a single slave in the RS485 segment. Tree or line structures (and any combination of the two) are network topologies supported by PROFIBUS with MBP transmission with up to 32 stations per segment and max. 126 per network. Fiber Optic transmission technology is used for fieldbus applications that place restrictions on wire-bound transmission technology, such as those in environments with very high electromagnetic interference or when particularly large distances need to be covered. The PROFIBUS guideline (2.021) for fiber optic transmission specifies the technology available for this purpose including multimode and single mode glass fiber, plastic fiber, and HCS® fiber. When determining these specifications, great care was naturally taken to allow problem-free integration of existing PROFIBUS devices in a fiber optic network without the need to change the protocol behavior of PROFIBUS. This ensures backward compatibility with existing PROFIBUS installations. The internationally recognized FISCO model (Fieldbus Intrinsically Safe Concept, developed in Germany by the PTB) considerably simplifies the planning, installation and expansion of PROFIBUS networks in potentially explosive areas. The model is based on the specification that a network is intrinsically safe and requires no individual intrinsic safety calculations when the relevant four bus components (field devices, cables, segment couplers and bus terminators) fall within predefined limits with regard to voltage, current, output, inductance and capacity. The corresponding proof can be provided by certification of the components through authorized accreditation agencies, such as PTB and BVS (Germany) or UL and FM(USA). If FISCO-approved devices are used, not only is it possible to operate more devices on a single line, but the devices can be replaced during runtime by devices of other manufacturers or the line can be expanded - all without the need for time-consuming calculations or system certification. So simple plug & play is possible, even in hazardous areas!

Communication Technologies

Communication protocol At the protocol level, PROFIBUS, with the protocol DP (Decentralized Peripherals) and its versions DP-V0 to DP-V2, offers a broad spectrum of options, which enable optimum communication between different applications. Historically speaking, FMS was the first PROFIBUS communications protocol. It is not longer supported by PROFIBUS International. DP has been designed for fast data exchange at field level. Data exchange with the distributed devices is primarily cyclic. The communication functions required for this are specified through the DP basic functions (version DP-V0). Geared towards the special demands of the various areas of application, these basic DP functions have been expanded step-by-step with special functions, so that DP is now available in three versions; DP-V0, DP-V1 and DP-V2, whereby each version has its own special key features. All versions of DP are specified in detail in the IEC 61158. Version DP-V0 provides the basic functionality of DP, including cyclic data exchange, station, module and channel-specific diagnostics and four different interrupt types for diagnostics and process interrupts, and for the pulling and plugging of stations. Version DP-V1 contains enhancements geared towards process automation, in particular acyclic data communication for parameter assignment, operation, visualization and interrupt control of intelligent field devices, parallel to cyclic user data communication. This permits online access to stations using engineering tools. In addition, DP-V1 has three additional interrupt types: status interrupt, update interrupt and a manufacturer-specific interrupt. Version DP-V2 contains further enhancements and is geared primarily towards the demands of drive technology. Due to additional functionalities, such as isochronous slave mode and lateral slave communication (DXB) etc., the DP-V2 can also be implemented as a drive bus for controlling fast movement sequences in drive axes. System configuration and device types DP supports implementation of both mono-master and multi-master systems. This affords a high degree of flexibility during system configuration. A maximum of 126 devices (masters or slaves) can be connected to a bus. In mono-master systems, only one master is active on the bus during operation of the bus system. The figure above shows the system configuration of a mono-master system. The PLC is the central control component. The slaves are decentrally coupled to the PLC over the transmission medium. This system configuration enables the shortest bus cycle times. In multi-master systems several masters are connected to one bus. They represent either independent subsystems, comprising one DPM1 and its assigned slaves, or additional configuration and diagnostic devices. DP master class 1 (DPM1) is a central controller that cyclically exchanges information with the distributed stations (slaves) at a specified message cycle. Typical DPM1 devices are programmable logic controllers (PLCs) or PCs. A DPM1 has active bus access with which it can read measurement data (inputs) of the field devices and write the setpoint values (outputs) of the actuators at fixed times. This continuously repeating cycle is the basis of the automation function. DP master class 2 (DPM2) are engineering, configuration or operating devices. They are implemented during commissioning and for maintenance and diagnostics in order to configure connected devices, evaluate measured values and parameters and request the device status. A DPM2 does not have to be permanently connected to the bus system. The DPM2 also has active bus access . Slaves are peripherals (I/O devices, drives, HMIs, valves, transducers, analyzers), which reads in process information and/or uses output information to intervene in the process. There are also devices that solely process input information or output information. As far as communication is concerned, slaves are passive devices, they only respond to direct queries. This behavior is simple and cost-effective to implement (in the case of DP-V0 it is already completely included in the hardware). Cyclic (DP-V0) and acyclic (DP-V1) data communication Cyclic data communication between the DPM1 and its assigned slaves is automatically handled by the DPM1 in a defined, recurring sequence . The user defines the assignment of the slave(s) to the DPM1 when configuring the bus system. The user also defines which slaves are to be included/excluded in the cyclic user data communication. Data communication between the DPM1 and the slaves is divided into parameterization, configuration and data transfer. Before the master includes a DP slave in the data transfer phase, a check is run during the parameterization and configuration phase to ensure the correct configuration. In addition to the station-related user data communication, which is automatically handled by the DPM1, the master can also send control commands to all slaves or a group of slaves simultaneously. These control commands are transmitted as multicast commands and enable sync and freeze modes for event-controlled synchronization of the slaves. For safety reasons, it is necessary to ensure that DP has effective protective functions against incorrect parameterization or failure of transmission equipment. For this purpose the DP master and the slaves are fitted with monitoring mechanisms in the form of time monitors. The monitoring interval is defined during configuration. Acyclic data communication is the key feature of version DP-V1. This forms the requirement for parameterization and calibration of the field devices over the bus during runtime and for the introduction of confirmed alarm messages. Transmission of acyclic data is executed parallel to cyclic data communication, but with lower priority. Slave-to-Slave Communications (DP-V2) enables direct and thus time-saving communication between slaves using broadcast communication without the detour over a master. In this case the slaves act as "publisher", i.e., the slave response does not go through the coordinating master, but directly to other slaves embedded in the sequence, the so-called "subscribers". This enables slaves to directly read data from other slaves and use them as their own input. This opens up the possibility of completely new applications; it also reduces response times on the bus by up to 90 %. Isochronous mode (DPV-2) enables clock synchronous control in masters and slaves, irrespective of the bus load. The function enables highly precise positioning processes with clock deviations of less than a microsecond. All participating device cycles are synchronized to the bus master cycle through a "global control" broadcast message. A special sign of life (consecutive number) allows monitoring of the synchronization. Clock control (DP-V2) synchronizes all stations to a system time with a deviation of less than a millisecond. This allows the precise tracking of events. This is particularly useful for the acquisition of timing functions in networks with numerous masters. This facilitates the diagnostics of faults as well as the chronological planning of events. Upload and download (DP-V2) allows the loading of any size of data area in a field device with a single command. This enables, for example, programs to be updated or devices replaced without the need for manual loading processes. Addressing with slot and index is used both for cyclic and acyclic communication services. When addressing data, PROFIBUS assumes that the physical structure of the slaves is modular or can be structured internally in logical functional units, so-called modules. The slot number addresses the module and the index addresses the data blocks assigned to a module. Compact devices are regarded as a unit of virtual modules. These can also be addressed with slot number and index.

Application Profiles
Profiles are the specifications defined by manufacturers and users regarding specific properties, performance features and behavior of devices and systems. Profile specifications define the parameters and behavior of devices and systems that belong to a profile family due to "profile-conform“ development, thus facilitating device interoperability and, as far as possible, device interchangeability on a bus. Profiles take into account application and type-specific special features of field devices, controls and means of integration (engineering). The term profile ranges from just a few specifications for a specific device class through to comprehensive specifications for applications in a specific industry. The generic term for all profiles is application profiles. A distinction is drawn between general application profiles with implementation options for different applications (this includes, for example, the profiles PROFIsafe, Redundancy and Time stamp), specific application profiles, which are developed especially for a specific application, such as PROFIdrive, Encoder, Ident Systems or PA Devices, and system and master profiles, which describe specific system performances that are available to field devices. Thus, these are the opposite of the application profiles. PROFIBUS offers a wide range of such application profiles, which allow application-oriented implementation.

General Application Profiles
PROFIsafe is a comprehensive, open fieldbus solution for safety-relevant applications without the use of a second layer or distributed over special buses. PROFIsafe defines how fail-safe devices (emergency stop pushbuttons, light arrays, overfill cutouts, etc.) can communicate over PROFIBUS with fail-safe controllers so safely that they can be used for safety-related automation tasks up to KAT4 compliant with EN954, AK6 or SIL3 (Safety Integrity Level). It implements safe communications over a profile, i.e. over a special format of user data and a special protocol. PROFIsafe is a single-channel software solution, which is implemented in the devices as an additional layer "above" layer 7; the standard PROFIBUS components, such as lines, ASICs or protocols, remain unchanged. This ensures redundancy mode and retrofit capability. Devices with the PROFIsafe profile can be operated in coexistence with standard devices without restriction on one and the same bus (cable). PROFIsafe uses acyclic communication and can be used with RS485, fiber optic or MBP transmission technology. This ensures both fast response times (important for the manufacturing industry) and intrinsically safe operation (important for process automation).


HART on PROFIBUS DP integrates HART devices installed in the field, in existing or new PROFIBUS systems. It includes the benefits of the PROFIBUS communication mechanisms without any changes required to the PROFIBUS protocol and services, the PROFIBUS PDUs (Protocol Data Units) or the state machines and functional characteristics. This profile is implemented in the master and slave above layer 7, thus enabling mapping of the HART client-master-server model on PROFIBUS. The cooperation of the HART Foundation on the specification work ensures complete conformity with the HART specifications. The HART-client application is integrated in a PROFIBUS master and the HART master in a PROFIBUS slave, whereby the latter serves as a multiplexer and handles communication to the HART devices.

The Time Stamp application profile provides certain events and actions with a time stamp, which enables precise time assignment. Precondition is a time-of-day management in the slaves through a time-of-day master. An event can be given a precise system time stamp and read out accordingly. A concept of graded messages is used. The message types are summarized under the term "Alerts" and are divided into high-priority "alarms" (these transmit a diagnostics message) and low-priority "events". In both cases, the master acyclically reads the time-stamped process values and alarm messages from the alarm and event buffer of the field device.

The Slave redundancy application profile provides a slave-redundancy mechanism: Slave devices contain two different PROFIBUS interfaces that are called primary and backup (slave interface). These may be either in a single device or distributed over two devices. The devices are equipped with two independent protocol stacks with a special redundancy expansion. A redundancy communication (RedCom) runs between the protocol stacks, i.e. within a device or between two devices, that is independent of PROFIBUS and whose performance capability is largely determined by the redundancy reversing times.Only one device version is required to implement different redundancy structures and no additional configuration of the backup slave is neccessary. The redundancy of PROFIBUS slave devices provides high availability, short reversing times, no data loss and ensures fault tolerance.

Specific Application Profiles
The PROFIdrive application profile defines device behavior and the access procedure to drive data for electric drives on PROFIBUS, from simple frequency converters through to highly dynamic servo-controls. The integration of drives in automation solutions is highly dependent on the task of the drive. For this reason, PROFIdrive defines six application classes, which cover the majority of applications. With standard drives (class 1), the drive is controlled by means of a main setpoint value (e.g. rotational speed), whereby the speed control is carried out in the drive controller. In case of standard drives with technological function (class 2), the automation process is broken down into several subprocesses and some of the automation functions are shifted from the central programmable controller to the drive controllers. PROFIBUS serves as the technology interface in this case. Slave-to-slave communication between the individual drive controls is a requirement for this solution. The positioning drive (class 3) integrates an additional position controller in the drive, thus covering an extremely broad spectrum of applications (e.g. the twisting on and off of bottle tops). The positioning tasks are passed to the drive controller over PROFIBUS and started. The central motion control (classes 4 and 5) enables the coordinated motional sequence of multiple drives. The motion is primarily controlled over a central numeric control (CNC). PROFIBUS serves to close the position control loop as well as synchronize the clock. The position control concept (Dynamic Servo Control) of this solution also supports extremely sophisticated applications with linear motors. Distributed automation by means of clocked processes and electronic shafts (class 6) can be implemented using slave-to-slave communication and isochronous slaves. Sample applications include "electrical gears", "curve discs" and "angular synchronous processes". In contrast to other drive profiles, PROFIdrive only defines the access mechanisms to the parameters and a subset of approx. 30 profile parameters, which include fault buffers, drive controllers, device identification, etc. All other parameters (which may number more than 1,000 in complex devices) are manufacturer-specific, which provide drive manufacturers great flexibility when implementing control functions.

The PA Devices profile defines all functions and parameters for different classes of process devices that are intrinsically intelligent and can execute part of the information processing or even the overall functionality in automation systems. The profile includes all steps of a typical signal flow - from process sensor signals through to the preprocessed process value which is read out at the control system together with a measured value status, see Fig. .... The PA Devices profile is documented in a general data sheet containing the currently valid specifications for all device types and in device data sheets containing the agreed specifications for specific device classes. The PA device profile version 3.0 includes device data sheets for Pressure and differential pressure, level, temperature, flow rate, valves and actuators, analyzers, and analog and digital inputs and outputs. In process engineering it is common to use blocks to describe the characteristics and functions of a sensor or actuator at a certain control point and to represent an automation application through a combination of these types of blocks. Therefore, the specification of PA Devices uses a function block model to represent functional sequences. The blocks are implemented by the manufacturers as software in the field devices and, taken as a whole, represent the functionality of the device. The following three block types are used: A Physical Block (PB) contains the characteristic data of a device, such as device name, manufacturer, version and serial number, etc. There can only be one physical block in each device. A Transducer Block (TB) contains all the data required for processing an unconditioned signal delivered from a sensor for passing on to a function block. If no processing is required, the TB can be omitted. Multifunctional devices with two or more sensors have a corresponding number of TBs. A Function Block (FB) contains all data for final processing of a measured value prior to transmission to the control system, or on the other hand, for processing of a setting before the setting process. Different FB are available: Analog Input Block (AI, delivers the measured value from the sensor/TB to the control system), Analog Output Block (AO, provides the device with the value specified by the control system), Digital Input (DI, provides the control system with a digital value from the device), and Digital Output (DO, provides the device with the value specified by the control system).


Ident systems is a profile for barcode readers and transponder systems. These are primarily intended for extensive use with the DP-V1 functionality. While the cyclic data transmission channel is used for small data volumes to transfer status/control information, the acyclic channel serves the transmission of large data volumes that result from the information in the barcode reader or transponder. The definition of standard function blocks has facilitated the use of these systems and paves the way for the application of open solutions on completion of international standards, such as ISO/IEC 15962 and ISO/IEC18000.

Specific Application Profile Summary
PROFIdrive The profile specifies the behavior of devices and the access procedure to data for variable-speed electrical drives on PROFIBUS. PA Devices The profile specifies the characteristics of devices of process engineering in process automation on PROFIBUS. Robots/NC The profile describes how handling and assembly robots are controlled over PROFIBUS. Panel Devices describes the interfacing of simple human machine interface devices (HMI) to higher-level automation component s. Encoders describes the interfacing of rotary, angle and linear encoders with single-turn or multi-turn resolution. Fluid Power describes the control of hydraulic drives over PROFIBUS. SEMI describes characteristics of devices for semiconductor manufacture on PROFIBUS. Low-voltage Switchgear describes data exchange for low-voltage switchgear (switchdisconnectors, motor starters, etc.) on PROFIBUS. Dosage and Weighing Devices describes the implementation of weighing and dosage systems on PROFIBUS. Ident Systems describes the communications between devices for identification purposes (bar codes, transponders). Intelligent Pumps describes the implementation of liquid pumps on PROFIBUS DP. Remote I/O for PA Devices describes a different device model (due to the special place of RIO in bus operations).

Master and System Profiles
Master Profiles for PROFIBUS describe classes of controller, each of which support a specific "subset“ of all the possible master functionalities, such as Cyclic and Acyclic Communications, Diagnostics, Interrupt handling, Time-of-day management, Slave-to-slave communication, Isochronous mode, and Safety. System Profiles for PROFIBUS go a step further and describe classes of systems including the master functionality, the possible functionality of Standard Program Interfaces (FB in accordance with IEC 61131-3, safety layer and FDT) and integration options (GSD, EDD and DTM). The figure below shows the standard platforms available today. In PROFIBUS system, the master and system profiles provide the counterpart to the application profiles. Master and system profiles describe specific system parameters that are made available to the field devices, application profiles require specific system parameters in order to simplify their defined characteristics. By using these profiles the device manufacturers can focus on existing or specified system profiles and the system manufacturers can provide the platforms required by the existing or specified device application profiles.

Integration Technologies
Modern field devices in both Factory and Process Automation provide a wide range of information and also execute functions that were previously executed in PLCs and control systems. To execute these tasks, the tools for commissioning, maintenance, engineering, and parameterization of these devices require an exact and complete description of device data and functions, such as the type of application function, configuration parameters, range of values, units of measurement, default values, limit values, identification, etc. The same applies to the controller/control system, whose device-specific parameters and data formats must also be made known (integrated) to ensure error-free data exchange with the field devices. PROFIBUS has developed a number of methods and tools ("integration technologies", GSD, EDD and DTM) which enable standardization of device management. The performance range of these tools is optimized to specific tasks (simplest handling, device-tuning at runtime, ...), which has given rise to the term scaleable device integration. GSD and EDD are both a sort of "Electronic device data sheets", developed with different languages, whilst a DTM (Device Type Manager) is a software component containing specific field device functions for parameterization, configuration, diagnostics and maintenance, generated by mapping and to be used together with the universal software interface FDT (Field Device Tool), which is able to implement software components.

GSD
GSD is a readable ASCII text file and contains both general and device-specific specifications for communication (Communication Feature List) and network configuration. Each of the entries describes a feature that is supported by a device. By means of keywords, a config-uration tool reads the device identification (ID number), the adjustable parameters, the corresponding data type and the permitted limit values for the configuration of the device from the GSD. Some of the keywords are mandatory, for example Vendor_Name. Others are optional, for example Sync_Mode_ supported. A GSD replaces the previously conventional manuals and supports automatic checks for input errors and data consistency, even during the configuration phase. Distiction is made between a device GSD (for an individual device only) and profile GSD, which may be used for devices that comply exactly with a profile such as PROFIdrive or PA Devices. GSD for compact devices, whose block configuration is already known on delivery, can be created completely by the device manufacturer. GSD for modular devices, whose block configuration is not yet conclusively specified on delivery, must be configured by the user in accordance with the actual module configuration using the configuration tool. The device manufacturers are responsible for the scope and quality of the GSD of their devices. Submission of a profile GSD (contains the information from the profile of a device family) or an individual device GSD (device-specific) is essential for certification of a device.


EDD (Electronic Device Description)
EDD is, like a GSD, an electronic device data sheet, but developed by using a more powerful and universal language, the Electronic Device Desciption Language (EDDL). A EDD typically describes the application-related parameters and functions of a field device such as configuration parameters, ranges of values, units of measurement, default values, etc. A EDD is a versatile source of information for engineering, commissioning, runtime, asset management, and documentation. It also contains support mechanisms to integrate existing profile descriptions in the device description, allow references to existing objects, to access standard dictionaries and to allow assignment of the device description to a device. An EDD is independent of operating systems and supports the user by its uniform user and operation interface (only one tool, reliable operation, reduced training and documentation costs) and also the device manufacturer (no specific knowledge required, existing EDDs and libraries can be used). The EDD concept is suitable for tasks of low to middle complexity.

DTM (Device Type Manager)
DTM is a software which is generated by mapping the specific functions and dialog of a field device for parameterization, configuration, diagnostics and maintenance, complete with user interface, in a software component. This component is called DTM and is integrated in the engineering tool or control system over the FDT interface. A DTM uses the routing function of an engineering system for communicating across the hierarchical levels. It works in field devices as a 'driver', similar to a printer driver, which the printer supplier includes in delivery and must be installed on the PC by the user. The DTM is generated by the device manufacturer and is included in delivery of the device.

DTM generation
DTM generation may be performed using one of the following options: Specific programming in a higher programming language, Reuse of existing component or tools through their encapsulation as DTM, Generation from an existing device description using a compiler or interpreter, or Use of the DTM toolkit of MS VisualBasic. With DTMs it is possible to obtain direct access to all field devices for planning, diagnostics and maintenance purposes from a central workstation. A DTM is not a stand-alone tool, but an ActiveX component with defined interfaces. The FDT/DTM concept is protocol-independent and, with its mapping of device functions in software components, opens up interesting new user options. The DTM/FDT concept is very suitable for tasks of middle to high comlexity.

Quality Assurance
In order for PROFIBUS devices of different types and manufacturers to correctly fulfill tasks in the automation process, it is essential to ensure the error-free exchange of information over the bus. The requirement for this is a standard-compliant implementation of the communications protocol and application profiles by device manufacturers.To ensure that this requirement is fulfilled, the PNO has established a quality assurance procedure whereby, on the basis of test reports, certificates are issued to devices that successfully complete the test . Basis for the certification procedure is the standard EN 45000. The PROFIBUS User Organization has approved manufacturer-independent test laboratories in accordance with the specifications of this standard. Only these test laboratories are authorized to carry out device tests, which form the basis for certification. The test procedure, which is the same for all test laboratories, is made up of several parts: The GSD/EDD Check ensures that the device description files comply with the specification. The Hardware Test tests the electric characteristics of the PROFIBUS interface of the device for compliance with the specifications. This includes terminating resistors, suitability of the implemented drivers and other modules and the quality of line level. The Function Test examines the bus access and transmission protocol and the functionality of the test device. The Conformity Test forms the main part of the test. The object is to test conformity of the protocol implementation with the standard. The Interoperability test checks the test device for interoperability with the PROFIBUS devices of other manufacturers in a multivendor plant. This checks that the functionality of the plant is maintained when the test device is added. Operation is also tested with different masters. Once a device has successfully passed all the tests, the manufacturer can apply for a certificate from the PROFIBUS User Organization. Each certified device contains a certification number as a reference. The certificate is valid for 3 years but can be extended after undergoing a further test.

Implementation
For the device development or implementation of the PROFIBUS protocol, a broad spectrum of standard components and development tools (PROFIBUS ASICs, PROFIBUS stacks, monitor and commissioning tools) as well as services are available that enable device manufacturers to realize cost-effective development. A corresponding overview is available in the product catalog of the PROFIBUS User Organization. PROFIBUS Interface Modules are ideal for a low/medium number of devices. These credit card size modules implement the entire bus protocol. They are fitted on the master board of the device as an additional module. PROFIBUS Protocol Chips (Single Chips, Communication Chips, Protocol Chips) are recommendable for an individual implementation in the case of high numbers of devices. The implementation of single-chip ASICs is ideal for simple slaves ( I/O devices). All protocol functions are already integrated on the ASIC. No microprocessors or software are required. Only the bus interface driver, the quartz and the power electronics are required as external components. For intelligent slaves, parts of the PROFIBUS protocol are implemented on a protocol chip and the remaining protocol parts implemented as software on a microcontroller. In most of the ASICS available on the market all cyclic protocol parts have been implemented, which are responsible for transmission of time-critical data. For complex masters, the time-critical parts of the PROFIBUS protocol are also implemented on a protocol chip and the remaining protocol parts implemented as software on a microcontroller. Various ASICs of different suppliers are currently available for the implementation of complex master devices. They can be operated in combination with many common microprocessors. Modem Chips are available to realize the (low) power consumption, which is required when implementing a bus-powered field device with MBP transmission technology. Only a feed current of 10-15 mA over the bus cable is available for these devices, which must supply the overall device, including the bus interface and the measuring electronics. These modems take the required operating energy for the overall device from the MBP bus connection and make it available as feed voltage for the other electronic components of the device. At the same time, the digital signals of the connected protocol chip are converted into the bus signal of the MBP connection modulated to the energy supply.

FIBER OPTICS
An optical fiber (or fibre) is a glass or plastic fiber designed to guide light along its length by confining as much light as possible in a propagating form. In fibers with large core diameter, the confinement is based on total internal reflection. In smaller diameter core fibers, (widely used for most communication links longer than 200 meters) the confinement relies on establishing a waveguide. Fiber optics is the overlap of applied science and engineering concerned with such optical fibers. Optical fibers are widely used in fiber-optic communication, which permits transmission over longer distances and at higher data rates than other forms of wired and wireless

communications. They are also used to form sensors, and in a variety of other applications.
The term optical fiber covers a range of different designs including graded-index optical fibers, step-index optical fibers, birefringent polarization-maintaining fibers and more recently photonic crystal fibers, with the design and the wavelength of the light propagating in the fiber dictating whether or not it will be multi-mode optical fiber or single-mode optical fiber. Because of the mechanical properties of the more common glass optical fibers, special methods of splicing fibers and of connecting them to other equipment are needed. Manufacture of optical fibers is based on partially melting a chemically doped preform and pulling the flowing material on a draw tower. Fibers are built into different kinds of cables depending on how they will be used.
Contents[hide]
1 History
2 Applications
2.1 Optical fiber communication
2.2 Fiber optic sensors
2.3 Other uses of optical fibers
3 Principle of operation
3.1 Multimode fiber
3.2 Singlemode fiber
3.3 Special-purpose fiber
3.4 Materials
3.5 Fiber fuse
4 Manufacturing
5 Optical fiber cables
6 Termination and splicing
7 See also
8 Notes
9 References
10 External links

History
The light-guiding principle behind optical fibers was first demonstrated in by Daniel Collodon and Jaques Babinet in the 1840s, with Irish inventor John Tyndall offering public displays using water-fountains ten years later.[1] Practical applications, such as close internal illumination during dentistry, appearing early in the twentieth century. Image transmission through tubes was demonstrated independently by the radio experimenter Clarence Hansell and the television pioneer John Logie Baird in the 1920s. The principle was first used for internal medical examinations by Heinrich Lamm in the following decade. Modern optical fibers, where the glass fiber is coated with a transparent cladding to offer a more suitable refractive index, appeared in the 1950s.[1] Optical fibers became practical for use in communications in the late 1970s, once the attenuation was reduced sufficiently; since then, several technical advances have been made to improve the attenuation and dispersion properties of optical fibers (i.e., allowing signals to travel farther and carry more information), and lower the cost of fiber communications systems. In 1952 physicist Narinder Singh Kapany conducted experiments that led to the invention of optical fiber, based on Tyndall's earlier studies. Later development, in the early-to-mid twentieth century, focused on the development of fiber bundles for image transmission, with the primary application being the medical gastroscope. The first fiber optic semi-flexible gastroscope was patented by Basil Hirschowitz, C. Wilbur Peters, and Lawrence E. Curtiss, researchers at the University of Michigan, in 1956. In the process of developing the gastroscope, Curtiss produced the first glass-clad fibers; previous optical fibers had relied on air or impractical oils and waxes as the low-index cladding material. A variety of other image transmission applications soon followed.

Applications

Optical fiber communication
Main article: Fiber-optic communication
Optical fiber can be used as a medium for telecommunication and networking because it is flexible and can be bundled as cables. It is especially advantageous for long-distance communications, because light propagates through the fiber with little attenuation compared to electrical cables. This allows long distances to be spanned with few repeaters. Additionally, the light signals propagating in the fiber can be modulated at rates as high as 40 Gb/s, and each fiber can carry many independent channels, each carried by a different wavelength of light. In total, a single fiber-optic cable can carry data at rates as high as 14444 Tb/s. Over short distances, such as networking within a building, fiber saves space in cable ducts because a single fiber can carry much more data than a single electrical cable. Fiber is also immune to electrical interference, which prevents cross-talk between signals in different cables and pickup of environmental noise. Because they are non-electrical, fiber cables can be used in environments where explosive fumes are present, without danger of ignition.
Although fibers can be made out of transparent plastic, glass, or a combination of the two, the fibers used in long-distance telecommunications applications are always glass, because of the lower optical attenuation. Both multi-mode and single-mode fibers are used in communications, with multi-mode fiber used mostly for short distances (up to 500 m), and single-mode fiber used for longer distance links. Because of the tighter tolerances required to couple light into and between single-mode fibers; single-mode transmitters, receivers, amplifiers and other components are generally more expensive than multi-mode components.

Fiber optic sensors
Optical fibers can be used as sensors to measure strain, temperature, pressure and other parameters. The small size and the fact that no electrical power is needed at the remote location gives the fiber optic sensor advantages to conventional electrical sensor in certain applications.
Optical fibers are used as hydrophones for seismic or SONAR applications. Hydrophone systems with more than 100 sensors per fiber cable have been developed. Hydrophone sensor systems are used by the oil industry as well as a few countries' navies. Both bottom mounted hydrophone arrays and towed streamer systems are in use. The German company Sennheiser developed a microphone working with a laser and optical fibers[2].
Optical fiber sensors for temperature and pressure have been developed for downhole measurement in oil wells. The fiber optic sensor is well suited for this environment as it is functioning at temperatures too high for semiconductor sensors (Distributed Temperature Sensing).
Another use of the optical fiber as a sensor is the optical gyroscope which is in use in the Boeing 767 and in some car models (for navigation purposes) and the use in Hydrogen microsensors.

Other uses of optical fibers

A frisbee illuminated by fiber optics
Fibers are widely used in illumination applications. They are used as light guides in medical and other applications where bright light needs to be shone on a target without a clear line-of-sight path. In some buildings, optical fibers are used to route sunlight from the roof to other parts of the building (see non-imaging optics). Optical fiber illumination is also used for decorative applications, including signs, art, and artificial Christmas trees. Swarovski boutiques use optical fibers to illuminate their crystal showcases from many different angles while only employing one light source. Optical fiber is an intrinsic part of the light-transmitting concrete building product, LiTraCon.
A fiber-optic Christmas Tree
Optical fiber is also used in imaging optics. A coherent bundle of fibers is used, sometimes along with lenses, for a long, thin imaging device called an endoscope, which is used to view objects through a small hole. Medical endoscopes are used for minimally invasive exploratory or surgical procedures (endoscopy). Industrial endoscopes (see fiberscope or borescope) are used for inspecting anything hard to reach, such as jet engine interiors.
An optical fiber doped with certain rare-earth elements such as erbium can be used as the gain medium of a laser or optical amplifier. Rare-earth doped optical fibers can be used to provide signal amplification by splicing a short section of doped fiber into a regular (undoped) optical fiber line. The doped fiber is optically pumped with a second laser wavelength that is coupled into the line in addition to the signal wave. Both wavelengths of light are transmitted through the doped fiber, which transfers energy from the second pump wavelength to the signal wave. The process that causes the amplification is stimulated emission.
Optical fibers doped with a wavelength shifter are used to collect scintillation light in physics experiments.
Optical fiber can be used to supply a low level of power (around one watt) to electronics situated in a difficult electrical environment. Examples of this are electronics in high-powered antenna elements and measurement devices used in high voltage transmission equipment.

Principle of operation
An optical fiber is a cylindrical dielectric waveguide that transmits light along its axis, by the process of total internal reflection. The fiber consists of a core surrounded by a cladding layer. To confine the optical signal in the core, the refractive index of the core must be greater than that of the cladding. The boundary between the core and cladding may either be abrupt, in step-index fiber, or gradual, in graded-index fiber.

Multimode fiber

The propagation of light through a multi-mode optical fiber.
Fiber with large (greater than 10 μm) core diameter may be analyzed by geometric optics. Such fiber is called multimode fiber, from the electromagnetic analysis (see below). In a step-index multimode fiber, rays of light are guided along the fiber core by total internal reflection. Rays that meet the core-cladding boundary at a high angle (measured relative to a line normal to the boundary), greater than the critical angle for this boundary, are completely reflected. The critical angle (minimum angle for total internal reflection) is determined by the difference in index of refraction between the core and cladding materials. Rays that meet the boundary at a low angle are refracted from the core into the cladding, and do not convey light and hence information along the fiber. The critical angle determines the acceptance angle of the fiber, often reported as a numerical aperture. A high numerical aperture allows light to propagate down the fiber in rays both close to the axis and at various angles, allowing efficient coupling of light into the fiber. However, this high numerical aperture increases the amount of dispersion as rays at different angles have different path lengths and therefore take different times to traverse the fiber. A low numerical aperture may therefore be desirable.
In graded-index fiber, the index of refraction in the core decreases continuously between the axis and the cladding. This causes light rays to bend smoothly as they approach the cladding, rather than reflecting abruptly from the core-cladding boundary. The resulting curved paths reduce multi-path dispersion because high angle rays pass more through the lower-index periphery of the core, rather than the high-index center. The index profile is chosen to minimize the difference in axial propagation speeds of the various rays in the fiber. This ideal index profile is very close to a parabolic relationship between the index and the distance from the axis.

Singlemode fiber

A typical single-mode optical fiber, showing diameters of the component layers.
Fiber with a core diameter less than about ten times the wavelength of the propagating light cannot be modeled using geometric optics. Instead, it must be analyzed as an electromagnetic structure, by solution of Maxwell's equations as reduced to the electromagnetic wave equation. The electromagnetic analysis may also be required to understand behaviors such as speckle that occur when coherent light propagates in multi-mode fiber. As an optical waveguide, the fiber supports one or more confined transverse modes by which light can propagate along the fiber. Fiber supporting only one mode is called single-mode or mono-mode fiber. The behavior of larger-core multimode fiber can also be modeled using the wave equation, which shows that such fiber supports more than one mode of propagation (hence the name). The results of such modeling of multi-mode fiber approximately agree with the predictions of geometric optics, if the fiber core is large enough to support more than a few modes.
The waveguide analysis shows that the light energy in the fiber is not completely confined in the core. Instead, especially in single-mode fibers, a significant fraction of the energy in the bound mode travels in the cladding as an evanescent wave.
The most common type of single-mode fiber has a core diameter of 8 to 10 μm and is designed for use in the near infrared. It is notable that the mode structure depends on the wavelength of the light used, so that this fiber actually supports a small number of additional modes at visible wavelengths. Multi-mode fiber, by comparison, is manufactured with core diameters as small as 50 microns and as large as hundreds of microns.

Special-purpose fiber
Some special-purpose optical fiber is constructed with a non-cylindrical core and/or cladding layer, usually with an elliptical or rectangular cross-section. These include polarization-maintaining fiber and fiber designed to suppress whispering gallery mode propagation.

Materials
Glass optical fibers are almost always made from silica, but some other materials, such as fluorozirconate, fluoroaluminate, and chalcogenide glasses, are used for longer-wavelength infrared applications. Like other glasses, these glasses have a refractive index of about 1.5. Typically the difference between core and cladding is less than one percent.
Plastic optical fiber (POF) is commonly step-index multimode fiber, with core diameter of 1 mm or larger. POF typically has much higher attenuation than glass fiber (that is, the amplitude of the signal in it decreases faster), 1 dB/m or higher, and this high attenuation limits the range of POF-based systems.

Fiber fuse
At high optical intensities, above 2 megawatts per square centimetre, when a fiber is subjected to a shock or is otherwise suddenly damaged, a fiber fuse can occur. The reflection from the damage vaporizes the fiber immediately before the break, and this new defect remains reflective so that the damage propagates back toward the transmitter at 1–3 meters per second [3],[4],[5]. The open fiber control system, which ensures laser eye safety in the event of a broken fiber, can also effectively halt propagation of the fiber fuse [6]. In situations, such as undersea cables, where high power levels might be used without the need for open fiber control, a "fiber fuse" protection device at the transmitter can break the circuit to prevent any damage.

Manufacturing
Standard optical fibers are made by first constructing a large-diameter preform, with a carefully controlled refractive index profile, and then pulling the preform to form the long, thin optical fiber. The preform is commonly made by three chemical vapor deposition methods: inside vapor deposition, outside vapor deposition, and vapor axial deposition.
With inside vapor deposition, a hollow glass tube approximately 40 cm in length known as a "preform" is placed horizontally and rotated slowly on a lathe, and gases such as silicon tetrachloride (SiCl4) or germanium tetrachloride (GeCl4) are injected with oxygen in the end of the tube. The gases are then heated by means of an external hydrogen burner, bringing the temperature of the gas up to 1900 Kelvin, where the tetrachlorides react with oxygen to produce silica or germania (germanium oxide) particles. When the reaction conditions are chosen to allow this reaction to occur in the gas phase throughout the tube volume, in contrast to earlier techniques where the reaction occurred only on the glass surface, this technique is called modified chemical vapor deposition.
The oxide particles then agglomerate to form large particle chains, which subsequently deposit on the walls of the tube as soot. The deposition is due to the large difference in temperature between the gas core and the wall causing the gas to push the particles outwards (this is known as thermophoresis). The torch is then traversed up and down the length of the tube to deposit the material evenly. After the torch has reached the end of the tube, it is then brought back to the beginning of the tube and the deposited particles are then melted to form a solid layer. This process is repeated until a sufficient amount of material has been deposited. For each layer the composition can be varied by varying the gas composition, resulting in precise control of the finished fiber's optical properties.
In outside vapor deposition or vapor axial deposition, the glass is formed by flame hydrolysis, a reaction in which silicon tetrachloride and germanium tetrachloride are oxidized by reaction with water (H2O) in an oxyhydrogen flame. In outside vapor deposition the glass is deposited onto a solid rod, which is removed before further processing. In vapor axial deposition, a short seed rod is used, and a porous preform, whose length is not limited by the size of the source rod, is built up on its end. The porous preform is consolidated into a transparent, solid preform by heating to about 1800 Kelvin.
The preform, however constructed, is then placed in a device known as a drawing tower, where the preform tip is heated and the optic fiber is pulled out as a string. By measuring the resultant fiber width, the tension on the fiber can be controlled to maintain the fiber thickness.
This manufacturing process is accomplished by numerous optical fiber companies like Corning, OFS, Sterlite Optical Technologies, Furukawa, Sumitomo, Fujikura and Prysmian. In addition, various fiber optic component manufacturers, assembly houses, and custom fiber optic providers exist.

Optical fiber cables
In practical fibers, the cladding is usually coated with a tough resin buffer layer, which may be further surrounded by a jacket layer, usually plastic. These layers add strength to the fiber but do not contribute to its optical wave guide properties. Rigid fiber assemblies sometimes put light-absorbing ("dark") glass between the fibers, to prevent light that leaks out of one fiber from entering another. This reduces cross-talk between the fibers, or reduces flare in fiber bundle imaging applications.[7]
For indoor applications, the jacketed fiber is generally enclosed, with a bundle of flexible fibrous polymer strength members like Aramid (e.g. Twaron or Kevlar), in a lightweight plastic cover to form a simple cable. Each end of the cable may be terminated with a specialized optical fiber connector to allow it to be easily connected and disconnected from transmitting and receiving equipment.
For use in more strenuous environments, a much more robust cable construction is required. In loose-tube construction the fiber is laid helically into semi-rigid tubes, allowing the cable to stretch without stretching the fiber itself. This protects the fiber from tension during laying and due to temperature changes. Alternatively the fiber may be embedded in a heavy polymer jacket, commonly called "tight buffer" construction. These fiber units are commonly bundled with additional steel strength members, again with a helical twist to allow for stretching.
Another critical concern in cabling is to protect the fiber from contamination by water, because its component hydrogen (hydronium) and hydroxyl ions can diffuse into the fiber, reducing the fiber's strength and increasing the optical attenuation. Water is kept out of the cable by use of solid barriers such as copper tubes, water-repellant jelly, or more recently water absorbing powder, surrounding the fiber.
Finally, the cable may be armored to protect it from environmental hazards, such as construction work or gnawing animals. Undersea cables are more heavily armored in their near-shore portions to protect them from boat anchors, fishing gear, and even sharks, which may be attracted to the electrical power signals that are carried to power amplifiers or repeaters in the cable.
Modern fiber cables can contain up to a thousand fibers in a single cable, so the performance of optical networks easily accommodates even today's demands for bandwidth on a point-to-point basis. However, unused point-to-point potential bandwidth does not translate to operating profits, and it is estimated that no more than 1% of the optical fiber buried in recent years is actually 'lit'.
Modern cables come in a wide variety of sheathings and armor, designed for applications such as direct burial in trenches, dual use as power lines [1], installation in conduit, lashing to aerial telephone poles, submarine installation, or insertion in paved streets. In recent years the cost of small fiber-count pole mounted cables has greatly decreased due to the high Japanese and South Korean demand for Fiber to the Home (FTTH) installations.

Termination and splicing
ST fiber connector on multimode fiber
Optical fibers are connected to terminal equipment by optical fiber connectors. These connectors are usually of a standard type such as FC, SC, ST, LC, or MTRJ.
Optical fibers may be connected to each other by connectors or by splicing, that is, joining two fibers together to form a continuous optical waveguide. The generally accepted splicing method is arc fusion splicing, which melts the fiber ends together with an electric arc. For quicker fastening jobs, a "mechanical splice" is used.
Fusion splicing is done with a specialized instrument that typically operates as follows: The two cable ends are fastened inside a splice enclosure that will protect the splices, and the fiber ends are stripped of their protective polymer coating (as well as the more sturdy outer jacket, if present). The ends are cleaved (cut) with a precision cleaver to make them perpendicular, and are placed into special holders in the splicer. The splice is usually inspected via a magnified viewing screen to check the cleaves before and after the splice. The splicer uses small motors to align the end faces together, and emits a small spark between electrodes at the gap to burn off dust and moisture. Then the splicer generates a larger spark that raises the temperature above the melting point of the glass, fusing the ends together permanently. The location and energy of the spark is carefully controlled so that the molten core and cladding don't mix, and this minimizes optical loss. A splice loss estimate is measured by the splicer, by directing light through the cladding on one side and measuring the light leaking from the cladding on the other side. A splice loss under 0.1 dB is typical. The complexity of this process is the major thing that makes fiber splicing more difficult than splicing copper wire.
Mechanical fiber splices are designed to be quicker and easier to install, but there is still the need for stripping, careful cleaning and precision cleaving. The fiber ends are aligned and held together by a precision-made sleeve, often using a clear gel (index matching gel) that enhances the transmission of light across the joint. Such joints typically have higher optical loss, and are less robust than fusion splices, especially if the gel is used. All splicing techniques involve the use of an enclosure into which the splice is placed for protection afterward.
Fibers are terminated in connectors so that the fiber end is held at the end face precisely and securely. A fiber optic connector is basically a rigid cylindrical barrel surrounded by a sleeve that holds the barrel in its mating socket. It can be push and click, turn and latch, or threaded. A typical connector is installed by preparing the fiber end and inserting it into the rear of the connector body. Quick set glue is usually used so the fiber is held securely, and a strain relief is secured to the rear. Once the glue has set, the end is polished to a mirror finish. Various types of polish profile are used, depending on the type of fiber and the application. For singlemode fiber, the fiber ends are typically polished with a slight curvature, such that when the connectors are mated the fibers touch only at their cores. This is known as a "physical contact" (PC) polish. The curved surface may be polished at an angle, to make an angled physical contact (APC) connection. Such connections have higher loss than PC connections, but greatly reduced backreflection, because light that reflects from the angled surface leaks out of the fiber core; the resulting loss in signal strength is known as gap loss.
Various methods to align two fiber ends to each other or one fiber to an optical device (VCSEL, LED, waveguide etc.) have been reported. They all follow either an active fiber alignment approach or a passive fiber alignment approach.
In 1965, Charles K. Kao and George A. Hockham of the British company Standard Telephones and Cables were the first to suggest that attenuation of contemporary fibers was caused by impurities, which could be removed, rather than fundamental physical effects such as scattering. They speculated that optical fiber could be a practical medium for communication, if the attenuation could be reduced below 20 dB per kilometer (Hecht, 1999, p. 114).This attenuation level was first achieved in 1970, by researchers Robert D. Maurer, Donald Keck, Peter Schultz, and Frank Zimar working for American glass maker Corning Glass Works, now Corning Inc. They demonstrated a fiber with 17 dB optic attenuation per kilometer by doping silica glass with titanium. A few years later they produced a fiber with only 4 db/km using germanium oxide as the core dopant. Such low attenuations ushered in optical fiber telecommunications and enabled the Internet.
On 22 April, 1977, General Telephone and Electronics sent the first live telephone traffic through fiber optics, at 6 Mbit/s, in Long Beach, California.
The erbium-doped fiber amplifier, which reduced the cost of long-distance fiber systems by eliminating the need for optical-electrical-optical repeaters, was invented by David Payne of the University of Southampton, and Emmanuel Desurvire at Bell Laboratories in 1986. The two pioneers were awarded the Benjamin Franklin Medal in Engineering in 1998.
The first transatlantic telephone cable to use optical fiber was TAT-8, based on Desurvire optimized laser amplification technology. It went into operation in 1988.
TAT-8 was developed as the first transatlantic undersea fiber optic link between the United States and Europe. TAT-8 is more than 3000 nautical miles in length and was the first oceanic fiber optic cable. It was designed to handle a mix of information. When inaugurated, it had an estimated lifetime in excess of 20 years. TAT-8 was the first of a new class of cables, even though it had already been used in long-distance land and short-distance undersea operations. Its installation was preceded by extensive deep-water experiments and trials conducted in the early 1980s to demonstrate the project's feasibility.
In 1991, the emerging field of photonic crystals led to the development of photonic crystal fiber (Science (2003), vol 299, page 358), which guides light by means of diffraction from a periodic structure, rather than total internal reflection. The first photonic crystal fibers became commercially available in 1996 [2]. Photonic crystal fibers can be designed to carry higher power than conventional fiber, and their wavelength dependent properties can be manipulated to improve their performance in certain applications.