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Articles and Technical Papers

The FSK Advantage: New Techniques Support Enhanced Data Acquisition and Control

ISA (July 1995)

Michel E. Maes, President
James R. Steffey, Vice President
Data-Linc Group
2635 151st Place NE
Redmond, Washington 98052
The United States of America
Frequency Shift Key (FSK) signaling technology and signal conditioning provides significant advantages and benefits for economical and versatile distributed data acquisition and control requirements including communications accuracy, reliability and integrity in the transmission of digital, analog, discrete and pulse stream signals for monitoring and control applications. Principle benefits include: immunity to "adverse environment" conditions (i.e. electromagnetic interference, noise, surge, ground loop/ground plane shift problems); the ability to transmit data across commutators or sparking sources (sliding contacts, slip rings, rolling wheels, etc.): the transmission of any type of data over any two conductor wire, shielded or unshielded, twisted or untwisted, including existing in-place writing (up to 600 volt AC or DC power lines, PBX telephone wires, instrument loop wiring, etc.). Applications include polling, addressing, multi-drop and multiplexing installations. In summary, the FSK Advantage results in user-friendly ease of installation, operation and reconfiguration as well as significantly enhanced range, accuracy, reliability, and data integrity in hostile, harsh environments with the utmost in transparency (no software, protocol matching, baud rate settings, signal conditioning or PC required).

Have you ever needed to monitor or control several channels of digital, discrete or analog I/O at some distance for the PLC, RTU or host computer and experienced either a) wiring problems, b) noise problems, or c) both? Connectivity challenges, including the escalating cost of wire installation, are becoming more troublesome as the complexity of data acquisition and control presents instrumentation engineers and technicians with technology overload headaches. As one harried engineer put it: "I just don't have the time to troubleshoot equipment breakdowns or data transmission interruptions>" Unfortunately, the complexity of today's digital technology has reached a point where it has become increasingly difficult to identify the problem let alone getting operations back on-line following a costly shutdown. An alternative is now available, however, in which advanced FSK technology can be applied to conditions requiring the utmost reliability and "user friendly" ease of installation, operation and reconfiguration, often at significantly lower cost. The technology supports data transmission for a wide variety of applications, such as process control, data acquisition, monitoring, energy management, access control, etc. Modular, portable units may be used for point-to-point, multi-drop, polling/addressing, multiplexing, of full LAN applications transmitting discrete, analog (4-20 mA, 0-5 volt, etc.), pulse stream or digital (RS232, 422, or 485) data.

Applications for the technology include: Data communication between PLCs, RTUs and PCs; security system and access control monitoring; automated materials handling and warehousing (i.e. transmitting data from control computers to stacker cranes over the AC/DC lines supplying such devices, across the sliding contacts); remote display in driver/electronic sign control; mine safety data gathering; waste water treatment remote monitoring/control tasks; and multi-station process control/monitoring including SCADA. In summary, the principal features and benefits include:

  1. Operate on Virtually Any Wires Available: The FSK modules can communicate on existing wires, such as PBX telephone lines (without interfering in any way with simultaneous telephone use), AC or DC power lines (up to 600 volts, carrying no more than 200 amps), 4-20 mA instrument loop wires or spare pair, as well as any other two conductor wire (twisted or untwisted, shielded or unshielded) with excellent data accuracy, reliability and integrity.
  2. High Data Rates/Long Range: Serial data rates up to 230 Kbaud are available. The units accept any square wave or logic level input and faithfully reproduce it at the receiving end. The distance capability is 20 miles on PBX telephone lines or unloaded twisted pair. Since the devices can typically tolerate three orders of magnitude (-30 dB) signal attenuation between the transmitter and receiver, smaller wire size can be used in many installations.
  3. Excellent Immunity to Harsh Environments: The FSK devices are designed with transformer and capacitive isolation from the data communication lines, and utilize advanced FSK technology, making them very tolerant of voltage and current surges and shifting ground planes. This provides "insurance" against current and future adverse environments (i.e. high EMI or noise conditions, ground loop problems and data over sliding contacts, slip rings, rolling wheels or other sparking sources and inherent circuit protection against electrical damage.
  4. Universal Interfaces - Digital, Analog, Discrete, Pulse, Voice: The devices can accept and deliver virtually and signal format, from the conventional TTL, RS-232, 422 and 485 to variable pulse width and rate square waves of any magnitude. Also available are interfaces which can directly accept and deliver analog signals at substantially lower costs than standard Al/D conversion methods, transmit discrete (switch open or close) signals, and send voice over power lines or instrumentation wires.
  5. Multiple Operating Modes: Point-to-Point, Multidrop, addressing, polling and Multiplexing:
    • The units are easily interconnected in point-to-point or multidrop networks.
    • IN multidrop polling applications, the units can also be connected to smart devices for address code recognition and addressable daughter boards can even be incorporated.
    • Switching of data communications lines in star network and several central unit configurations for compact termination of substantial numbers of multiple data lines is also provided.
    • An example of a typical multidrop installation supporting both polling and multiplexing is illustrated herein.
  6. Transparency: The modules are designed for true "plug and play" installation, eliminating any requirements for software, baud rate settings, signal conditioning, protocol matching, etc. PCs and/or software are employed only if needed for monitoring, control or display purposes.
  7. Elimination of Enable Signal Wires: Proprietary modules now exist allowing EIA/RS-485 networks to operate without the need for separate unable signal lines to control the drivers or receivers. For multi-point multidrop or addressable, pollable RS-422/485 data installations, this "Auto-Enable" capability is consistent with the utilization of existing in-place wiring as an alternative to expensive cable installation by also eliminating the need for separate enable wiring.

In Summary, the ability to move any data (digital, analog, discrete or pulse stream) on any two conductor already existing or installed wiring can result in substantial saving over the expense of traditional cable installation, particularly for difficult applications or in hostile environments.

The FSK Advantage
Traditionally, data is transmitted between computers and/or peripheral devices such as Process Logic Controllers (PLCs) and Remote Telemetry Units (RTUs), either by direct multi-wire cable or through a "modem". Direct multi-wire cable is large (9-25 wires), bulky, cumbersome, and frequently difficult and expensive to install. It has a range limited to about 100 feet without line drivers. A modem is a device that converts digital signals into a format that can be transmitted much longer distances over single or double wire pairs, or telephone lines. Most modems on the market are made to transmit data over the public telephone lines, and the telephone cannot be used for voice while data is being transmitted. Special modems are also available, however, that allow voice and data to be transmitted simultaneously without interfering with each other over private, in-facility PBX telephone lines, or to transmit data over facility 4-20 mA instrument loop wires or power lines. Other versions can send data over any tow conductor wire at distances of over five miles, full duplex (data traveling in both directions simultaneously) and up to twenty miles half duplex. In addition, add-on single coupling modules permit data to be easily transferred from on wore pair to another such as PBX telephone to AC power wiring. Finally, other modules allow the modems to be used in multi-drop mode so that data can be obtained from many remote monitoring locations over any existing or to-be-installed single two wire network. In such cases the FSK signal transmitted form each station is turned on and off locally so only one FSK signal is on the line at one time.

Currently, the vast majority of data transmitted in industrial/commercial monitoring and control applications is in digital form, typically either RS-232, 422 or 485, except where extensive ISDN type installations involving Computer Integrated Manufacturing (CIM) networks are required. Nevertheless, much of the equipment is often analog in which A/D and D/A conversions and instrument loop wire installations are a typical necessary expense in moving the data from where it originates to where it is needed. Analog to digital conversion for the sole purpose of data transmission to other locations (where additional D/A converters must be installed if sensor outputs are being sent, for example, to analog recorders or totalizers) can prove to be more expensive and less reliable than the now available advanced "modem" devices providing a number of distinct advantages as itemized in the Introduction.

The FSK Advantage is based upon the ability to convert ASCII data streams, or any other digitally encoded information, into frequency equivalents prior to transmission from point-to-point. Digital information is coded in a series of 1s and 0s (high and low or space and mark) which are represented by two voltage levels. It travels between electronic devices such as computers or PLCs as a flow of logic signals that shift back and forth between the two voltage levels. If electrical noise, changing ground voltage or signal attenuation generates a "threshold" level difference between the two voltages, data error can result.

An FSK "MODEM" converts the data stream from changing voltage levels to changing frequencies. One frequency is assigned to the "1" and another to the "0". The FSK transmitter converts digital signals to frequency (Modulation) and the receiver converts it back to digital form (DEModulation). With FSK, data is not transmitted as a series of voltage step changes, but instead as two alternating frequencies. Data communications problems are dealt with effectively because of this unique characteristic of the FSK signal. Clearly, electromagnetic interference, shifting ground planes, ground loops and long transmission distances all create problems that compromise data integrity. FSK is a communication technology largely immune to such problems also providing exceptional flexibility in the selection of a communication path.

  1. Noise – frequency is far less prone to interference than changing voltage levels. FSK frequencies can be selected to avoid electrical noise and the receiver circuits designed to reject out-of-band noise.
  2. Unstable Ground – The shifting FSK signals take the form of an AC signal. Since an AC signal has no ground reference, it is unaffected by ground plane changes.
  3. Range – An FSK receiver can detect frequency changes even if they are of very low amplitude. FSK signals can be of millivolt levels while direct digital data must be maintained at several volts to be read reliably. Data transmission distances of as much as ten to twenty miles are therefore typical.
  4. Data Paths – Using capacitors and transformers, the FSK signal can be coupled to other conductors such as telephone, instrumentation or power lines. Selective frequency filters are used to separate the FSK signal from other signals on the line. FSK is also particularly well suited for data transmission over sliding or rolling contacts which are prone to changing resistance caused by dirt and scale. The FSK Advantage is ideally suited to conditions where the transmission of digitally encoded data streams compromises data integrity as a result of the sensitivity of digital data to so-called "adverse environment" conditions. In addition, the technology can be utilized not only to transmit discrete and digitally encoded data as frequency equivalents, but to move analog data as a variable pulse rate without the traditional A/D and D/A conversions. The technology not only enhances data integrity but also provides an alternative to cable installation by superimposing discrete, digital, analog or pulse stream data on in-place power, PBX, instrumentation or other existing wiring without affecting what those wires are already being used for.

Industrial data acquisition and process control challenges in high value-added operations are such that equipment reliability and dependability are essential in preventing costly down time incidents. In this digital era, equipment malfunctions can result in extensive and lengthy troubleshooting efforts before a problem can be detected and corrected. Too often, the inability to rapidly identify data transmission reliability as the culprit can significantly compromise product quality or delay resumption of operations. The care taken, therefore, in the selection of computer hardware, software and peripheral equipment such as PLCs or RTUs should also be applied to the recognition that data communications between devices is every bit as critical, not only in controlling installation costs, but also toward insuring efficient, reliable on-going operations.

The ultimate objective of any data communication device is to provide error free data. A further practical objective is trouble free operation and ease of installation. The inherent characteristics of FSK technology allow these objectives to be achieved in many instances when straight digital communications is potentially marginal or unreliable. A properly designed FSK modem can not only be "plug and play" but also deliver on-going maintenance free service. FSK sidesteps most basic communications problems and its versatility results in many innovative data communications solutions.

Comprehensive Monitoring and Control System

The Comprehensive Monitoring and Control System (CMCS) illustrates how the technology may be applied in a typical installation, superimposing discrete, digital and analog data on the in-place facility PBX wiring for remote data acquisition and control between a host computer, PLCs and RTUs. As more and more data are transmitted with increasingly complex SCADA type systems, requirements are proliferating not only for monitoring and control functions, but for additional data as well such as access control, energy management, etc. Installation of independent datacom systems to deal with these requirements results in cumbersome, expensive and difficult to maintain wiring networks subject to on-going periodic interruptions or degraded performance, too often even involving undetected error. The CMCS illustrates how significant cost, reliability and other advantages may be obtained with application of the FSK advantage to large-scale distributed monitoring and control tasks. A typical installation is shown herein.

Typical Installation

FSK, Frequency Shift Key, wire line modem installation 1

Interfacing FSK Modems With PLC Processor Equipment

Advanced technology FSK Modems may be used to significantly enhance the accuracy, reliability and data integrity of PLC Processor equipment in the transmission of data between separate locations. Whereas the PLC Processor companies expend considerable effort to optimize their equipment for bulletproof reliability, accuracy and data integrity, the requirement to transmit data from point to point imposes real-world limitations and problems. Typical industrial adverse environment conditions such as noise, surge, and electromagnetic interference as well as ground loop/shifting ground plane problems are often encountered. Range limitations may also arise. Modems employing The FSK Advantage support the transmission of data, on any two-conductor wire, to and from virtually all PLC Processors. User-friendly ease of installation results from the well-deserved reputation for simple plug and play connections that is the hallmark of FSK technology.

Selected Typical Examples

FSK, Frequency Shift Key, wire line modem installation 2

FSK, Frequency Shift Key, wire line modem installation 3

FSK, Frequency Shift Key, wire line modem installation 4

FSK, Frequency Shift Key, wire line modem installation 5

FSK, Frequency Shift Key, wire line modem installation 6

FSK, Frequency Shift Key, wire line modem installation 7

Intelligent RTU Systems

The FSK modems, in combination with special RTU multiplexing signal conditioning boards, can provide full systems capability, not only transmitting the data on any two conductor wire. The ultimate in transparency serves to support ease of on-going operations. There is no start up or restart procedure. Even when power may be lost intermittently, the system automatically restarts, without any operator intervention at all. A typical Polling/Multiplexing Installation is illustrated herein.

Typical Polling/Multiplexing Installation

 Polling/Multiplexing Installation


For decades, modules based on the inherently rugged, robust FSK technology have been plugged into the AC electrical outlets at home for the purpose of remote operation of appliances. Advanced FSK technology can now provide important and critical advantages where reliable data communications are essential to optimize accuracy and reliability for industrial data communications. Clearly, any significant improvement in the inherent immunity to EMI, noise, surge, ground loop or shifting ground plane headaches provides present and future freedom from the "hassle" problems that can plague instrumentation technicians and other operations personnel in their efforts to insure on-going data reliability and integrity. In summary, while many features and characteristics of the technology serve to facilitate dependable, reliable and economical data communications even in hostile environments, the important "bottom line" advantage of this technical approach to industrial data communications is the ease of installation, reliability of on-going operations and ease of reconfiguration, with the also important cost savings as frosting on the cake.

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