Articles & Technical Papers
How to Improve Industrial Data Communications
Control Engineering (March 1996)
Industrial data communication is characterized by its operating environment. Electromagnetic interference (EMI), long distances and physical barriers set industrial communications apart from typical business office requrements. All too often, however, conventional data communications equipment is called upon to function properly in industrial environments with unsatisfactory results. Conventional equipment usually lacks the versatility to adapt to the unique requirements of data monitoring and process control. In response to the growing needs in industrial data communications, a number of purpose developed industrial data communications devices have entered the marketplace. their designs are a result of field experience in solving difficult data communications problems and optimizing characteristics for all aspects of reliability and economy.
As a part of examining the requirements for these devices, it is necessary to define the industrial environment more precisely. In the context of this discussion, industrial communications includes both relatively short range, in-plant connnections as well as medium and long range communications to remote locations and between facilities. EMI covers many kinds of effects such as noise caused locally by electric motors and starters to longer range effects such as shifting ground planes and surge created by weather conditions. Physical barriers involves not only actual physical impediments to installing data cable but also economic considerations in burying or stringing wire over extended distance. All these factors pose challenges to data transmission and the new industrial data communications devices have evolved in response to such challenges.
Industrial data, in its raw form, is usually either analog or discrete. Analog is a changing signal which is generally proportional to changing physical phenomena such as flow, pressure, temperature or level. Discrete (also sometimes called digital, but not to be confused with digitized, digital data streams) is typically a position, motor start/stop or valve on/off. When monitoring, controlling or processing groups of these signals, it is more convenient to digitize them and to communicate them as digital data streams. Such digital data is formatted as a series of carefully timed voltage pulses or bits, logic high and logic low, which form coded messages that microprocessors can manipulate. The faster the data or data rate, the shorter the individual bits and the more important becomes the resolution of those bits to regenerate the data accurately. Since digital dat streams are made up of step changes in voltage, measuring them at the receiving end is the challenge. Usually the received voltage must be a least 10% or more of the transmitted voltage. Such accurate reception can be achieved on directly connected data cable if the environment is stable and distances are a few thousand feet or less. Introduce EMI and/or longer distances and digital data streams run into trouble.
One of the most common methods of overcoming the basic limitations of digital data streams involves using the digital data logichighs and lows to modulate a carrier signal. Aproperly selected and configured carrier signalcan overcome most of the EMI anddistance problemsthat affect direct digital data stream communication. Since the carrier is an AC signal with no ground reference, and because reception of the carrier involves detecting frequency rather than voltage level, carrier modulation is a much more noise immune and versatile data communication technique. Modulated carriers can be transmitted many miles on wires (or worldwide over the public telephone networks), impressed on power and instrumentation lines, sent through sliding contacts and also transmitted wireless. Carrier modulation is the key to successfull industrial data communications in harsh environments. A more complete discussion of carrier modulatiois found in the section below entitled Terminology.
While carrier modulation is essential to solving most industrial comTop of Page ^munications problems and a broad range of products use carrier modulation in one form or another, it is not the only factor in arriving at the proper communication solution. the correct implementation of carrier modulation involves more than the selection of an appropriate carrier and modulation technique. Of equal importance are a number of other practical factors which take the total job to be done into consideration.
In the real world of industrial data communications, data is usually transmitted between Process Logic Controllers (PLCs) or between PLCs and computers. Each major brand of PLC has its own specific communications protocol (roughly translated, its own code). This means that data communications equipment must be compatible with or adjustable in order to interface and function with many different devices.In addition to standard RS-232, many devices use RS-422 or 485 digital interfaces.The communications devices must also deal with different word lengthsand special attention pulses. Decisions regarding which communications device to use must take these factors into account if simple istallation and interoperability is to be accomplished.
Some data communication tasks are only point-to-point but many more involve multiple remotes controlled from a central location.The data communications devices must operate in a multi-drop mode, in most instances providingcarrier control so that only one responding carrier is on at one time. In some instances, it is desirable to operate two tier multi-drops. This requires special capability since there is no means for providing an enable signal to control the carriers on the first tier devices.
Many systemcommunications tasks require combinations of different kinds of modems. For example, a long range communication link using dial-up modems can be combined with a short range in-facility link using power line modems to monitor the performance of a group of water pumps from across the country. With communication devices that are designed to work together, many useful combinations can be devised to optimize any communications task.
In addition to satisfying technical requirements, total cost must also be considered. In some instances, there is a cost trade-off. An example is the use of relatively expensive radio modems instead of lower cost leased line modems which involve an ongoing monthly line charge. In other cases an analysis must be mead comparing the expense of installing cable as an alternative to the use of modems on existing wires. In all cases, however, the cost if installing and configuring the communications devices must be estimated. To the extent that the modems cen be acquired factory pre-configured for simple installation, a substantial cost savings can often be realized. It is not unusual that the cost associated with installing (or, if not done properly, re-installing) the devices is greater than the price of the hardware, not even taking into account the time expended.
As industrial data communications requirements grow and communications device capability expands, many new features are becoming available. High performance spread spectrum radio data communications is a particularly exciting area. Until recently, spread spectrum radio capability was limited to a one or two mile range at 9600 baud, point-to-point. A new generation of these devices are now available with a range capability to 20 miles, line-of-sight , although at a distance of up to one mile the line-of-sight restriction does not apply. With Repeaters, range can be extended to 60 miles. The radios deliver full duplex uncompressed data rates of up to 115.2 Kbaud and operational capabilities including point-to-point, multi-drop and Repeater configurations; transparent PLC and individual radio addressing: two tier multi-dropping and multiple digital interfacing. Other new data communications features include combining data with real time video, wireless dial-up monitoring using the celular telephone network and a new generation of advanced PLC rack-mounted and imbedded modems.
Purpose designed industrial data communications devices are heavy-duty tools. They can be configured to do a specific job and yet be versatile enough to adapt to multiple applications, operate under many different conditions, function reliably in harsh environments and use a wide variety of communications paths. A broad family of modems, designed to work together as seamlessly as possible, is an essential ingredient for advanced industrial control.
Terminology (return to article)
Carrier Modulation Defined
Data is usually transmitted in digital form. Information is coded in a series of 1s and 0s (i.e. High and Low or Space and Mark) which are represented by two voltage levels. It travels between electronic devices such as computers or process logic controllers (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 alters the difference between the two voltages excessively, data errors can result.
Carrier Modulation techniques convert the data stream from changing voltage levels to changing frequencies. One frequency is assigned to the "1" and another to the "0". The CM transmitter converts digital signals to frequency (modulation) and the receiver converts it back to digital form (demodulation). MOdulation/DEModulation is, of course, the derivation of MODEM. Utilizing CM, data is transmitted as two alternating frequencies instead of a series of voltage step changes. Data communications problems are thereby dealt with effectively because of this unique characteristic of a CM signal.
Noise - Frequency is much less prone to interference than changing voltage levels. The CM frequencies can be selected to avoid electrical noise and the receiver circuits designed to reject out-of-band noise.
Frequency Shift Key (FSK): As described above, FSK uses two different frequencies to represent the space and mark.
It is beyond the scope of this paper to explore all the differences in the Carrier Modulation techniques described above. Therefore, FSK has been selected to illustrate the basic principles of Carrier Modulation as used for industrial data communications and to provide application examples. Most of the points made are applicable to the other techniques.
The practical implementation of FSK is typically in two frequency bands. The first is from about 300Hz to 3000Hz, referred to as the "voice band," and the second from about 100KHz to 400KHz. The voice band is designed to function on telephone lines or any twisted pair. Voice band FSK modems have data rates up to 1200 baud and ranges of 20 miles or more. The higher frequency modems can operate at up to 230Kbaud but range is usually only a few miles. Higher frequencies also allow convenient coupling to power, instrument loop and even PBX telephone lines (for data over voice wire sharing).
FSK can interface with virtually any digital signal including RS-232, 422 and 485. Some modems have intelligent front ends with error checking and retransmission. These usually require that baud rate and digital data protocol be set with jumpers or switches. When used with industrial PLCs with their own error detection schemes the modem's error detection can interfere with and reduce data throughput. Other FSK modems are "transparent" and pass data directly. These are typically easier to install and operate with the added advantage of maximum data throughput. Non-standard data can even be transmitted such as variable pulse streams.
FSK modems can be operated in point-to-point or multi-drop mode. Point-to-point can be simplex (one way only), half duplex (first one direction and then the other, sequentially) and full duplex (simultaneous communication in both directions). Full duplex often requires four wires but some modems can accomplish it on two wires. Half duplex operation is useful for multi-drop data acquisition and control in which one Master unit can control many remote stations. In such cases, the FSK signal transmitted from each station is turned on and off locally so only one FSK signal is on the line at one time.
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