November 2008

Communicating with SCADA

By Stuart Boyer

Supervisory control and data acquisition (SCADA) enables a user to collect data from one or more distance facilities and to send limited control instruction to those facilities. SCADA includes the operator interface and the manipulation of application-related data. But it is not limited to that. Some manufacturers are building software packages they call SCADA, while these are often well suited to act as parts of a SCADA system, because they lack communications links and other necessary equipment, they are not complete SCADA systems.

Communications is the movement of data or intelligence from one location to another. For communications to happen, several things must be in place. First, a communication path must exist; the data must travel over some selected medium. Second, equipment must exist at the sending end of the communications path to condition the data and to put it into a form that we can send over the communications medium. Third, equipment must exist at the receiving end of the path to extract the message from the medium and understand its meaning.

Given a SCADA system consists of one or more master terminal units (MTUs) sending instructions to and receiving data from one or more remote terminal units (RTUs), it is clear communications plays a vital role in the operation of the system.

Installing SCADA is usually justified because of the remoteness of a site and the difficulty or cost of manning it. In a few cases, it is dangerous, unhealthy, or otherwise unpleasant for a person to be at a site. In most cases , it is simply too expensive to have an operator stay at the site for extended periods of time or even to visit the site on a once-a-shift or once-a-day basis.

As long as you can establish some type of communications path between the remote sites and the central or master site, you can pass data. If you cannot establish a communication link, you cannot develop a SCADA system.

In analog-to-digital conversion, all data that moves between the MTU and the RTUs is binary data. It may have originated that way as a status condition of an on-off switch, or it may have converted to binary form from analog form.

Long distance communication is serial. All data that moves between the MTU and the RTUs is serial. That means a single string of binary characters goes one after another. The alternative to serial is parallel. Parallel buses see use within computers and from computers to printers, but the cost of the extra communications medium (wire) becomes prohibitive for long distance communications paths. To communicate the digital word from the analog-to-digital converter in serial format, you need to define some convention to transmit first the most significant bit, then the next smaller, then the next smaller until you send all bits. Or you need to define some convention to transmit first the least significant bit followed by the next larger, followed by the next larger, and so on. This convention would be part of the communications protocol.

Protocol

A protocol is a set of rules that defines the meaning of a pattern of binary words. The messages you send from the MTU to the RTUs are a series of binary digits. But what will the first bit represent? What about the second or the 247th? Protocol tells us-it supplies the code to create this long series of ones and zeros.

The same code allows the receiving station to decode it. The same code the sender uses, the receiver must also use. This is not to say only one protocol is available; there are dozens. Equipment manufacturers developed them before any standards organizations became interested. Many equipment manufacturers continued to use their proprietary protocols even after the standards organizations had developed open standards. And some even developed new proprietary standards after these open standards were available. Some are better for certain applications than others; some are worse for all applications than others. The important thing to know is you must have the same protocol at the RTU as at the MTU.

Modems

The modem is at the lowest two levels in the ISO/OSI seven-layer model. It checks to determine use of the communications medium and turns the radio transmitter on. It changes the low-power binary signals as they feed to it from the MTU or RTU into a form that will travel to the other end of the medium, and another modem receives them.

Early attempts to send direct-current (DC) signals long distances over wireless demonstrated that resistance reduced the signal. Attempts to send more and more pulses per second down the line demonstrated inductance, and capacitance effects also affected the signals. We reached limits to data rate and distance early because we affected the shape of the pulses.

We can separate a wave form mathematically into a series of sine waves. Sharp-edged pulses contain more high-frequency components than do round-edged pulses. The inductive reactance of a long pair of wires will selectively attenuate the high-frequency components, effectively rounding off the pulse.

The communications modulator varies one of three characteristics of the carrier. It may change the amplitude, the frequency, or the phase. Amplitude modulation (AM) varies the amplitude of the relatively high-frequency carrier by multiplying it by the amplitude of the data. The result is a series of sine waves at the carrier frequency that vary in amplitude at the data rate. Frequency modulation (FM) varies the frequency of the carrier according to the amplitude of the data. Output amplitude is constant. Because most atmospheric noise is amplitude-related, and FM does not receive any intelligence from the signal amplitude, FM signals are not affected by atmospheric noise as much as are AM signals.

ABOUT THE AUTHOR

Stuart Boyer is president of Iliad Development Co. in Calgary, Alberta, Canada. His e-mail is stuart_linda@telus.net.