Analytical Instrumentation & Environmental Monitoring

A guide to cableless scada communication

March 2000 Analytical Instrumentation & Environmental Monitoring

It will come as no surprise to the systems design engineer that the cost of installing process signal cables in scada systems can be a major portion of the total project cost. The 'connectivity' of modern radio data products means that it has never been easier to utilise this time and time saving technology. Indeed, many telemetry systems are in use today where radio is the primary communication medium.

The use of radio links in relation to scada systems falls into two categories:

* Radio modems communicating between proprietary I/O systems of PLCs that would otherwise use cable.

* Standalone radio telemetry with their own I/O.

Radio modems

Radio modems are most often used in a "broadcast mode". This mode of operation can be likened to a multidrop cable. Any unit in radio range (subject to security measures) will ‘hear’ the messages and output them on their serial ports. The messages contain destination information and it is up to the device connected to the modem to recognise its own messages and respond. Since most systems operate on a single RF channel, half duplex communication is the norm. This is achieved by the host system operating a polling regime, thus avoiding data clashes on the radio channel.

Most systems of this kind operate on the deregulated UHF telemetry band, which imposes bandwidth and hence speed limitations. Purchasers should ensure that data rates quoted are ‘over the air’ and not merely interface rates. The latest products available support approximately 8 kbaud over the air data rate which approaches 4,8 kbaud data throughout when forward error correction (FEC) is applied, (at 12,5 kHz channel spacing).

As important as transmission rate is "turnaround time". This is a fixed overhead delay that takes place every time the modem changes from transmit to receive. The purchaser should seek the shortest times. The latest market offerings have a turnaround in order of 10 ms. If the system is to operate on RS485, it is important that the radio modem has its own RS485 as 232/485 converters seem to present more than their fair share of problems when used with radio modems.

Some systems will require synchronous transmission and some asynchronous. Different systems will have different hand-shaking requirements. The first time purchaser should contact the modem vendor, who will assist.

The radio modem/PLC combination is particularly suited to systems where the control facilities of the PLCs are required at the outstation location. An example of such a system is to be found at a British Steel plant, where about 20 radio modems are used to communicate between Allen Bradley PLCs on moving plant around coke ovens and a PC custom written software.

Standalone telemetry systems

The standalone telemetry systems differ from the modems described in that they have their own process signal I/O and their own radio communication protocol. Generally, the outstations of such systems have little or no control capability of their own, behaving purely as remote I/O for PC-based scada.

Bandwidth limitations and shared RF channels mean that this type of radio-based scada is seldom suitable for any high-speed applications, but is ideal for such applications as water treatment, energy and leakage audit.

Versanet offers continuous RF power level adjustment (from 500 mW downwards), to maximise local bandwidth availability. Each transceiver can use any of the 64 channels available.

A system such as RDTs Versanet consists of a number of nodes, each comprising a communications controller and the requisite I/O cards. Each node also has a serial port which may communicate using a MODBUS protocol. System architecture is usually a PC-based MMI with a number of outstations providing the I/O. The PC may directly read or write to any input or output with radio range. Versanet is unusual in that each node will act as a repeater passing messages for any other node, allowing range to be extended or obstacles avoided.

Communications security is handled by the radio system, which looks after error checking, requests for retransmission, etc all invisible to the users.

General requirements

When selecting a radio system of either of the above general types, the purchaser should look for a few fundamentals. First, the radio should be multichannel. Usage of the band is increasing and to deny oneself the ability to select the operating channel can be a false economy. Secondly, the selected units should have, as a minimum, a means of displaying received signal strength, since this will be found invaluable at the time of commissioning and for diagnostic purposes.

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