Radio telemetry can offer a time and money saving solution to effective SCADA systems.
As any systems design engineer knows, the cost of installing process signal cables in SCADA systems can amount to a significant share of the overall project cost. Thankfully, the `connectivity’ of modern radio data products means that it has never been easier to utilise this time and money saving technology instead. Indeed, many telemetry systems are in use today where radio is the primary communication medium.
Typically, the use of radio links with SCADA systems falls into two categories: radio modems communicating between proprietary I/O of PLCs that would otherwise use cable; and stand-alone radio telemetry systems with their own I/O.
Radio modems are more often than not used in `broadcast mode’. This can be likened to a multi-drop cable. Any unit in radio range (subject to security measures) will `hear’ the messages and output them on its serial port. 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 operating a polling regime, which is designed to avoid data clashes.
In the UK, most systems in this category operate on the deregulated UHF telemetry band, which imposes bandwidth and, as a result, speed limitations. When choosing a system, selection on the basis of interface rates alone should be avoided, and purchasers should instead ensure that data rates quoted are `over the air’.
Just as important is the `turnaround time’, a fixed overhead delay that takes place every time the modem changes from transmit to receive. Look for the shortest; the latest devices on the market have a turnaround of some 10msec. If the system is to operate on RS485, the radio modem should have its own RS485 port as 232/485 converters can be fraught with problems.
Modem cum PLC
Radio modem/PLC combinations are particularly suited to systems where the facilities of the PLCs are required at an outstation. An example is 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 running custom-written software.
The stand-alone telemetry system differs from the modems described so far in that it has its own I/O and radio comms 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 application, but it is ideal for applications such as water treatment, energy and leakage audit.
A system such as RDT’s VersaNet consists of a number of nodes, each comprising a comms controller and I/0 cards. Each node also has a serial port which can communicate using Modbus. System architecture is usually a PC-based MMI and a number of I/O outstations. The PC can directly read or write to any input or output within radio range. VersaNet is unusual in that each node acts as a repeater, passing messages to other nodes, and allowing the range to be extended. Security is handled by the radio system, which looks after error checking, requests for re-transmission, etc, all of which are invisible to the user.
Rememeber, when selecting a radio system, you should look for a few fundamentals. First, the radio should be multi-channel. Usage of the band is increasing, and to forego the ability to select the operating channel is a false economy. Second, the selected units should have, as a minimum, a means of displaying received signal strength, since this will be found invaluable for commissioning and diagnostics.
* Author is with Radio Data Technology.