The UK Radio Teleswitch Service is a little known facility which is used to remotely control elecrical power switching devices within the UK, and operates from the BBC's Long Wave transmitters by means of phase shift keying the main RF carrier. The RTS is used primarily by electricity companies for switching loads and meter rates for 'economy 7' purposes and also to load-shed in order to even out demand at peak times. There is anecdotal evidence that the system is also used by the Environment Agency for disseminating flood warnings¹.

The purpose of this series of articles is to summarise the state of knowledge from a radio perspective, to show how certain data can be decoded, and to present some Java software that I've written to capture and analyse teleswitch data. As the latter includes 'current time' messages every minute, it's quite within the realms of possibility to build a time clock using RTS signals, perhaps using a PIC microcontroller, and information is presented here on how this might be done.

Sangamo RC unit as supplied by SEEBOARD (the former South Eastern Electricity Board).

The system consists of a central computer system, the 'Central Teleswitch Control Unit' (CTCU), to which customers are connected by user terminals and modems. The assembled data is transmitted by the LF data system, which includes the BBC long wave transmitters at Droitwich, Westerglen and Burghead, and at subscriber premises are the receiver-controllers (RC's) that actually switch loads or control meters.

The serial PSK data runs at 50 symbols/sec. Manchester coding is used, which preserves the integrity of the BBC Radio 4 transmission for use as a frequency standard. The resulting data stream is at 25 bits/sec.

The outgoing data is broken down into 50-bit frames (messages), consisting of a 5 bit header and a 13 bit Cyclic Redundancy check (CRC) leaving 32 bits for the message payload. This may consist of the following:

• 'Filler' frames, repetitive 101010... pattern sent when there's nothing else to do.
• Time messages
• User messages

More on these later...

A RC unit has to comply with BS and be able to switch

As stated above, the RTS data is sent as phase shift keying of the BBC Radio 4 200kHz transmission. The phase deviation is +/- 45 degrees. This is fairly small and in order to receive it, we will need to construct receiver with a phase detector of sufficient sensitivity.

Motorola application note AN1597² describes a clock constructed with MC68HC711 microcontroller and a receiver using an MC3371 FM system IC. This appears to still be obtainable, but an MC3357, 3359 or 3361 or similar would possibly do the job just as well. It needs a high-Q quadrature coil and the output will be low.

Alternatively, try to obtain a circuit board from a teleswitch unit. A photograph of mine is shown below.

Circuit board from defunct teleswitch. It's marked RTS II and has a build date of Nov 1994

This board has, as you can see, an on-board microcontroller and relays for driving the output contactor which was rated at 45 Amps.It was not difficult to find the point in the circuit where the Manchester encoded data from the receiver is fed to an input on the micro; the transistor you can see at the bottom of the picture interfaces this point to the PC parallel port for use with my Java decoding software.

The next article describes the data transmission format.

References:

1. Teleswitch web site
2. Motorola Application Note AN1597 (pdf)

Last Modified 17/05/2009