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.
- Teleswitch web site
- Motorola Application Note AN1597 (pdf)
Last Modified 17/05/2009