Well I have completed some code for the transponders now. I can transmit any identifier, plus battery status and other data (e.g. a CRC).

The actual code which sends the modulated data is a tight ASM loop of 7 instructions. Finding the right combination to keep within 4 cycles (I am running the PIC at 20Mhz) was good fun. This does mean I can vary the modulated data in any way though.

Unfortunately I am still waiting for my PCBs, so I'll be going back to the decoder for a short time.

Thanks again Howard, this has been keeping me busy and entertained!