Hi all,
This may be off topic, but since the project will be using PICs, perhaps it isn't?

What is the simplest way to initiate two way, ie point to point, comms with the remote end recovering power from the loop?
Protocal would be master interogate, slave answer, repeat ad infinitum.
I have tried RS485 over 500 metres and it works great, but am unable to recover DC from the loop to drive the circuitry at the remote end.
This comprises 2 X 12F675s ONLY to be powered. (fleapower really.)
Years ago I did have details for modified 4-20mA loop with DC recovery but have long lost all info.
Can anyone suggest a solution or point me at a site with one. Have checked a lot of circuit archives without success. BTW the method MUST be real cheap, or is cheap and good mutually exclusive?

Speed is not really important, even 300baud would be OK, anything PIC generatable is OK, up to about 4800.