There has been some discussion about sending data via a powerline.  National
Semiconductor make the LM1893/LM2893 Carrier-Current Transceiver.  The
datasheet lists the following features:

Noise resistant FSK modulation
User-selected impulse noise filtering
Up to 4.8 kBaud data transmission rate
Strings of 0's or 1's in data allowed
Sinusoidal line drive for low RFI
Output power easily boosted 10-fold
50 to 300 kHz carrier frequency choice
TTL and MOS compatible digital levels
Regulated voltage to power logic
Drives all convential power lines

Sounds pretty good to me.  Maybe someone  has had experience with this IC.   I
would like to use it to transmitt digitised temperature data 250 metres across
a farm.

Andre Lategan