2.1 RS485 on the MyIO project
Rather than attempt to describe RS485 in gratuitous detail here, just check out google on the matter or read the wikipedia description.
Essentially I'm using RS485 because it gives me me an electrically simple interface which can be implimented in a few components and allows long cable lengths (1200m) at high speed (up to 100kbps) on simple twisted pair cable. What more could a man ask for?
I'm using the max485 RS485 drivers, and intend to use 19.2kbps, 8 bits, 1 stop bit, no parity.
The basic RTU circuit, showing the Max485 driver IC with jumper selectable 120 ohm termination resistor and 100Ohm current limit in the RS485 ground to limit earth loop current, as per the RS485 spec.
This is the basic building block of the RTU design. If I can find a good source of cheap isolation transformers I may switch to using fully isolated RS485 in all of the RTU's, but as at the time I wrote this I've not found anything locally available.