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What would be a reliable and simple tranceiver interface/protocol for board to board serial communication at about 100ft. [9600bps or lower is fine.]

The two-lead communication wiring cannot be changed and is un-twisted, unshielded wire [~25pf/ft] in commercial buildings.

12VDC rail is already available.

Specifically, I've looked at Modbus/RS485 and RS232. Most available and low cost [~$0.50] chips seem to either run 3V3 or use capacitor charge pumps to level shift to 6V.

What's typically used in these scenarios?

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    \$\begingroup\$ RS-232 works at this distance but depends on isolated ground on one side with charge pump for -12V. Have you tested the wires for noise yet? \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 4 '18 at 23:15
  • \$\begingroup\$ @TonyStewart.EEsince'75 Assume noise exists. Hundreds of sites , not one set of wires. The boards do share a common ground. I'm concerned the charge pump not being 'strong' enough for the long wires. \$\endgroup\$ – MandoMando Feb 5 '18 at 1:07
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    \$\begingroup\$ Any differential balanced driver matched to cable impedance with 10V swing ought to have highest SNR. But most reliable will have error free transport protocol. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Feb 5 '18 at 4:06
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You might consider a 20 mA current loop, used back in the day. I believe mark was 20 mA and space was 0 mA. Not affected by line loss and twisted pair not required since it's operating in current rather voltage mode.

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