[GreenKeys] A reliable current-loop-to-RS232 converter

[Jim Haynes]

On Sat, 15 Aug 2015, Justin Scott wrote:
>
> On the circuit I've outlined above - one thing to keep in mind is that 
> on the 232 RX side, it will not drop below 0v for obvious reasons (i.e. 
> will not go to -12v).  Most 232 interfaces are fine with this, and this 
> circuit does work with TTL level 232 signals.

One place where this could cause trouble is in a radio installation where
you are transmitting.  I tried using TTL voltages on some things and
found that with any appreciable wire length the wires acting as antennas
would pick up RF during transmit, and it would be rectified by the
solid-state parts and displace the switching voltage.  I imagine that's
why RS-232 and MIL-STD-188 both specified bipolar voltages for signals,
to give some margin against zero voltage not being quite zero.