Just a plain (20ma) current loop. The 4-20 thing is for analog signals.
Maybe use an optoisolator at the receiving end for each direction.

~ Bob Ammerman
RAm Systems

> -----Original Message-----
> From: piclist-bounces@mit.edu [mailto:piclist-bounces@mit.edu] On Behalf
> Of John Ferrell
> Sent: Tuesday, December 02, 2014 2:23 PM
> To: Microcontroller discussion list - Public.
> Subject: Re: [EE] Line drivers
>=20
> The old fashioned way would be a 4-20ma current loop. Other than power
> consumption, I cannot think of a simpler way.
> On 12/2/2014 1:07 PM, Neil wrote:
> > I have not, but I did coincidentally spend the last 15 minutes looking
> > up termination resistors.  Everything points to RS232 and RS485
> > though, so I want to find a good intro to how to calculate the ideal
> > termination resistor value for my individual-conductors system.
> >
> > Cheers,
> > -Neil.
>=20
> --
> John Ferrell W8CCW
>     Julian NC 27283
>   It is better to walk alone,
> than with a crowd going the wrong direction.
>                    --Diane Grant
>=20
>=20
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