We may be thinking about different conditions. In the 20 ma teletype=20
loop a mark was 4 ma, a space was 20 ma. A line that is 20 ma and open=20
has unsafe transients due to the unknown inductance in the line. Neither=20
the 20ma condition or the 4ma condition is high impedance or saturated=20
logic. The implementation is analog, but the data is digital.
This is likely another time I should have remained silent!
Your suggestion to implement with optoisolators is a winner and the=20
actual current values are not important.

On 12/2/2014 7:56 PM, Bob Ammerman wrote:
> 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
>
>

--=20
John Ferrell W8CCW
    Julian NC 27283
  It is better to walk alone,
than with a crowd going the wrong direction.
                   --Diane Grant


--=20
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