> Something like, yes, in that it is a circuit that regulates current, 
> rather than voltage.  However, the teletype 20 mA loop had only two 
> output values, 0 and 20 mA, and was often only crudely regulated 
> (sometimes just with a big series resistor and a high voltage power 
> supply).



I work in industry in the USA that uses 20ma and it is 4ma to 20ma with
regulated power supplies. The signal can be any value between those two
numbers, quite often 0 signal is 4ma and maximum signal is 20ma. Machine
computer control is very effective at sending the exact values via a d/a
module.
This may have nothing to do with this thread but I wanted to comment on the
technology.

Cheers

Richard

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