Look here....
http://www.bb-elec.com/tech_articles/current_loop_app_note/basics_of_current_loop.asp#21

"Rick C." wrote:

> I used to use 4N28 optoisolators and a few resistors and diodes for 20 ma
> current loop designs. This was a common type of interface back when the
> old Teletype printers were being converted to all electronic printers.

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