[GreenKeys] Affordable current loop adapter

[Tim McNerney]

I need to test my 28RO.
Most of the RS-232 to 20 mA current-loop adapters are "wicked expensive."
Plus nowadays, RS-232 is not exactly easy to find as an interface.
So I'll be up to $200 before I finally connect to a computer via USB.
And of course USB to current-loop... good luck finding that adapter...

Can anyone recommend a circuit for a simple 20 mA current loop transmitter?
I have EE and soldering chops.  Happy to build my own prototype.

BTW, I've noticed lots of affordable 4-20 mA current loop transmitters 
for remote industrial sensors.
Can these be adapted to serve as a 20 mA digital transmitter?
Intuition makes me think yes, but I want to double-check.
Maybe 4 mA does not equal "0".

Thanks.
--Tim McNerney