Creating 4 to 20MA ?

[amgen]

thankyou for links which I read all.

Would like to make PIC ckts to output the 4-20 MA stuff.

what I know (maybe), recieve ma. is easy, just read V across a 100 to 250 ohm resister and scale/calculate or limit the a/d reading. PIC can do 10 bit but many products reach to 12 or 14 bits.
Send or creating ma signal seems a little more involved, apparently needed is 12 to 24 vdc supply to be able to adjust for wire and other loop resistance ??
Getting a good DC level (0 to 5) volts from PIC with 1000 or more resolution --- not so easy. See diagram, and how to set limits of 4 and 20 ma with resisters ???
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motor t-stat
used simple process of elimination (mental process that is) the 2-A and F wires are generally all thats needed to run motor in both directions and more times than not, two small wires will be for temp sw.

don