RS232 to 20ma current loop converter

[grsparks]

I am looking for a schematic for an RS232 (from a PC serial port) to a
serial receipt printer that uses 20ma current loop on a DB25 connector.
If I am not mistaken, it should be able to be done with a couple opto-
isolators (transistor). 

[Staigen]

Hi

This is a facility already built into a true RS232 serial interface, if i recall things right! Check the RS232 standard, i'm not sure! If you had asked me 20-25 years ago i could have given you a straight answer. 20 mA loop is a very old standard, seldom used anymoore, where to heck do you use that today? It must be a very old printer! As i recall it, its just to plug the connectors right in.

*Observe*

[grsparks]

After AT, PC com ports no longer support the current loop option.
The current loop is used for higher speeds and
longer distances than regular RS232. It might be in the original
specification, but not in all the products.  There are lots of devices
that use this today because higher distance,  noise immunity and speed it provides.
It is also used in sensors and control applications.

I see lots of commercial products out there which perform this function.
I just wanted to build one.  I am not sure of the difference between
current loop and RS485 (long wire extended or 2 wire RS232) but one
company offering products had a version for both.

Incidently, because I need this for a printer, I could get by fine with
a half duplex version, 1200 baud - not really speed critical.