@Duane ... Current loop...now there's a can of worms.
In RS232, -ve voltage (-3 to -15V to be exact) means 1, and +ve means 0.
In the old teleprinter current loop world, 20mA meant 1 and no current meant 0.
(That is if you're using single current. If you're using double current, it's +20mA and -20mA, but that's not much used so let's keep things simple).)
So Max's ASR33 has two sets of terminals, receive which wants a 20mA / 0 current supplied to it, and transmit which usually provides 20mA or 0. These would often be provided at up to 60 or 80V, for transmission down long lines (Sounds awful but don't forget they only work at 50,60,100 or 110 baud depending on the standard where you are). You'd usually common one of each sets of terminals to earth (ground) so you only need 2 wires (at least that's how we did it). When you're not using long lines then 12V is usually sufficient to provide 20mA. We used to have a nice multi-turn wire-wound pot in each line to adjust the current to exactly 20mA and I still have a couple of +/-20mA meters lying around.....
In old machines like I used to work with, and I think Max's ASR, the receive device is a magnet which, on the first 0 which is a start bit, trips a mechanical process which then selects a character to print based on the next 7 bits. A similar mechanical process sends the start, data and stop bits of the character corresponding to a key press. These mechanical devices were things of wonder to me, being an electronics guy with the benefit of shift registers and stuff like that, but teleprinters were around long before TTL was a gleam in someone's eye....
You could use half-duplex, where the receive magnet and transmit contact were in series. As long as you only did one thing at a time you wer fine. Then you could have local battery (you provide the current) or remote battery (the other end provides the current). And you only needed one wire and ground.
Yes, B&B's converter has a lot inside it and you can do it a lot more simply, especially if you don't want isolation (B&B's has opto-isolators on both circuits). it also has U4 to provide a - voltage for the RS-232 side. But for 69 bucks I don't think I could do anything better in less than a good few hours work..... You note that the transmit circuit just switches current, you'd need to wire it in with the RES12V and RES1/2 terminals to actually provide a current for the ASR. You'd have to look at the ASR circuitry to work out exactly how to do it
I'd love to get this going for Max but I think the postage for an ASR to Australia would be prohibitive..... :-)
A wonderful blast from the past. In my junior year of highscheel (1970-71) I discovered computers and found that I loved them. We started out with marked-sense cards (BASIC) then moved to punched cards (FORTRAN) that had to be sent elsewehere for procsesing. Next, the school district got an HP9800(?) and each school had a dedicated leaased line to a room with an ASR33. Boy I spent a lot of time in there, even when the school was officially closed for the day. Fortunately, I didn't cause any problems and neither did the janitor. Fun memories.
David, Max - That's a lot of circuitry for a 20mA to RS232 converter. I have to believe that it could be done with an MCU, a MAX232 and a few discrete components. I could be wrong though. I really don't know the principles behind the current loop.
@terrapindundee: One thing to look out for is the rubber print hammer in front of the cylindrical print wheel, they go off with age and can destroy the print cylinder which are now very hard to obtain.
Thanks for reminding me -- in fact Leo (the original owner) did menton this and we looked at the rubber print hammer .... like all of the other rubber element sof thsi device (belts etc) the print happer is actually in incredibly good condition.
All I really need to do is sort out the linefeed and bell problems -- and I bet those can be solved with a basic cleaning (de-gunking).