2 May
2023
2 May
'23
1:05 p.m.
On Tue, May 2, 2023 at 1:34 PM Paul Koning via cctalk
<cctalk(a)classiccmp.org> wrote:
It seems surprisingly hard to find that, though this
https://forum.vcfed.org/index.php?threads/rs-232-to-current-loop-interface.… is
close.
Conceptually it's really easy. The main tricky part, certainly if you're driving
a mechanical terminal like a Teletype, is the current source. You need something that can
drive 20 mA into an inductive load. The classic answer was to use a fairly high voltage
with a big series resistor so the inductive impedance is much less than the resistance. A
current regulator would be a modern replacement. Also, for the case of the inductive
load, you'd need a shunt diode across the switch to absorb the voltage spike from the
inductor when the current is switched off, otherwise the switching transistor will be
quickly destroyed.
The common version of the Model 33 Teletype with a current loop
interface [1] has a bit of electronics inside. A little driver PCB
with a couple of transistors on it, a power transformer for it and so
on. The result is that although the interface is 20mA or 60mA current
loop you are not driving the receive magnet directly and there are no
high voltage spikes at the interface terminals.
[1] This is by far the most common version of the Model 33 in the UK.
If you do have to drive the solenoid directly then (as you're in the
UK) look out for the RSGB [2] Teleprinter Handbook. Although, not
surprisingly, this is biased towards amateur radio RTTY operation, it
has a lot of information on mechanical teleprinters, how to drive them
(with circuit diagrams) and so on.
[2] Radio Society of Great Britain. Basically our equivalent of the ARRL.
-tony