On Tue, 6 Oct 2026, Harold Hallikainen via GreenKeys wrote:
> What are people using as a 60 mA loop to PC interface?
>
A concept I tried and failed to sell at Teletype was the following.
Every selector magnet runs at low voltage with a power transistor
driver. Input to the driver is something like +/- 6 volts. Input
impedance of the driver is fairly high so that one signal source
can feed multiple selector magnets.
Every signal generator has a circuit producing a +/- 6 volt signal.
The result is you can make an ASR set or whatever you want where the
internals are all low voltage and are voltages, not current loops.
If the external signal source and sink are current loop or polar or
anything else use a polar relay or an electronic circuit to convert
to the voltage inside the set.
The reason for +/- polar signals is that if you are operating near
a radio transmitter there will be rectified RF inside the set that
offsets the intended voltages. Or at least it tends to make the set
RF proof.
I had in mind a simple saturated transistor driver and 12 volts 600
ma for the selector magnet driver. Whomever took the idea and ran
with it for Model 32-33-35 used a more complicated circuit with the
driver transistor in constant-current region. This was not a bad
idea since we were pushing the circuits into 110 or 150 baud where
faster response was useful. I think it turned out to be 20 volts
and 500 ma. I never understood why Bell Labs wanted a current loop
interface to the TWX modem when a voltage interface would make
things so much simpler. And RS-232 made the polar voltage signal
standard. Around the same time the military was standardizing on
low polar voltages on patch panels, as was Western Union.
A consideration for signal generators is that electrical contacts
tend to get covered with oil in teleprinters, so the circuit needs
to have enough voltage and current to break through the oil, or
needs to have dry gold plated contacts or maybe a relay with mercury
wetted contacts, I was willing to use 120 V and 20 ma just for the
health of the signal generators. There's another story of how Teletype
got into deep trouble with oily signal generator contacts.
A further example of the rightness of the low voltage set concept.
The original dial TWX sets used the 101 series of modems, which
required putting most of a telephone into the Teletype machine
to make and answer calls. The modems and sets were expensive and
there was a 99 wire cable between the Teletype set and the modem.
We found that several of the Bell operating companies were buying
the much cheaper private-line Teletype models and connecting them
to the 100 series Data Sets (modems) that included the telephone
and could be configured to talk to TWX sets. With that approach
Teletype didn't need to make a whole series of call control units
featuring rotary dial, Touch Tone keypad, card dialers, earphones
so the user could hear dial tones and call progress tones. What
a mess of a product line!
Getting back to the question of interfacing a loop current machine
to a computer, the last thing I used was some kind of opto-isolator
MOS switch. I think whatever I was using has been discontinued but
there were several things out there. Various things you can use to
prevent the inductive "kick" on turnoff from exceeding the voltage
rating of the device.
Jim W6JVE
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