ASR33 Teletype interfacing

From: Tom Jennings <>
Date: Thu Oct 9 16:10:10 2003

>I have been attempting to get my ASR33 teletype connected to something
>and communicating, but so far I have not been successful. I have built
>the interface here :

It wont work, sorry...

Teletypes are inductive loads. Though they only want 20 mils, the
voltage needs to be high to get the initial magnet pull-in (basic RL
theory). ASR33 loops were generally run at 100V or so, but I run my
Model 28 at 14V, with non-perfect error rate, and I don't use the

The keyboard and printer are IN SERIES. If you hit keys while it's
printing you foul it up. Normal.

Because it's inductive, it makes a spike when yuo turn the voltage off.
You need to suppress this with a diode, a resistor and capacitor, for

They're not subtle interfaces, and weren't meant to be.

If you just want to print, you can rig up a power transistor, two
resistors, a diode, and a high-voltage DC power supply to do the trick,
and drive it from the serial port.

If you want to receieve also, you can use another transistor and
resistor to pick off the change in loop current that happens when you
press keys which open the loop, and drive the serial port.

I've done one of these fairly recently, and if poked with a
not-too-sharp stick, I'll scan the schematic and pu on my website.
Received on Thu Oct 09 2003 - 16:10:10 BST

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