ASR33 Teletype interfacing

From: Frank Smith <>
Date: Fri Oct 10 11:40:39 2003

--On Thursday, October 09, 2003 13:52:27 -0700 Tom Jennings <> wrote:

>> 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
> keyboard.
> 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
> example.
> 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.

For the time and/or solder impaired, you can buy RS232 <-> current loop
converters. I bought one from B&B Electronics <>
when I needed one quickly, but there are probably other (and possibly
cheaper) places that sell them also.

Received on Fri Oct 10 2003 - 11:40:39 BST

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