ASR33 Teletype interfacing

From: Tom Jennings <>
Date: Mon Oct 13 13:22:57 2003

> Date: Sat, 11 Oct 2003 09:04:11 -0400
> From: Ian Primus <>
> Subject: Re: ASR33 Teletype interfacing

> >> 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

> > 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.
> I take it that the voltage isn't that crucial, just the current?

Yes, but higher voltage means the in-rush current will reach 20mA faster
(the RL time constant doesn't change, just absolute values).(I kinda
lied when I said it wasn't subtle; there are subtleties. But they're not
of the pee-cee interface kind. The Old Guys who worked all this stuff up
were as smart or smarter than us; it's best to read old source material
for this kind of trivia.)

> > The keyboard and printer are IN SERIES. If you hit keys while it's
> > printing you foul it up. Normal.
> What about on a full duplex machine? Is it the same, or are they
> separate?

Yes, but most teletype equip was half-duplex (two wires).

> > 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.
> *poke*
> <grin>
> If it's not too much trouble, I'd like to see it. From what I have
> heard, there are lots of ways to do this, and i would be very
> interested to see how you did it.

OK, I'll get to it this week and post URL here.

Everyone's interface is better than everyone else's, mine included :-)
Be skeptical. Simplest isn't always most reliable, it depends on what
you're trying to do: punch 1000 feet of error-free tape a mile away? Or
just make the tty dance for an afternoon of fun?

I cobbled up a PIC-based board that does ITA2 (aks "baudot") to ASCII,
motor control, etc for a Model 28 teletype. It fits a particular project
of my own. The source for it is at scroll down
for "PIC BASED PROJECTS" or something.

Received on Mon Oct 13 2003 - 13:22:57 BST

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