11 Oct
2003
11 Oct
'03
1:04 a.m.
On Thursday, October 9, 2003, at 04:52 PM, Tom Jennings wrote:
Somehow I'm not surprised. Something told me that it was too simple to
work properly.
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 :
http://www.daedalus.co.nz/~don/computing/20mahack.html
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.
I take it that the voltage isn't that crucial, just the current?
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?
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.
*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.
Thanks!
Ian Primus
ian_primus(a)yahoo.com