--On Thursday, October 09, 2003 13:52:27 -0700 Tom Jennings <tomj_at_wps.com> 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 :
>> 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.
>
> 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 <
http://www.bb-elec.com/>
when I needed one quickly, but there are probably other (and possibly
cheaper) places that sell them also.
Frank
Received on Fri Oct 10 2003 - 11:40:39 BST