Gee, nobody has talked about the ASR33 in a while...
ard at p850ug1.demon.co.uk
Sun Mar 18 15:11:10 CDT 2007
> Well, as I reorganize and clean, I managed to
> sidetrack myself from the aforementioned cleaning by
> playing with my ASR33 TeleType. I have it working
> properly, and it prints and types correctly - although
> I have never managed to interface it to anything. I
> remember a year ago, trying some quickie kludge I
> found on the 'net, and having it not work. I am
> struggling to remember what I thought I used to know
> about TeleType interfaces. Correct me if I'm wrong:
> The teletype's serial output is compatible with RS232
> at the protocol level, and since it's a 33, it's ASCII
Yep. It's 110 baud, 8 bits, 2 stop bits. AFAIK all Model 33s ignore the
high bit when printing, punch all 8 bits on the tape, read and transmit
all 8 bits from the reader. There are 2 versions of the keyboard, one
always sends the top bit as a 0, the other sends it as an even parity
> The TeleType is a passive device with a 20ma current
> loop interface. In order to work, it needs a loop
It can either be a 20mA or 60mA loop, depending on a jumper setting IIRC.
> current source (I had to use a 9v radio battery for
> The signal levels, being 20ma current loop, are NOT
> compatible directly with RS232C. That being said, it's
> seemingly possible in some circumstances to interface
> the two with a diode and a resistor and have it work.
Well, I'd use a little more than that :-)
> In interfacing this thing, I basically want to be able
> to construct a simple device that 1) supplies loop
> current, 2) optically isolates the current loop from
> the RS232 host, and 3) provides proper RS232 signal
> levels (MAX232, what a wonderful little chip) so that
> I can connect it to whatever without worrying about
> blowing up the transceivers.
Seems reasonable. It's been done many times, BTW. I've built several
devices to do the rerse (that is, to use an RS232 terminal on a host that
expects a current loop interface)
> I should be able to scrounge up some optoisolators, I
> know I have MAX232's and I should have no trouble
> constructing a simple, transformer based power supply
> to supply power for both the MAX232 and the loop
> current. But, I have forgotten some key elements, such
DOn't forget if you isolate the data lines with optoisolators, you also
need sapearate PSUs for the loop side and the RS232 side. Using one
transformer with 2 secondary windings would be fine, I think.
> as I don't know what an acceptable voltage is to
> supply the loops, and although I remember reading that
> the RX and TX used different amounts of juice, I don't
The transmitter side of a Model 33 is just an array of switch contacts
(including the carbon brush contacts on the distributor disk, of course).
It's pretty easy to deal with.
> remember the specs, and lastly - AC or DC loops? DC,
> right? What's an acceptable way to create loop current
Oh yes, it's certainly DC
> without driving something too hard or blowing
> something up? Also, my TeleType manual doesn't appear
> to have information on what screw terminals do what on
> the terminal strip at the back of the machine - I
You want the scehamtics. I think they're on Bitsavers, there's also the
HP version over on hpmuseum.net.
BTW, IIRC, there's 115V mains on 2 of those terminals. Just waiting to
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