18 Mar
2007
18 Mar
'07
1:11 p.m.
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
too.
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
bit.
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
testing)
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
zap you.
-tony