TTY and current loop questions (was Re: 20mA cabling questions)

From: Ethan Dicks <>
Date: Wed Dec 15 23:31:48 1999

--- Tony Duell <> wrote:
> >
> >
> > The PDP-8/L restoration continues. I have two devices to attach to my -8/L
> > for testing - a VT220 (all have 20mA current-loop connectors) and an
> > ASR-33.
> OK, first thing to do is to clean and oil (correctly!) that ASR33. If
> parts are sticking, there's a reason for it, either no oil or old,
> gummed-up oil. Carrying on running it like this will cause wear and/or
> damage.

I have intentionally not run the TTY for more than a few lines just for this
> Do you have the 3 grey books for the ASR33? Volume 1 of the maintenance
> manual covers lubrication.

If I do, it's long buried. What's the synopsis for the area around the print
hammer/type cylinder? Some letters print OK, some don't. The ones that don't
do OK for the first one, but as I said, I have to manually press the cylinder
back down to its locking position.

How many types/grades of lubricants are there for a proper lube job on a TTY?

> Reader run is another loop (and it's not part of the 'standard' loop
> setup).

Right. I located the conversion manual in the back of some 11/20 docs - how
to turn a factory-fresh TTY into a DEC-modified ASR-33.

> According to a PDP8/e printset, the pinout of the 8-pin mate-n-lock...
> I seem to remember that's 'standard' on DEC devices. You may well need
> the equiavlent of a null-modem cable to link a VT220 to the card (swap
> (2,3) and (5,7)).

That's the kind of thing I was looking for.

> A word of warning. THere are several versions of the COM8116. The
> 'standard' one, as used by Radio Shack in the TRS-80 uses a 5.0688MHz
> clock.

I have the standard one. We used it in an optional modem eliminator option
for our sync serial boards. I have the xtals, too.

Thanks for all the good info, Tony. Now if I could only locate the box of
20mA cables in the attic...


Infinet has been sold. The domain is going away in February.
Please send all replies to
Do You Yahoo!?
Thousands of Stores. Millions of Products. All in one place.
Yahoo! Shopping:
Received on Wed Dec 15 1999 - 23:31:48 GMT

This archive was generated by hypermail 2.3.0 : Fri Oct 10 2014 - 23:31:55 BST