Re: [COFF] DEC terminal line driver chips?

[Clem Cole <clemc@ccc.com>]

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If you look at the schematics for the VT-100 (which are available on
bitsavers) as well as most terminals of the day used the
Motorola MC1488/1489 driver pairs [early on, TI marked them as 75488/75498
but I doubt you'll see that old].

Anyway, for any terminal of those times, I'd be surprised if the later
devices (like VTx20 series) did not them [the issue with these devices is
you need all of 5 and +/- 12 volts].   These terminals are new enough that
they might have used MAX232 (single 5 v power - there are even 3.3v
versions these days).  All of these devices are very available online.  I
would suggest putting high quality (*i.e.* auget style / machined pins)
sockets.  The nice thing about 1488/1489 is that they were doped to fuse
the output section on failure, so just replacing them will in fact
resurrect most serial port failures.  Frankly, I never understood why more
manufacturers did not put them in sockets for easy replacement.

That said before you open up the case, I make sure you are cabled
correctly.   If you are using so-called 3-wire cables, then you need to
ensure RTS/CTS [4-5] are strapped and DSR/DCD/DTR [6/8/20] are also
strapped.   The terminal will ignore any I/O unless it sees these pins
asserted.

On Thu, Apr 29, 2021 at 3:11 AM Tom Ivar Helbekkmo via COFF <
coff@minnie.tuhs.org> wrote:

> I've got a number of DEC terminals, ranging from the VT220 to the VT520
> (sadly, I got rid of my VT100 and VT102 many years ago, before I started
> collecting DEC equipment instead of just using it), and some of them
> have one or more burned out serial ports.  Before I start taking them
> apart to find out what chips were used, I figured I'd check if any of
> you folks happen to know.  I'd like to order a stash of replacements,
> and it would be nice to have them handy before I clear the work bench to
> start dismantling terminals...
>
> Oh, and for the record: the Q-bus PDP-11/23 uses 9636ACP and 9637ACP for
> output and input, respectively, while the VAX-11/630 substitutes a
> 9639ATC optocoupler for the 9637ACP differential receiver.  (I have a
> couple of spare CPU boards with damaged ports, as well, so these are all
> on my shopping list already.)
>
> -tih
> --
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