[COFF] DEC terminal line driver chips?

[COFF] DEC terminal line driver chips?

[Tom Ivar Helbekkmo via COFF]

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
-- 
Most people who graduate with CS degrees don't understand the significance
of Lisp.  Lisp is the most important idea in computer science.  --Alan Kay
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Re: [COFF] DEC terminal line driver chips?

[Brad Spencer]

Tom Ivar Helbekkmo via COFF <coff@minnie.tuhs.org> writes:

> 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

I will not speak specifically about DEC serial terminals, but rather an
experience I had many years ago with Data General terminals at the
University I was at that may or may not be helpful.

I spent part of a summer fixing the DG terminals.  The problems mostly
in that case were the last set of driver chips after the CPU that blew.
The University was running a current loop set up with very long runs all
over campus and lightening would tend to take out a serial terminal from
time to time.  We later build our own lightening suppressors, but that
is another story.  Only in one case I remember did we blow a CPU.  In
the case of the DG, the CPU was an 8085 or some variant that had a
serial port on it.  This then was sent to a set of driver level chips
that did the 5v TTL serial to whatever you needed conversion, be it
regular RS-232 or the current loop specification that was used.  The
terminal took both and it was this last set of drivers that would often
burnout.  We usually found out what was really wrong by just replacing
the drivers and if that didn't work, replace the CPU.  We also saw many
cases where a terminal would transmit but not receive, or vise versa.
We tested for this too with a cross over cable from another known
working terminal.

You did not mention the age of these terminals, but I would also suspect
caps going bad somewhere.


In any case, good luck.  If you have a cheap scope, available for just a
small bit of $ on Amazon, you may be able to determine where the problem
actually is in a much simpler way if you can't get the schematics.
Serial ports are pretty slow and pretty simple to see with a scope.  Of
course, if you can find schematics it will be a whole lot simpler.




-- 
Brad Spencer - brad@anduin.eldar.org - KC8VKS - http://anduin.eldar.org
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Re: [COFF] DEC terminal line driver chips?

[Clem Cole]

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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
> --
> Most people who graduate with CS degrees don't understand the significance
> of Lisp.  Lisp is the most important idea in computer science.  --Alan Kay
> _______________________________________________
> COFF mailing list
> COFF@minnie.tuhs.org
> https://minnie.tuhs.org/cgi-bin/mailman/listinfo/coff
>

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Re: [COFF] DEC terminal line driver chips?

[Dave Horsfall]

On Thu, 29 Apr 2021, Brad Spencer wrote:

> I will not speak specifically about DEC serial terminals, but rather an 
> experience I had many years ago with Data General terminals at the 
> University I was at that may or may not be helpful.

[...]

We had a network of serial lines extending for several km over a few 
suburbs; after every lightning strike I would get to replace the 1488/1489 
chips which were *soldered* in.

Eventually I just bought a set of high-quality sockets, and merely swapped 
the chips as each terminal came in after being zapped by Thor.

As an aside, not only did I chuck out the bad chips but I took care
to bend their pins over backwards, in case some idiot saw them...

-- Dave, who once destroyed a $US10,000 Ethernet card for the CCI Power 
6/32
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Re: [COFF] DEC terminal line driver chips?

[Dave Horsfall]

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On Thu, 29 Apr 2021, Clem Cole wrote:

> 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. 

I used to do that so often for so many terminals that I could practically 
do it in my sleep...  Having the correct crimping tool also helped :-)

-- Dave, who has yet to see a full implementation of RS-232 i.e. the 
secondary channel etc

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Re: [COFF] DEC terminal line driver chips?

[Tom Ivar Helbekkmo via COFF]

Brad Spencer <brad@anduin.eldar.org> writes:

Thanks for your advice, Brad!

> The University was running a current loop set up with very long runs
> all over campus and lightening would tend to take out a serial
> terminal from time to time.

That would do it.  In my case, it's carelessness with connections
between different in-house power circuits.  I live in an older house,
with three phases in, and pairs of them used for the individual 240v
circuits, of which I have two in my hobby room.  Imbalances (caused by
exterior faults) can cause ugly spikes when connecting or disconnecting;
I've become very careful about making sure all computer equipment in the
room is on the same circuit, with good ground connections.

...which reminds me: I once worked for a company whose offices stretched
throughout the same floor in two adjacent city buildings.  Never really
gave that fact a second thought, until I grabbed a grounded metal RS232
connector with one hand, rested the other on the grounded metal chassis
of the machine it was connected to, and pulled the connector.  The other
end of the cable was grounded in the other building.  I needed a short
break after that before continuing work...

> You did not mention the age of these terminals, but I would also
> suspect caps going bad somewhere.

Mm.  Old caps do that.  I recently had one blow in a DEC BA23 power
supply, which I pulled, and replaced with a spare one I had on hand.
That lasted all of five minutes before the same capacitor blew in the
replacement.  Turns out they have noise suppression caps on the line
input, and these go bad and explode.  Doesn't affect anything else,
though, so I just swapped in new ones, and the supplies work fine.

-tih
-- 
Most people who graduate with CS degrees don't understand the significance
of Lisp.  Lisp is the most important idea in computer science.  --Alan Kay
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Re: [COFF] DEC terminal line driver chips?

[Tom Ivar Helbekkmo via COFF]

Clem Cole <clemc@ccc.com> writes:

> [...] Motorola MC1488/1489 driver pairs [...]

Thanks!  That's useful info.  I'll add a small pile of each to my
shopping list.

> [...] they might have used MAX232 [...]

Ah, yes, I remember those.  Checking Mouser now, I see that there's an
awful lot of variants of them, though.  I think maybe I should open up
my VT520 (the newest terminal I have), and check what it's using.  It's
down from four to one working serial interface, and it's such a nice
terminal, I'd really like to have it fully operational.

> If you are using so-called 3-wire cables, [...]

I usually call those RS-237.  :)

-tih
-- 
Most people who graduate with CS degrees don't understand the significance
of Lisp.  Lisp is the most important idea in computer science.  --Alan Kay
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Re: [COFF] DEC terminal line driver chips?

[Derek Fawcus]

On Thu, Apr 29, 2021 at 09:17:27AM -0400, Clem Cole wrote:
> 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.

Well even in 1988 when I entered industry for a year (as an EE), the MAX232
was quite new (looks ike it was introduced in '87), and plenty of designs
still used the 1488/1489 chips.  I do recall that those I worked with at the
time were often produced with turned-pin sockets, specifically to allow the
chips to be easily replaced.

It seemed to be around 91-92 before the MAX232 was common in new designs,
certainly I used it for adhoc test equipment around that period.

I guess familarity, and the the time for the design pipeline to flush out
may have had a effect, only using it on new designs, or reworks where the
PCBs had to be redesigned due to chips going out of production.

DF
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