From: lasner@watsun.cc.columbia.edu (Charles Lasner)
Subject: Re: NS 8641 chips *available*!
Keywords: spares 8641 buffers
Reply-To: lasner@watsun.cc.columbia.edu (Charles Lasner)
Organization: Columbia University
Distribution: na
Date: Sun, 22 Nov 1992 10:27:58 GMT

In article <1ej84rINNl7f@function.mps.ohio-state.edu> dicks@function.mps.ohio-state.edu (Ethan Dicks) writes:
>cjl mentioned the usefulness of various components for repairing
>PDP-8 boards.  The place where I work uses 8641 chips in manufacturing
>Unibus and Q-Bus boards.  We paid $7.50/chip for a bunch of National-
>Semiconductor 8641 chips.  I think that some could be made available,
>perhaps at less than cost.  These parts are new and in the tube.

That's a bit high, but could be useful to some of us who maintain/repair the
Omnibus/Qbus/Unibus cards that use them.  Unfortunately, this is the least
needed non-74xx chip relative to the PDP-8, as I believe it's only used in
a few 8/A memory boards while the rest of the list is more commonly used in
this class of boards.

There is no equivalent, as DEC just accepts the NS or Signetics numbering 
system for these chips, and most manufacturers went along with the scheme.

I once did find some chips that DEC soldered into boards as SP-384, yet they
were actually made by/for TI, and instead had a bizarre and lengthy number on
them, perhaps being a special-order number as DEC approached TI as a second
source for the chips, and TI didn't want to be identified as "following in
the footsteps" of Signetics or National Semiconductor, even though these chips
were never the forefront of any competition between these companies, etc.

If anyone designs new Omnibus peripherals, there are many choices of
chips to do pretty much the same thing.  Some of the CESI designs take
advantage of newer chips, although in some cases they are taking slight
liberties with the design, and even DEC has occasionally done so in a 
few cases of 8/a board.

The "worst offender" I am aware of is the use of 7438 as a buss driver,
which may not pass the design spec re leakage allowance for an Omnibus driver.
Certainly, when this chip was available, DEC passed over it in favor of the
7401 with leakage being measured and graded out, etc.  It is possible that
more modern versions of all of these chips just work due to better
manufacturing techniqes, etc.  In any case, the cards I saw the chips used in
were presumed to be used in 8/a boxes where usually (but *not* definitely) the
buss tends to be 20 slots or less, while the Omnibus design spec is for an
80-slot machine (two full 8/e boxes with a BC08H cable pair between them).

My own machine is a 40-slot 8/e box with a 20-slot 8/a box attached using the
obscure BC-80C cable to properly connect it.  (You can also use two BC08H
cables, but it won't "fold" very pretty, and you must ensure to disconnect the
Power-OK jumpers on the cable in either case.  This is always the case when
there are two un-like boxes such as 8/e, 8/m or 8/e, 8/a, etc.)

So, Ethan's offer is welcome, but only seems to solve a small portion of the
problem; anyone else out there with some ability to dredge up the other
non-standard chips short of cannabalizing one board to fix another?  (That
method ultimately fails.)

cjl
Newsgroups: alt.sys.pdp8