Subject: Looking for schematics for DEC M702 and M720 modules

From: Ethan Dicks <[email-masked]>
Date: Mon Jun 9 14:09:00 2003

Both of these are likely to be labelled "memory detection".  I have
seen both appear in module listings (and, from reports, in the real
world) in slot B10 of a PDP-8/i.  I keep entertaining delusions that
I will be able to upgrade my -8/i to 8K someday.  This module is the
only one that I cannot borrow from another machine or my pile of spare
parts.

ISTR the M702 contains a delay line and some buffering TTL.  I'm told
that the M720 contains some TTL chips and a few transistors.

My goal is to understand them well enough to fabricate a substitute,
either from scratch, or by co-opting some blank spaces near slot B10 and
using DEC M-series modules.

Thanks,

-ethan

---

From: Andreas Freiherr <[email-masked]>
Date: Wed Jun 11 04:07:00 2003

Ethan,

there's a M720 circuit diagram on microfiche in my collection. I'll try 
to take a picture of the fiche viewer's screen with my digital camera, 
but it's a bit difficult due to illumination varying widely across the 
screen. In addition, the newer rev level is printed blurry on the film. 
I'll try again tonight, if I get out of the office in time.

As far as I can remember from looking at the fiche yesterday, there are 
three SN7400N chips and one SN7460N in the circuit, together with four 
transistors, a couple of diodes and passive parts. Doesn't look too 
complicated.

Andreas

--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com

---

From: Ethan Dicks <[email-masked]>
Date: Wed Jun 11 15:21:15 2003

> I'll try again tonight, if I get out of the office in time.

That's OK.  Al Kossow sent me a copy.  It's in his scanned docs for the
PDP-12.

> transistors, a couple of diodes and passive parts. Doesn't look too 

Nope... it looks pretty straightforward.  All I need are some 7460s.  :-)
 
Thanks for looking, though.

-ethan

---

From: Andreas Freiherr <[email-masked]>
Date: Thu Jun 12 05:09:00 2003

Hello Ethan,

(M720 circuit diagram)
> That's OK.  Al Kossow sent me a copy.  It's in his scanned docs for the

And it's certainly better quality than what I got from the fiche reader 
screen. In case somebody else wants to try: I got readable quality with 
a few simple measures. In a dark room to avoid reflections, set the 
camera's shutter control to "dark night mode" and put the camera on a 
tripod. Since mine doesn't have a remote trigger capability, I used the 
timer function to avoid touching the camera, so it would be steady while 
taking the picture. For my setup, it didn't make a difference in the 
final picture whether I set the fiche reader to "high" or "low" light.

Even though the picture gets darker towards the corners, in a 1,600 by 
1,200 (2M pixel) .JPG you can read about everything that is readable on 
the screen.

> Nope... it looks pretty straightforward.  All I need are some 7460s.  :-)

These are certainly harder to get than the 7400s, but it appears to me 
that the 7460 is actually used a a kind of delay line: since all inputs 
of each gate are tied together, there is no logic function involved, so 
maybe you can get away with something completely different.

Since I had the fiche looked up already: the only difference between the 
original schematics and Rev. A is C7: it had ".005MFD" originally and 
was changed to ".02MFD" in Rev. A. Looks like the one-shot made from E3 
needed a longer trigger pulse.

> Thanks for looking, though.

You're welcome! - I also used the opportunity to test what the camera 
can do for me. Now that I have the procedure: If you need anything else, 
let me know.

---

From: Ethan Dicks <[email-masked]>
Date: Thu Jun 12 11:01:03 2003

--- Andreas Freiherr <[email-masked]> wrote:
> >>three SN7400N chips and one SN7460N in the circuit...
> > 

That is logically consistent with what I recall about the M702 - It
has a delay line similar to what's on the M360 and M310 modules.  I
don't know exactly why, but there needs to be some amount of delay
in the memory field detection process... perhaps to let some other
part of the circuit stabilize first.

> needed a longer trigger pulse.

Good to know.  Thanks for the tip.

*Now* I need to find a spec sheet for an SN7460 and see what its
propogation delay is.  I can probably estimate based on other members
of its family, but in this particular case, the specifics will matter.
Obviously, I won't be substituting a 74HCT part or a 5ns GAL.  :-)

-ethan