9 Jun
2003
9 Jun
'03
10:09 a.m.
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
11 Jun
11 Jun
12:07 a.m.
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
Ethan Dicks wrote:
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
--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com
11:21 a.m.
--- Andreas Freiherr <Andreas.Freiherr(a)Vishay.com> wrote:
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.
That's OK. Al Kossow sent me a copy. It's in his scanned docs for the
PDP-12.
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.
Nope... it looks pretty straightforward. All I need are some 7460s. :-)
Thanks for looking, though.
-ethan
12 Jun
12 Jun
1:09 a.m.
Hello Ethan,
(M720 circuit diagram)
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.
That's OK. Al Kossow sent me a copy. It's in his scanned docs for the
PDP-12.
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.
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.
Nope... it looks pretty straightforward. All I need are some 7460s. :-) Thanks for looking, though.
-ethan
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.
--
Andreas Freiherr
Vishay Semiconductor GmbH, Heilbronn, Germany
http://www.vishay.com
7:01 a.m.
--- Andreas Freiherr <Andreas.Freiherr(a)Vishay.com> wrote:
:-)
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.
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.
As far as I can remember from looking at the fiche
yesterday, there are
three SN7400N chips and one SN7460N in the circuit...
Nope... it looks pretty straightforward. All I need are some 7460s. 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.
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
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