HP 82929A
Programmable ROM Drawer
The HP 82929A Programmable ROM Drawer is one of the many accessory modules available
for the HP Series 80 personal computers.
The module fits into one of the rear expansion slots and allows you to add your own
code resident in PROM or EPROM, or any of the old original ROM modules for which images are
available on-line.
After removing the enclosure of the HP 82929A module you will find two 28-pin DIP
sockets that can accommodate either two 4KByte (2732) or two 8KByte (2764) EPROMs.
Each ROM module image is 8KBytes, then using 2764 EPROMs you can add two additional
ROM module images with the HP 82929A module.
Project Description
The primary goal of this project is to reverse engineer an original HP 29829A
module and obtain a full set of schematics. Since the module does not require
or use any specific custom HP parts only standard (some of the original ICs
may be obsolete today and hard to find) CMOS and TTL parts it won't be hard
having the schematics to make a clone or equivalent module.
Also having the schematics and a better understanding about how the HP-85 I/O
bus works will provide valuable information for another project in the list
that is to emulate many expansion modules and interfaces with a microcontroller
based board.
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HP 82929A Component Side
HP 82929A Solder Side
[Click Images to Enlarge]
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This process has been done in the past, but besides some high level description
of the PRM-85
developed by Bill Kotaska with support from John Shadbolt as a replacement for the
HP 82929A, I didn't find much information available on-line.
John was quick to reply to my request for information, and I owe him a great deal
of gratitude for his help and encouragement for this an other projects we discussed
via e-mail. I also recently got in touch with Bill, we are already exchanging some
information and ideas.
I also need to mention Everett Kaser who while at HP was involved somehow in the HP-85
development and provided additional information. When I was almost done with
drawing the complete schematics Everett found and scanned the schematics for an
old EPROM board that HP used to run the diagnostics code and for a
3 8-bit Digital Output ports. Both schematics helped to confirm and validate
the one I was putting together.
Project Progress
I started putting together the schematics by following the traces using two
high resolution pictures of the HP 82929A top and bottom sides posted
by Vassilis Prevelakis in 2003 in the
ClassicCMP mailing list.
Doing the solder or bottom side was obviously quite easy since you can see all
the traces in front of your eyes, but getting the traces on the component or top
side was a challenge. While the picture had good resolution some shadows on the
ICs pins made it very difficult.
I was almost ready to hang the towel and keep searching for help on the net
when John came to the rescue and let me put my hands on an actual and
working HP 82929A module.
What I did first was to take high resolution pictures of both sides of the board.
The pictures above on the right are a medium resolution version of the actual board.
You can find a pair of higher resolution version in the links section below.
Now with the new pictures I sized them and mirrored the bottom side to match the
top side. Using Adobe Pothoshop I created different layers for different elements
such as pads and vias, top and bottom traces and legend for component IDs.
For the complicated and hidden traces on the component side I recurred to an
ohm meter and also the logic that was showing up on the schematic I
was putting together.
The following two pictures show the top and bottom sides and the pads, vias and
trace layers for each side of the HP 82929A board.
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[Click on the images for a larger view]
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And turning off the layers with both pictures, the result was a complete layout diagram
showing all the top and bottom traces for the board as shown below.
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HP 92829A Board Layout
[Click on the image for a larger view]
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Now with the complete board layout, time and patience was not too difficult to put
the complete schematic together. It took me a couple of passes while I moved gates
here and there to make the final schematic more easy to read and logically consistent.
As I do for many other projects I used Eagle CAD
to enter the schematics and Adobe Acrobat to produce a pdf print out.
Project Status
This project is complete and the full schematics and additional information
can be found in the links below.
CORRECTION/UPDATE:
In the process of testing the Programmable ROM module clone I found two minor
mistakes in the previous version of the schematics.
- The legend on page 4/6 said that S1 selects the ROM bank for U19 and S2 for
U18.
The correct association is S1 for U18 and S2 for U19.
- On page 2/6, U15, the signal legends for pins 14 and 15 were permuted
The correct signals are pin 15 (Y0) is U18-CE and pin 14 (Y1) is U19-CE
The link to the schematics pdf file has been changed to point to a new version (3.2)
that includes these two corrections.
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