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Last updated Mar 10 2024. This document copyright Herbert R. Johnson 2024,
doucments from others copyright by them.
Summary: There's two SSM ROM boards which support the 2708 EPROM, the MB-8 and the MB-8A. The MB-8, is marked "8K/16K" to the left of the PROMs. It has two sets of 4-position DIP switches, to either side of the PROM array. The Serial A version has issues, the Serial B version corrects them. The MB-8A, is marked "16K EPROM MB8A"to the left of the EPROM array. It has two sets of 8-position DIP switches, to either side of the line of logic IC's. It's appreantly marked Serial C.
April 2021, reported by Reisen Thomas: The MB-8 board has at least two really big design faults on the pcb. The first bug was a "swapping" of the data bits D4 and D5. The second bug was a short circuit!!! from pin 1 to 3 on U11 (7812) , so, the full 16V input went to the output, there is the second row of EPROM's connected. I think the board was never ever used fully populated with 2708 EPROM's, 16V is definitely too high and results in a damage. But now [repaired], it seems to work well." He reports no problems with the schematic.
Feb 2024 Here's an MB-8A rev C board, the component side and the the solder side. - Herb
Feb 2024, Rich Camarda: So, it looks like the board I have (MB8 SERIAL B) is working as designed. The manual with the schematic you sent helped me understand it better. I assumed, before getting the manual, that the unused EPROM sockets would allow memory overlap, which they do not. That is the main difference between this board, and the MB8A. The MB8A was a much improved version allowing memory overlap in the used sockets, and also the use of the Phantom line. The MB8 does not have the Phantom line, and when you address either the 8K block, or the 16K block, it then makes that block unavailable for RAM, even if you only populate 1 or 2 EPROMS. Obviously not as useful.
My board is the "SERIAL B" version which does not have the problems of the "SERIAL A" shown in the write up on your website (no shorts, or swapped data lines). The serial number is etched on the ground plane just below the "SERIAL B" legend. It shows it is # 103.
So that is what I found out, and unfortunately the MB8A would be of more use to me, however this board works fine for reading EPROMS, if you disable RAM for the 8K, or 16K block. Rich Camarda
June 2021, Reisen Thomas: this board looks a little bit like my other MB-8 board but it's named 2K/4K and have some 1702 EPROM's mounted." Well, it's an MB-3 board! I don't think "MB-3" is etched on the card. - Herb
Herb Johnson
Copyright © 2024 Herb Johnson
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