MS11-JP MOS Memory SN#1958348/78

The MS11-JP (M7847-DJ) is a MOS memory board for Unibus PDP11's. In the PDP11/04 I am restoring there are two fully populated boards, each with 32K words of dynamic memory.... and neither board works when accessed from the front panel.

The first board I worked on is SN#1958348/78.

Using the front panel the board did respond to read and write requests to RAM in the assigned memory block but reading data did not show the value that had just been written.  This behaviour was consistent at any address I tested.

The voltages (+5v, -5v and +12v) were correct on the dynamic RAM chips so I used a logic probe to check for activity on the dynamic RAM chips with both no front panel activity and when reading and writing to memory.  

When not attempting to access the memory using the front panel there was activity on CAS (Column Address Strobe) and RAS (Row Address Strobe) which suggested that the refresh circuit was doing something and may even be working.

Attempting to write to memory from the front panel showed activity on WR (Write) which was also expected.

What I did not see was activity on CS (Chip Select).  If CS does not go low then the chips are not selected which means the write does nothing and the read shows random data from the Unibus.

 

Working backwards from CS through the schematic shows that CS is generated by a 7440 NAND Buffer (E114).  I could see activity on the input (pins 9,10,12 and 13) but no activity on output (pin 8).

Replacing E114 with a "new" 7440 sourced from eBay (with what appears to be a 1973 date code) fixed the problem.  It is now possible to write data to RAM and read it back.  I haven't done a full RAM test yet and can't do this until I get the processor board operational.  

These notes are written with the benefit of having solved the problem. The MS11 memory board is a lot more complex than I am used to in microcomputers.  It has it's own refresh circuit independant of the processor that identifies and gives priority to processor requests.  There are also circuits for address mapping (configurable) and Unibus bus interfacing so a lot of time was spent trying to understand how the board worked.  The EK-MS11E-OP-001_Oct76 and MP00019_1104_EngrDrws_Feb78 documents were invaluable in understanding the board and eventually fixing it.

The other MS11 does not exhibit the same symptoms.  Attempts to access the board give a Bus Error.  Initial indications are that the issue is in the refresh generation circuit.  We will see.....

 

 

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