When configured as a Mod 8, the there are no issues. This only happens when the system is configured as a Mod 80. The only difference between the two configurations is the following.
- The Buffer Board (Mod8-3) is replaced with the Mod 80-1 CPU Board
- The Mod8-1 Board is replaced with a Jumper Board (Mod80-3)
- The Monitor 8 EPROM Board (Mod8-4) is replaced with two EPROM Boards containing Monitor 80 code
I have three different Mod 80-1 boards, one is a reproduction board, and two are original boards made by two different people at different times. All three exhibit the same behavior, so I doubt the problem has to do with the CPU board. If it were, there would have been some sort of bug fix published, or something in the notes I have, I suspect.
Next I checked the Jumper Board. I thought maybe I missed a jumper that affects only this function. However, when the board was double checked with the bus diagram from the Moducomp Mod-8 Supplement no errors were found.
As Mr, Sherlock Holmes is fond of saying, "when you have eliminated the impossible, whatever remains, however improbable, must be the truth". This left the two Mod 8-4 boards, or more specifically, the Monitor 80 code I was using,
Monitor 8 and Monitor 80 address the ZIF socket slightly differently. In Monitor 8, the EPROM in the ZIF socket appears at addresses 200000 to 200377. Since 200000-200377 could be a real addresses in an 8080 system, Monitor 80, uses addresses P000 to P377 to refer to the EPROM in the ZIF socket.
My copy of Monitor 80 was burned using the copy of the code in the Moducomp Mod 80 Supplement. I have checked the ROM contents several times to ensure that they are correct, and found nothing wrong, but this is not an easy task to get right. I suspect there must be a bug in it somewhere. I can't think of any other reason. Can you?
I am hoping to get a copy of a original set of ROMs from a friend to compare, and see if that corrects the problem. Stay tuned.