80-Bus News

150uf, as I did not have anything else of suitable value handy, and put the card back into the case. This time the POR worked perfectly, and it has done ever since. (It should be equally valid to increase R13 to say 12K/15K.)

I have not analysed the action of the POR circuits of the N2, but it seems that the value of the time constant of C3/​R13 can be critical. The time constant C2/​R11 in the manual reset circuit is identical in component values. In consideration of the ‘cure’ for this fault it seems that C3/​R13 should have a longer time constant than C2/​R11 for correct circuit action. By the laws of averages, this should be true for 50% of the Nascom 2’s with 10K/68uf fitted in both positions, allowing for component tolerances. In any case, if this assumption is true, the fault will only be noticed in a proportion of those N2’s that are trying to reset to some other address than 0000H.

I am not an expert in digital logic. Perhaps someone can send in an article explaining the operation of the circuit, with some timing diagrams, as my cure may not be the best one for this problem. It does work though.

I would like to thank Alan Stevens and Phil Harvey for the loan of cards, which helped me to isolate the problem. I hope that this article may help others who have experienced similar problems with the power-on-reset on their system.

This note refers to the review of the MAP 256K RAM card on pages 40 to 44 of Vol 2 Issue 1 of 80-BUS News. A problem has arisen when using the Gemini GM813 CPU/64K RAM card with more than one MAP RAM card. The reason is that the GM813 does not provide an A19 signal – see “The Great A19 Debate” on page 43 of that issue for a detailed explanation. This means that the 64K of RAM on the GM813 is addressed both at the bottom of the one megabyte of addressable memory, and also half way through. This obviously causes problems, since if two 256K MAP cards are added, the last 64K of the second card is overlaid by the bottom 64K on the GM813.

The solution is to disable the 64K on the 813, and alter the header plugs on the MAP cards so that they provide memory starting at 0K, 256K, 512K and 768K. This is very simple and MAP will happily tell you how to do this.

The best way to disable the memory on the 813 is a simple hardware modification to permanently disable the RAM. MAP will provide a recommended alteration. Alternatively, the 813 RAM could be addressed on a different 64K page, using the original paging scheme which has four pages of 64K. This has the disadvantage that as well as the GM813 needing modifying, a patch to RP/M is needed, which makes it nonstandard. I don’t approve of that, but MAP may prefer to adopt this solution.

You may feel that it is incredibly wasteful to effectively throw away 64K of RAM, but if you have a system with 512K or more, you probably don’t mind!

Note that with Gemini’s own GM833 512Kbyte RAM-DISK board you can go upto 8 Megabytes with no mods. to either the GM813 or GM833!