80-Bus News


January–February 1984, Volume 3, Issue 1

Page 8 of 55

  1. Connect a wire from TP10 to LKS1 pin 4.
  2. Remove IC8 and bend pins 1, 2 and 3 (carefully) out straight so that they will clear the socket and replace IC8.
  3. Remove IC71 and do the same with pin 8.
  4. Connect a wire from IC71/8 to IC8/2.
  5. Touch solder a wire onto IC24/12 and connect the other end to IC8/1.
  6. Touch solder a wire onto IC18//13 and connect the other end to 1C8/3.

The effect of this mod is only to provide a chip select and a RAMDIS signal when the tape drive is on. Certainly on my machine, it is always on when powered up so SIMON is connected in and any RAM in the system occupying those addresses is disabled. Once SIMON is entered other than for booting up or once CP/M is loaded and started, port 0 is reset and SIMON disappears from the memory map. Plug in and power up and all should be as before (on a 60k max CP/M) provided SIMON is not used as a monitor. If the system is powered up without a disk in drive A, your machine will crash once port 0 is reset by SIMON.

Before a full 64k system can be installed, the cold boot loader must be modified to switch out SIMON before loading the CP/M system. To do this, perform the following sequence of operations:

  1. Use MOVCPM to generate a 64k system. Allow room for SYS if you use it. Save the result as CPM64.COM.
  2. Load CPM64.COM under DDT or ZSID.
  3. Use the S command to modify the addresses given below to the values indicated.
    090B 18     097C 3E
    090C 6C     097D 01
    0979 AF     097E 18
    097A D3     097F 8D
    097B 00
  4. Use ^C or G0 and save the modified CPM64.COM.
  5. Use SYSGEN to put the file on drive A. This can be done by typing:
    after which, specify drive A as the destination.
  6. Unplug & plug in again with the new system in the A drive and Hey Presto, you should have a 64K system up and running with no SIMON in the way. As an added bonus, it won’t get in the way when paging RAM either.

Faulty RAM Board

My second bit of first-aid is probably of no use to Kevin Weatherhead who has, by now, probably cured his faulty RAM board himself. If not then here goes.

Kevin doesn’t say which type of RAM board he is using, so I shall go into the Nascom RAM A & B boards and the Gemini GM802 64k RAM card. The problem is this. When Kevin powers up, his RAM card is full of rubbish as expected. He can write zeros to the card but not ones. Fortunately, all three RAM cards use the same principle so only a brief “how it works” is necessary.

When the Z80 wants to read data, the address lines contain the required address. This address is decoded by the RAM cards to select the appropriate RAM chips. The /WR signal from the Nasbus/80-bus provides the /WR signal to the RAM chips so that they accept data rather than send it. This signal will be a ‘1’ when reading and this side appears to work. As Kevin can also write to the chips, albeit in a limited fashion, this appears to be switching correctly. Data to be written to the RAM chips is gated into the RAM after

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