Scor­pio News


May 1989, Volume 3, Final Issue

Page 30 of 43

Lucas Hardware/Software – Points Arising

by Dennis McLaren

AVC and RAM mods

Ifyou have an AVC in your system, you’ll know that a “simple mod” has to be made to your RAM card before anything will work. You then find that (RAMDIS no longer works properly when any other card brings it low. The reason is that your simple mod has caused /RAMDIS to both read and write protect the RAM.

This is essential for the AVC, but contrary to the whole philosophy of /RAMDIS when used by EPROM cards etc.

Fortunately, the cure is simple. The idea is to restore /RAMDIS to its original read-disable role and to give the AVC its own /WRITEDIS line to the RAM card. Whenever the AVC wishes to read and write protect any RAM overlaid by the video planes, it will bring down /RAMDIS by way of a diode and /WRITEDIS via a separate lead to the RAM card. Any other card which brings down /RAMDIS will be prevented from bringing down /WRITEDIS by the diode.

This mod works for my Gemini 64K RAM card at 4MHz. It should work for other RAM cards but I don’t know anything about those.

GM802 64K RAM card mod.

  • Bend up pin 10 of IC 48 (74LS00) but don’t connect it to anything for the moment (a low on this pin will write protect the card).

Nas AVC mod.

  • Bend up pin 8 of IC 56 (7417) and solder a diode (I used a 1N4148) between pin 8 and TP 17 (/RAMDIS) immediately adjacent. Arrange for the cathode of the diode to be connected to pin 8 of the IC.
  • Make up a length of hook-up wire long enough to reach from pin 8 of the IC on the AVC to pin 10 of the IC on the RAM card.
  • Solder a soldercon to one end of the wire and solder the other end to the diode/pin 8 junction on the AVC.
  • Plug the cards into your system and clip the soldercon end of the hook-up wire to pin 10 of the IC on the RAM card.

This is an OCR’d version of the scanned page and likely contains recognition errors.

Page 30 of 43