Lucas Hardware/​Software – Points Arising

by Dennis McLaren

AVC and RAM mods

If you 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.