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.