hbit@PacBell.COM (Henry Bitter) (11/16/89)
After many months of fighting a particular problem using the Lucas
board and some brands of memory boards has been solved (at least for my
case). The problem was not being able to boot the machine with both the
Lucas and expanded memory connected even though with either one connected
the product worked fine.
The problem turned out to be a timing difference that the expanded
memory could not handle. With the lucas board active the two timing signals
UDS and LDS were getting to the external memory before it could handle them,
atleast in relation to the other timings such as *DSACK. With the installation
of a small delay on both of these signals by way of a 74x04 the problem has
been totally eliminated. I can now use expanded memory, Francis memory, or
both. Since I have also installed the 68000 hack I can also switch to 68000
mode with expanded memory still available just like it should be.
The data below shows the small change required to make it work.
The only negative part I have found is that I used to be able to run the
Lucas board at 20 meg (but with no external memory) and I found I had to
back off to 16 meg small price to pay for an extra 2 meg of ram. I have tested
the fix with everything I have available including A-MAX and all seem to work.
It is interesting to have a 6.5 meg MAC and see how that machine should have
worked.
The fix can be installed on either the Lucas board or on the
expanded memory board (which ever is easer). The chips (2) I used after
some experimenting were 74ALS04, but any of the F,AL,AS,ALS family can be
used. The one you need is determined by your external memory board and
its timing requirements. I used all 6 inverters in each chip for each of
the lines. You may not need that much. Just use the inverters in pairs and
experiment until you get it working. The chips are very cheap so you can
try many combinations at little cost.
from lucas--> |\o--|\o--|\o--|\o--|\o--|\o---> to external memory LDS line
|/ |/ |/ |/ |/ |/
Henry Bitter
* Pacific Bell * (415) 823-2836
San Ramon, Calif.
{ }!pacbell!pbhya!hbit