So to the cures:
None of these is technically elegant, but they do work. They
should be tried in order until the problem is cured. Don’t go
in for overkill, as this is unnecessary, and undesirable.
1). Go for a National 81LS97 in the IC2 position (AMD devices
seem to have lower noise immunity although AMD deny this).
You have 7 81LS97s to play with, 3 on the NASCOM, 1 on the
buffer, and 3 on the memory, one at least is likely to be
made by National. Swap these ICs about for the best
results in the IC2 position.
2). Grid off the Ground and +5 volt supplies. On the underside
of the pcb it will be noted that the GND and +5 rails
supplying the TTL ICs terminate at the end of each row. Wire
links can he fitted to connect these rails to the equivalent
rails supplying the RAM chips, thus completing the “grid”
on the power supply rails, thereby reducing power supply
noise. Take care not to short out the power supply rails
by “gridding” to the wrong tracks.
3). The 74LS04 on the buffer board may be replaced with 74S04.
Bit of a naughty one this, as far as loading goes, but it
does tidy up the MREQ waveform.
4). On ICs 4-11 only, fit a 4K7 resistors from pin 9 to pin 14
of each chip, thus pulling the outputs of the RAMs to +5V.
5). On ICs 4-11 only, in addition to 4 (above), fit 47pF ceramic
capacitors from pin 14 to pin 16, thus producing a time
constant on the RAM output.
Various combinations of these cures have been tried with
100% success on the few boards that have come our way, and
although not ‘elegant’ solutions have transformed
recalcitrant RAM boards into perfect working members of
Please write to the INMC if you have come across any other
oddities in the RAM or buffer boards.
We have recently discovered that noise on the NASCOM 1 itself can
cause problems with expansion, but fortunately this is easily cured.
If you look at the corner of the board where the modulator is
situated you will see an issue number. If it is “ISS.C” ignore these
comments. Otherwise you may find it worthwhile to add a few links
to the back of the board along the long edge of the board where the
power supplies are connected. The links should “bus up” the ground
and 5V supply rails and the easiest way to do this is to connect the
ground of each decoupling capacitor at the edge of the board to the
next decoupling capacitor at the edge. Similarly connect the 5V
sides. Be careful not to get them twisted!!
If you are at all uncertain about any of these modifications, please
contact your distributor, NASCOM, or the INMC. If in doubt, stop.
If you return the system for repair you should include the NASCOM,
memory and buffer boards to ensure that the system is totally
operational when returned to you.