80-Bus News


May–June 1984, Volume 3, Issue 3

Page 13 of 51

required if only the 64k version is to be used. It is then sufficient to tie the reset pin to the OR gate output as indicated by the dotted line. The inverter for the clock input to the flip-flop can be taken from pin 9 (input) and pin 8 (output) of IC 34 which does not appear to be used in the original design. Both the extra address line A7’ and the /CAS signal should be coupled to the RAM via 33R resistors to reduce electrical noise and ringing.


When it was first tested by writing a block of identical bytes using the NAS­SYS C command, the RAM showed a lot of corrupted bytes. The reason for this took me many hours to uncover. Eventually I discovered that the read/​write line on pin 1 of the data bus buffer IC 2 suffered from spurious negative pulses of about 50 nSec duration. These pulses occurred every time the /RAMDIS signal was activated. This line goes low about 50 to 60 nSec after /MREQ; thus, with /CS on P6 or P7 held low, the buffer is put into the read state as soon as /RD and /MREQ go active. Now if this is the start of a read-cycle of memory on the main board, the read buffer on the RAM board should not be activated, but it does not know that for 50 nSec or so, until /RAMDIS goes active. As designed, P6 or P7 are fed from the decoders IC 22 and IC 23, or IC 24, and these, together with the address latch IC 25, produce sufficient delay that /RAMDIS goes low before /CS and hence there is no timing problem. The solution which seemed to be most simple was to produce a version of /MREQ delayed by about 60 nSec, and apply it to the NAS­COM SEL pin (which is coupled directly to bus line 11); this stops IC 36 from going low until after /RAMDIS goes active, if it is going to. I used two gates from a CMOS 4049 inverter connected in series, which produces sufficient delay. This is the only additional chip which is required to complete the 64k conversion. I must confess that I thought I still had problems, even after applying the delay, but this turned out to be nothing to do with the modified board, but rather, that I was testing it using a 10 inch extension board to give me more ready access to the RAM card; it turned out that this introduced sufficient spurious fluctuations to the bus lines to cause corruption of some of the data. Finding this also wasted a considerable amount of time – so be warned. With all the bugs removed, the board functioned perfectly without wait-states at 4 MHz.

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