Radio Shack TRS-80 Reference Handbook page 28

RAS* is generated by the CPU at pin 19
(MREQ)' Whenever MREQ goes low, RAS* goes
low; and the RAMs will load the lower order
address into the row section. The CPU may be
looking at system ROM when MREQ goes low,
but RAM will still receive RAS* and thus be
"refreshed. "
Normally, you would not be too concerned
about this aspect of the RAMs. But you need to
be aware of the differences between a static
RAM and a dynamic RAM. Remember: Dynam-
ic RAM must be periodically addressed to enable
it to retain data. In the TRS-80, the RAM is
refreshed once every two milliseconds.
RAM Programming
You may have noticed Z71 during the discus-
sion of the RAM. Z71 is a DIP shunt. It is used
to program the size of memory in a system. Find
pin 13 on the RAM. Follow it down and you
will see that it is tied to two pins of DIP shunt
Z71. Pin 13 is the CE (Chip Enable) or the A6
address input. In a 41< system, pins 4 and 130f
Z71 are shunted. RAM* is on pin 4, so RAM* is
used to select RAMs. But in a 161< system, 4 and
13 are opened and pins 3 and 14 of Z71 are
shorted. Instead of RAM*, we'll get address line
A6 or A 12 (depending on multiplexer status) go-
ing to pin 13 of the RAMs. There are other parts
of Z71 shown on the left side of the multiplexer
(Z35 and Z51). Before troubleshooting a system,
you will need to know the size of RAM that the
system uses. If Z71 is "programmed" wrong,
you may find yourself with RAM problems.
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