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CUSTOM MmlORY MODULES
As technological developments make it feasible, RCA will
offer new RAM and ROM memory modules.
Nevertheless, some
users will want to experiment with modules of their own
design and construction.
Within the physical and electronic
constraints of the Nicrokit, they may do so.
The physical constraints are fairly obvious.
If the memory
is to reside in the kit, the cards containing it should be
no more than 6.5 inches deep and no more than 0.4
inch~s
in
thickness overall if adjacent slots need to be occupied.
If
the memory is to be external and serviced by a cable, then
the cable connector may plug into a Memory Bus slot.
This
arrangement may require the user to buffer the address bus
(A15-P to AO-P) and the data busses (D07-N to DOO-N and DI7-N
to DIO-N), depending on the length of cabling and the drive
required.
Another consideration in adding memory involves the available
d.c. power.
The Microkit power supplies, VDD=+5V and the
-9V supply, have some reserve capacity, namely, over 2 Amps
at +5V and 350 milliamps at -9V.
Power beyond this must be
provided by the user.
The COSMAC CPU ar£hitecture does not require memory cycles to
be contiguous, i.e., to immediately follow each other in time.
(See Appendix D of the COSMAC
~1ICROPROCESSOR
MANUAL for memory
timing.)
Thus the cycle-times of added memory modules are not
critical.
(Clearly, they must be less than the machine cycle
of approximately four microseconds.)
What are important are the
access and write times.
The read access time in the kit envir-
onment, at 1.95 MHz clock, should be
.8
microsecond or less.
The write pulse width (MWR-P) in the kit WIll be about .75 micro-
second, and the memory should take no longer than that time to
wrIte.
Slower memories may be incorporated by supplying a
slower
clock~
.
Example
Design and
~nstall
a 4K RAM card for address
space OOOO-OFFF, and move the lK RAM in slots
11 and 12 to address space lOOO-13FF.
Construct a 4K memory card as shown in the
functional diagram of Fig. 15 below.
Remove the wire-wrap connection between
RMDS-P and pin X at slots 11 and 12.
Choose an empty Memory Bus slot for the new
4K card.
Assume you select slot 9.
Wire slot 9 as follows:
pin 19 to RMDS-P (pin 20 of slot 16)
pin
X
to pin
X
of slots 11 and
12
Insert the 4K card in slot 9.
- 49 -
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