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32K16BUS.TXT
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2006-10-19
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PUBLISHED IN LIMA NEWSLETTER
FEBRUARY 1988
^^^^ MIKE BALLMANN'S 32K -- 16 BIT
BUSS PROJECT
^^^^^^^^^^^^^^^^^by John Clulow
The following is a step by step
description of how to add 64K of RAM
memory on the 16 bit bus. The
present modification uses only 32K.
This corresponds to the memory space
of the 32K Memory Expansion. The
modification yields a speed increase
of about 50%.
Mike Ballmann is currently
working on a circuit to allow CRU
decoding of the remaining 32K. This
will open up a whole new area of
software, including such
possibilities as a real DOS which
could be loaded into RAM from disk on
power-up. The 32K modification
described below can easily be
modified for full decoding upon
completion of Mike's work.
You will need two Hitachi
HM62256LP-12 RAMs. One source of
these is Microprocesors Unlimited.
They cost around $12. You'll also
need a 74LS21 and a 74LS153. These
can be obtained from various
electronics supply houses. All
wiring should be done with wire-wrap
wire. You should use a low wattage
soldering pencil with a fine pencil
type tip.
The modification is done on the
main board of the Black & Silver
console, and you'll need to refer to
the (accompanying) Logic Board
Component Location Diagram from the
99/4A console Technical Data book.
1. Remove the board from the
console and identify the two ROMs.
They are located between the GROM
connector and the 9900 IC. One is
parallel to the 9900 and the other is
perpendicular to it. They are U610
an U611 on the Component Location
Diagram.
2. Bend the pins on the HM62256
IC's closer so they will firmly
contact the ROM pins when
piggy-backed. One way of diong this
is to place the RAM in it's side on a
table and then move the body of the
IC toward the table to bend the pins
uniformly.
3. Bend out the following pins
on both HM62256 RAMs: 1 2 20 22 23 26
27 28. These pins will NOT be
soldered to anything on the ROMs.
Holding the IC with the notch up and
looking at the top, pin numbers start
with pin 1 on the upper left, go down
the left side, then across the and up
the right side. Pin 28 is opposite
pin 1 on the end with the notch.
4. Place one HM62256 over the
ROM that is parallel to the 9900.
Make sure th notch points toward the
9900 and that the writing on the 9900
and the 62256 can be read from the
same direction. Place the RAM such
that pins 1 2 27 and 28 extend beyond
the end of the ROM. The un-notched
end of the RAM should line up with
the un-notched end of the ROM. There
should be a sort of "spring tension"
that clamps the RAM pins onto
corresponding ROM pins below it.
This will help to insure good solder
joints. If the RAM doesn't fit
tightly, remove it and bend the pins
closer.
5. Solder all RAM pins not bent
out to the ROM pins below. Use a low
wattzge pencil with a fine pencil
type tip. Inspect each solder joint
carefully in good light under
magnification.
6. Place the second 62256 on the
ROM that is perpendicular to the
9900. The notch on the RAM points
away from the 9900 and toward the
edge of the board. As above, solder
and inspect all pins that were not
bent out.
7. Bend out the 74LS21 pins 1 2
4 5 6 8 10 12 14. Note that pins 1
and 14 are across from each other on
this 14 pin IC.
8. The 74LS21 will be
piggy-backed on the 74LS138 U504.
this IC is located adjacent to the
end of the board where the edge
connector is. There are two 138's
next to each other. U504 is the one
nearest the end of the board. You
will place the 74LS21 so that the
UN-NOTCHED end lines up with the
un-noched end of the 138 (pointing
toward the cassette connector). Pins
1 and 16 of the 138 will extend
beyond the notched end of the 74LS21.
9. Before positioning the
74LS21, solder 1/2" lengths of
wire-wrap wire to the 138 pins 7 and
9. Then position the 74LS21 on top
of the 138 and solder all pins not
bent out to the 138 pins below and
inspect the connections.
10. Bend out all of the 74LS153
pins EXCEPT 8 and 16.
11. Place the 153 over U613, a
74LS194. The notch will line up with
the 194 notch and point toward the
edge of the board away from the 9900.
Solder pins 8 and 16 of the 153 to
pins 8 and 16 of the 194 below.
12. At the end of the 9900
opposite to where the RAM's have been
piggy-backed, you will see a line of
three ICs. They are 74LS00, 74LS32,
and 74LS04. The 74LS00 is U606 and
the 74LS32 is U605. Turn the board
upside down so you can see the
traces. Find the trace that runs
from pin 11 of the 74LS00 (U606) to
pin 13 of the 74LS32 (U605). Double
check to make sure you're doing the
pin numbering correctly. When you've
found the trace, cut it with a knife
so there is no continuity between the
LS00 pin 11 and the LS32 pin 13.
13. Identify the piggy-backed
RAM that is perpendicular to the
9900. Solder wire-wrap wires
connecting every bent out pin on this
RAM to the corresponding bent out pin
on the RAM that is parallel to the
9900. Pin 1 to pin 1, pin 2 to pin
2, etc. There will be eight wires in
all to solder.
14. Solder wire-wrap wires to
make the following connections on the
RAM that is parallel to the 9900.
Pin 1 goes to pin 24 of the 9900
(solder the wire to the 9900 pin on
top of the board). Pin 2 goes to the
9900 pin 22. Pin 20 goes to two
places. Connect pin 20 of the RAM to
pin 22 of the RAM and also to pin 8
(bent out) of the 74LS21. There
should be three wires comming off pin
20 of the RAM. Pin 23 of the RAM
goes to pin 21 of the 9900. Pin 26
of the RAM goes to 23 of the 9900.
Pin 27 of the RAM goes to pin 61 of
th 9900 (fourth from the top on the
right side). Finally, connect pin 28
of the RAM to pin 20 of the 74LS244
(UJ10) adjacent ton the piggy-backed
74LS21.
15. Connect the following 74LS21
pins with a bare wire: 1 2 4 and 14.
Connect the short wire from the 138
pin 7 to the LS21 pin 5 (bent out).
Connect the LS21 pin 6 to LS21 pin
12. Connect LS21 pin 8 (bent out) to
the piggy-backed 153 pin 2. Connect
the short wire comming from the 138
pin 9 to LS21 pin 10. Finally,
connect the 74LS21 pin 14 to the
74LS244 pin 20 that you connected the
RAM pin 28 to.
16. OK, we're almost done, so
take a break and have a beer coke.
17. On the 153, connect pin 9 to
pin 13 on the 74LS32 (U605). Pin 10
of the 153 goes to pin 14 of the
74LS74 next to it (U607). Also
connect pin 10 of the 153 to pin 15
of the 153, and then connect pin 15
of the 153 to pin 7 of the 74LS00
U612 (next to the 74LS74). Connect
pin 14 of the 153 to pin 11 of the
74LS00 U606; that's the one you cut
the trace on.
That's it! Now have another
beer coke before putting your
computer back together. When you try
it out, remember that this version
isn't compatable with any other 32K
in the system. If you have any
problems with this I can't promise I
can help, but feel free to give me a
call or write EMAIL 419-874-8838.
Ask for John (or Hose-Head).
.PL 1