
29-03-99

MMU Expansion correction by Maciej 'YTM' Witkowiak

Not longer than 56 hours ago I've built a MMU expansion as described in Marko Makela's article. There is only one little error - the asciidram for MMU expansion is correct (fig.6) but the description should go as follows:

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2.4.1 Realizing the processor bus interface

First bend up the pins 3--15, 35 and 41 of the new 8722 MMU chip (IC5)
aside. Bend the rest of the pins slightly to the opposite direction so
that they are perpendicular to the surface of the chip. If the old MMU
is in socket, pop it off. Push the new MMU on the back of the old one.
Solder the pins together, but be careful not to heat the chips too
much. Now you can insert the MMU pair to the socket.

After wiring those pins, you must build the logic for interfacing the
system data bus to the new MMU. Locate U3, 74LS138, it is to the left of
the processor (U6, 8502). Take another 74LS138 (IC6) and bend its pins
1--3, 7 and 9--15 aside. Solder the rest of the pins to corresponding U3
pins. Connect the pins 1--3 of the new chip to the processor's pins 7--9,
respectively.

Before connecting the IC6's required outputs, you have to solder IC7 and
IC8 on the board. First locate U20, the 4066 to the right of the RF
shield covering the video circuitry. Get a new 4066 (IC8) and bend its
pins 1, 4--6 and 8--13 to the side and solder the four downwards pointing
pins to the U20. Wire the pins 1 and 4 together and connect them to the
new MMU's pin 35, the data lead D0. Similarly, connect the pins 9 and 10
together, and solder them to the topmost MMU's pin 41, D6. Solder the
IC8's pins 8 and 11 to the system data bus lines D6 and D7, respectively.
They are available in the original MMU's pins 41 and 42. It is a good
idea to locate through-put places for these signals and solder the wires
there.

The next chip to be mounted is IC7, the 74F00. A good place for it is the
14-pin chip next to the IC6 and the processor. Leave only the pins 7 and
14 down and solder them to the chip on the motherboard. Connect the pins
13 and 12 from the 74F00 to the IC6's pins 15 and 13, and connect the
74F00's pins 11 and 10 together. Then solder the pin 9 to the 8502's
pin 10, A3. Solder the IC7's pin 8 to its own pins 4 and 5, and connect
it also to the IC8 4066's pin 13. Connect the IC7's pin 6 to the IC8's
pin 5. Lead the 74LS138's pin 10 to the 4066's pin 6 and to the 74F00's
pins 1 and 2. Lead the 74F00's pin 3 to the 4066's pin 12.

After all these piggy-backings, it is wise to check if the computer
powers up any more. If not, check the solderings. When I built this
second version of the MMU expansion, I swapped the 4066 pins 9 and 10
by mistake, which resulted in a miserably black screen each time I
desperately tried to power the computer up.

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If you have a 'new' C128 - without cooling fan then when reading the next paragraph (2.4.2) keep in mind that when the author is reffering to U9's (or its socket) pins 8 and 11 - you are doing that with pins 11 and 3 (respectively). This DOES NOT reffers to IC9 - mount it as described.
If you have a 'new' C128 then you have also to use 4464 DRAM chips (4 bits x 64K). Their -CAS is on pin 16.

I don't know if it is possible to make 1024K expansion as I don't know if there are DRAMs with 4 bits x 256K that need only the old way of refreshing (not -CAS before -RAS).

Anyway, building this expansion was a real pain (I mean FUN!) and it is worth to do that. Just to see:
Internal ram: 256K
in ACE128 boot-up message...

contact:
Maciej Witkowiak
ul. Slowackiego 6/57
77-400 ZLOTOW
POLAND
or:
ytm@friko.onet.pl
