mikes@sir-alan.UUCP (01/26/87)
The following note is being reposted as apparently some interested parties
missed the first posting, and attempts to send the note by e-mail have
failed. The note was written by Paul Sutcliffe (paul@devon).
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Anyway, here's the info on the 1Meg upgrade. After finishing this
modification, the schematic of the board looks just like the real 6000
1Meg board, except that the 6000 board has jumpers to turn the board
back into a Model-16-type board:
You will need 150ns chips for a 6mHz 68000, and you should use
120ns chips for an 8mHz 68000 (150ns may not be fast enough).
I as able to purchase NEC 150ns 256K RAMs for $2.85 apiece from
Microprocessors Unlimited, Inc., 24000 So. Peoria Ave., Beggs,
Oklahoma, (918) 267-4961. The whole bill for 36 (I ordered spares)
was $106.85. I received the order in 3 days. This was in late
January, so the prices may have changed. At that time, 120ns NEC's
were $3.37 apiece.
The mod is as follows:
Parts Needed:
Description Qty
RAM chips (see above) 32
Resistor, 68 ohm 1
74S240 1
74S157 1
Cut the following traces:
BUS A17 --> U9.13 Cut top side between U9 and first via
BUS A18 --> U2.4 Cut bottom side between U2 and first via
BUS A19 --> U2.6 Cut bottom side between U2 and first via
New Parts:
Mount the 74S240 on top of U3 or U4 by bending up all pins EXCEPT
1, 10, 19, and 20. This chip will be called P1 for piggyback 1.
Mount the 74S157 on top of U29 by bending up all pins EXCEPT 1, 8,
15, and 16. This chip will be called P2 for piggyback 2.
Jumpers:
BUS A17 --> P1.11 Via from first cut Buffered A17
BUS A18 --> P1.13 Via from second cut Buffered A18
BUS A19 --> U9.13 Via from third cut Buffered A19 for bank
U2.4 --> U2.5 On U2 to default address selection
U2.6 --> U2.7 On U2 to default address selection
P1.9 --> P2.2 Buffered A17
P1.7 --> P2.3 Buffered A18
P2.4 --> resistor A17 MUXed with A18
resistor--> pin 1 of ALL RAM chips on the bottom of the board.
The resistor is in series to pin 1 of all the RAM chips.
That's All Folks! The mod could have been done a little differently
without the 74S157, but that chip makes it possible to use only 16
RAMs to get 512K (in case of memory failure, a 512K board is better
than a 0K board!).
Mike Squires
814-724-3360
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