On an unmodified PCB of the 3K RAM expansion or Super Expander, pin 21 of both 24 pin sockets is connected to the top-left pad of the jumper labelled "12". This in turn is connected with the bottom-left and bottom-right pad of the same jumper. You should be able to measure a connection between CA12 on the edge connector (PCB soldering side, pin R) and pins 21 of both sockets.chysn wrote:Now, I've got my 4K application on a 2732. When I plug the 2732 into the Block 3 socket, I only see the first 2K. Everything from $6800 - $6fff is invisible.
I think that this is because the A11 pin of a ROM chip is pin 18, while the A11 pin of the 2732 EPROM is pin 21. That would explain failure to access addresses with a high bit 11, like $6800.
It seems like pin 21 is unconnected on the 3K RAM PCB. Relying on both translucency and my meter, it seems to be isolated. So, I think that the solution here is to simply jump pin 18 to pin 21. Am I on the right track here?
I have a minor concern about the output enable pin, which is the 2732's pin 18. This will be pulled high at the high addresses, so my fix might not work unless I pull that pin low, which would involve diverting the PCB's line to pin 21 rather than simply jumping it...
It's no good idea to simply connect pins 18 and 21, as this will short CA11 and CA12.
Code: Select all
2332 2732
+----\/----+ +----\/----+
1 --|A7 Vcc|-- 24 1 --|A7 Vcc|-- 24
2 --|A6 A8|-- 23 2 --|A6 A8|-- 23
3 --|A5 A9|-- 22 3 --|A5 A9|-- 22
4 --|A4 /CE2|-- 21 4 --|A4 A11|-- 21
5 --|A3 /CE1|-- 20 5 --|A3 /OE|-- 20
6 --|A2 A10|-- 19 6 --|A2 A10|-- 19
7 --|A1 A11|-- 18 7 --|A1 /CS|-- 18
8 --|A0 D7|-- 17 8 --|A0 I/O8|-- 17
9 --|D0 D6|-- 16 9 --|I/O1 I/O7|-- 16
10 --|D1 D5|-- 15 10 --|I/O2 I/O6|-- 15
11 --|D2 D4|-- 14 11 --|I/O3 I/O5|-- 14
12 --|GND D3|-- 13 12 --|GND I/O4|-- 13
+----------+ +----------+To install the 2732 in the lower position (BLK1/BLK3) to appear at $6000, you would:
- open jumper "1" (already done, I suppose),
- close jumper "3" (also supposedly done),
- open pad 12 lower,
- open pad 11 upper,
- wire from pad 11 top left to pad 12 top right (connect CA11 to pin 21 of both sockets) and finally,
- when the 2732 is soldered in, add a (short) wire connecting pins 18 and 20.
You'll get the EPROM contents mirrored to $7000, but this should be of minor inconvenience.
Note this also connects pin 18 of the upper position to /BLK3 as select signal. If you want to use the upper position for anything else (particularly in BLK2/BLK5), you'll need to cut the trace connecting pins 18 of the two positions.
