Replacing the original ROMs with a 27C256
Back in time, during my studies, I replaced the level I ROM with 3x 2732, holding the level II ROM content.
Unfortunately one pin of that hardware extention broke off and my
But where do I get A13? My first try, to take it from the address bus directly, tied up the whole computer. Where do I get a buffered A13, a decoupled signal?
The solution is Z21. The address decoder devides A12 (A) and A13 (B) to four outputs. At Z21 pin 11 the inverted A13
With diodes I decoupled the outputs 9 and 10 to get the inverted A13 alone.
R61 pulls the input of Z74 to high level. The open collector outputs of Z21 delivers low signal only. This is not disturbed by the diodes.
The address decoder is Z21 and not Z31. I changed the text according to that.
The schematic and the picture are showing Z3. Pin 1 is in the upper left corner of the picture.
The diodes are standard 1N4148.
The IC Z37 is having a spare inverter. I disconnected pin 8 and pin 9 carefully. Pin 8 stays at ground level. As Z21 pin 11 is an open collector output, pin 9 gets a 4k7 pullup resistor. Now pin 10 is offering the orginal A13 signal.
If you left click on the picture and drawing, you get a larger version of it.
If someone is interested, one can get the PCB files as Gerber format.
Using a 27C128
I had a 27C128 with the only difference at pin 27. If I set A14 to 5V, with the jumper, I can use the same PCB with my 27C128 and that is what I am doing.