ROM replacements / 6128

[orzel]

What do you (or would you) use for ROM replacement on the CPC6128 ? I'd like to avoid "one-time" or old-school-uv-erasable stuff (like 27c256). 28c256(eeprom) are hard/expensive to source. I've got a couple at29c256 (atmel flash), but that doesn't work as is.

There seems to be an ambiguity for the pin 1 on those chips:
* on the official cpc schematics, it's "NC" on the 40025 and connected to +5v on the 40015
* on perlrun PCB, pin1 for 40025 is "GND" and pin1 for 40015 is +5V. That corresponds to the original PCB pictures (i assume that's what the layers User.BCu and User.FCu are)
* on my own 6128, pin1 for 40015 is indeed +5v but, but pin1 for 40025 seems to float. It's definitely not gnd or +5V

The AT29c256 probably wants 5V on pin1/VPP, but even then, the CPC wont start.

[McArti0]

You connect pin1 as you like and load the ROM into the upper or lower half of the 2XC256 as you like..

[orzel]

Well, as said, it just doesn't work. I've programmed the whole 32k, with  OS_6128.ROM in lower 16k and BASIC_1.1.ROM in upper 16k. The sha1sum of the whole corresponds to what people have posted on the net. The programmer didn't report any error, even after the mandatory re-reading/check.

But my main question was : which chip would you use ? I understand very well it should be a 2xc256, jedec-compatible stuff. But I have a hard time sourcing them in the usual suspects (digikey, mouser, aliexpress, lcsc).
 

[McArti0]

Is resistor R112 working?
What do you see on your screen?
Does the computer with a different ROM work?

[orzel]

r112 looks fine, and measures ~2.2k, which seems as it should.
The CPC works fine with the original ROM, both before, and after testing the at29.

The screen stays black.
I sometimes have a dark-greenish rectangle, but i'm almost sure it's a remnant of previous testing. It doesn't appear after a long off-state.

[McArti0]

Are you sure RESET on Z80 pin 26 is high? Try resetting it low on /BUSRESET

[orzel]

Are you sure RESET on Z80 pin 26 is high? Try resetting it low on /BUSRESET

I've checked and pin26 is high.
I tried connecting /busreset to gnd for a short time, nothing happened.

[McArti0]

Try WAIT signal to GND and turn on CPC.
On D0-D7 there should be byte &01 (first in ROM) and on A0-A15 address &0000.



You can also make a simple stepper from two or three diodes like here...

;schem: WAIT-----|>|---- xxx----M1
;            WAIT-----|>|---- xxx----IORQ
;            WAIT-----|>|---- xxx----WR
;            WAIT-----|>|---- xxx----RD

[orzel]

You can also make a simple stepper from two or three diodes like here...

Amazing !
You got centronics ports ? Or you did change your expansion port ? I have the usual pcb edge one, doesn't seem as easy to connect wires :-/

[McArti0]

you can hold the Z80 pins: WAIT-24, M1-27, WR-22, RD-21, IORQ-20