Orange, you talking about the following?: (forgot who wrote it but I cut & pasted it into an email to myself).
I had burned an EPROM (AMD AM27C400) with Kickstart 3.1 image for my A500+ but when I replaced the original ROM I got a red screen with flashing power LED. Very similar situation to the one described by orange.
The kickstart was the 40.063 version intended for A500/A600/A2000. I had verified the CRC-32 checksum before burning it. The image wasn't corrupted or modified. I had checked it with the SKick. Everything had worked fine.
With the CPU replaced by the turbo board (030 CPU) the kickstart booted correctly (I'm sure it wasn't a 020+ image because it had been soft-kicked earlier and had worked with the stock 68000 CPU, the EPROM was also verified if it contained the programmed data).
Strangely when my Action Replay III card was connected the kickstart booted correctly but required disabling of the cartridge (actually it was A2000 version of the Action Replay III connected through a bus board). I have burned another EPROM with the same data but the story repeats.
Then I started to suspect that something strange must be happening on the CPU bus. I noticed that the Action Replay card had pull-down resistros on the data bus but my rev 8A motherboard was missing data bus pull-ups. I populated the missing resistor packs (RP108 and RP110) but nothing happened. I tried both pull up and pull down configurations without success.
Then I started to measure signals at the CPU socket with my scope and found out that when I touched with a probe some low order address lines the power LED flashing stopped. The system hung. It was deterministic behaviour.
I realized that also low order address lines (A16-A1) were missing their pull-up resistor packs (RP106 and RP107) so I removed data lines pull-ups and moved them to low order address lines. Since that moment everything works flawlessly.
You can locate aforementioned missing resistor packs next to the CPU socket on the A500+ rev 8A motherboard.
I don't really know why it initially worked with the original mask ROM but didn't with the CMOS EPROM. One possible explanation is an input leakage current of the ROM/EPROM chip. The 68000 CPU was manufactured using NMOS technology with relatively weak sourcing current of high output state (0.4mA). According to the datasheet of the ROM chip its input leakage current is ten times larger than the one of the EPROM.
I hope someone will find my description helpful.
just to see if i understood this. you installed two resistor packs at RP106 and RP107 to make the "AMD AM27C400" eprom working. what are the types/specs of these resistor packs?
Yes, I installed two resistor packs (aka resistor networks or resistor ladders) as specified in the rev 8A schematics: 9 x 4k7 in SIP-10 package.
One of the suitable components is BOURNS 4610X-101-472LF. You can buy them at Farnell.
