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Amiga Hardware Issues and discussion / Re: Help wanted: Amiga 1200, black screen, no power led
« Last post by Castellen on October 26, 2025, 08:34:01 PM »As you've probably found already on the rev 2B board, _ROMCS is permanently pulled low (active) by R959, and R597 is not fitted on production boards - the schematic is slightly confusing. You're correct that XU1 generates _ROMOE, which should normally be high, and only strobe low while the system is reading the $F80000 ROM area.
Given your fault description, it sounds as though _ROMOE might not be generated at the correct time (or is it incorrectly being asserted all of the time?); or if that's working correctly, then the CPU is possibly just executing garbage - in which case are you absolutely sure that the ROM contents is valid, and U6A/B are not exchanged in the sockets? And both ROMs are correctly located at the bottom of the 42-pin sockets.
Regarding the function of XU1, I copied the A1200 programmable logic data and source code off a 5.25" disk from Commodore development many years ago, the contents of which is on my webserver: http://amiga.serveftp.net/eprom_binaries/A1200_PAL/
I've no idea if this is the same as what was used in production or not, or if the .JED files will work on a production board or not. But since the CUPL source is there, you'll get an idea of what they're making the PLD do.
Since XU1 is a big unknown at this point, i.e. the original device is suspect, and you don't know if reprogramming it has actually fixed it or introduced more problems, I'd suggest getting a known good XU1 from somewhere and try that.
Edit - the PLD source I referenced above is from XU9 on the rev 1 A1200. Not sure what the differences are between this and XU1 on the rev 2 boards, I don't have the source for XU1. The XU9 source may be similar enough to give you an idea of how the logic is intended to operate.
Given your fault description, it sounds as though _ROMOE might not be generated at the correct time (or is it incorrectly being asserted all of the time?); or if that's working correctly, then the CPU is possibly just executing garbage - in which case are you absolutely sure that the ROM contents is valid, and U6A/B are not exchanged in the sockets? And both ROMs are correctly located at the bottom of the 42-pin sockets.
Regarding the function of XU1, I copied the A1200 programmable logic data and source code off a 5.25" disk from Commodore development many years ago, the contents of which is on my webserver: http://amiga.serveftp.net/eprom_binaries/A1200_PAL/
I've no idea if this is the same as what was used in production or not, or if the .JED files will work on a production board or not. But since the CUPL source is there, you'll get an idea of what they're making the PLD do.
Since XU1 is a big unknown at this point, i.e. the original device is suspect, and you don't know if reprogramming it has actually fixed it or introduced more problems, I'd suggest getting a known good XU1 from somewhere and try that.
Edit - the PLD source I referenced above is from XU9 on the rev 1 A1200. Not sure what the differences are between this and XU1 on the rev 2 boards, I don't have the source for XU1. The XU9 source may be similar enough to give you an idea of how the logic is intended to operate.
