Mega ST Drive access = bombs

[frank.lukas]

The computer should be work fine without all the exxos fixes.

See the Mega ST Service manual ...

"EXCEPTIONS (may occur at any tine)
El--E5 not used
E6 Autovector error. IPLO is grounded or 68000 is bad.
E7 Spurious interrupt. Bus error during exception processing.
Device interrupted, but did not provide interrupt vector.
EB Internal Exception (generated by 68000).
E9 Bad Instruction Fetch.
EA Address error. Tried to read an instruction from an odd
address or read or write word or long word at an odd
address. Usually this error is preceded by a bus emor
or bad instruction fetch.
.--EB Bus error. Generated internally by the 68OOO or
externally by GJ-ue. Usually caused by device not responding.
Displays the address of the device being accessed."

The E8 error can be all. Has nothing to do with the FDC I think. Maybe the TOS Rom´s are faulty ?

[exxos]

The computer should be work fine without all the exxos fixes.

*should* yes, in reality 30+ year old machines need fixing when they were barely working properly from new!

[Badwolf]

I shall delve deeper, if anyone has any ideas....
Soph

I'm not particularly familiar with the Mega, but AIUI blitter, floppy and ASCI port are the full set of DMA peripherals. It sounds like a DMA issue.

Floppy and ASCI both use the DMA chip on the Mega, but I don't think the blitter does, does it? If not, That would suggest the problem are with the signals rather than the chips.

I'd start with the /BGACK line. If the CPU were getting a flaky signal on that line, I'd expect to see this kind of behaviour as the CPU would wake up early from a DMA handover and start talking on the bus at the same time as the blitter or DMA chip.

If you have a two channel scope or a logic analyser, try checking the signals at pin 12 of the 68000 and, simultaneously, at pin 29 on the blitter. They should be identical throughout the cartridge blitter test.

Good luck,

BW.

[SophieRose]

Wow it's fixed :) This fault it seems was a mix of bad ROMs even though they passed the diag checksum test and a broken trace on the BGACK line to the blitter, Thank you all so much for the pointers I'm so glad I got this working, it has sentimental value.

Soph

One happy ST user :)

[exxos]

:bravo: :girldance: :girldance: :cheer: :cheer:

[rubber_jonnie]

:girldance: :girldance: :girldance:

Well done :)

[Badwolf]

Winner, winner, Mega ST dinner. :bravo: