Arlen
2014-01-03 02:56:21 UTC
Another N8-2312 problem I finally debugged late last night was an
unresponsive on-board PPIDE interface.
I could not get the PPIDE drive to work in RomWBW 2.5.2. After checking
all the upstream hardware, I could not understand why the 8255's chip
select was staying stubbornly inactive.
But when I ran my own tiny test loop I could see the chip select waggling
properly on the scope. At this point I began to suspect that the BIOS code,
unlike my 5-byte test loop,wasn't actually talking to the 8255 parallel
port. In other words, a software glitch, not hardware (for a change :)
So I went back to config_n8_2312.asm in the RomWBW 2.5.2 distribution and
discovered a weirdness in the port number used by the software to set up
the PPIDE code. In the file I changed
PPIDEIOB .EQU $60 ; PPIDE IOBASE
to
PPIDEIOB .EQU $80 ; PPIDE IOBASE
which matches the 8255's base I/O port number of $80 -- as indicated by the
address decoding circuit on the N8-2312 schematic. After re-building the
ROM with this one change, PPIDE now works perfectly with both hard drives
and CF adapters.
Has anyone else encountered this? It had me running in circles for a while.
Arlen Michaels
unresponsive on-board PPIDE interface.
I could not get the PPIDE drive to work in RomWBW 2.5.2. After checking
all the upstream hardware, I could not understand why the 8255's chip
select was staying stubbornly inactive.
But when I ran my own tiny test loop I could see the chip select waggling
properly on the scope. At this point I began to suspect that the BIOS code,
unlike my 5-byte test loop,wasn't actually talking to the 8255 parallel
port. In other words, a software glitch, not hardware (for a change :)
So I went back to config_n8_2312.asm in the RomWBW 2.5.2 distribution and
discovered a weirdness in the port number used by the software to set up
the PPIDE code. In the file I changed
PPIDEIOB .EQU $60 ; PPIDE IOBASE
to
PPIDEIOB .EQU $80 ; PPIDE IOBASE
which matches the 8255's base I/O port number of $80 -- as indicated by the
address decoding circuit on the N8-2312 schematic. After re-building the
ROM with this one change, PPIDE now works perfectly with both hard drives
and CF adapters.
Has anyone else encountered this? It had me running in circles for a while.
Arlen Michaels
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