I have an Apple "Disk II interface card" from 1978. For some reason, it does not work in my Apple II+. If I insert it, the Apple won't boot at all. Seems like data bus is corrupted.
Funny thing is the card is probably OK. I tested it in an Apple IIe before. I discovered that the reason for data bus corruption in the II+ is that a PROM IC on the interface card is sucking current from the data bus, when it (the PROM) has no power supply (shut off when device is not selected). The PROM seems fine in all other aspects.
I'm starting to believe this card just is not designed for the Apple II+. Is this correct?
Woz designed it for the non-plussed Apple ][, actually. But (if it's good) it'll work fine in a ][+. Do you get a beep on startup? Anything on the screen? How does it differ when the card isn't inserted? What do you normally have for a disk controller if this isn't the one you use?
It may not have the auto start rom chip in it. May have to do a PR#6 to boot the machine. I seen to remember about the auto staet chip.
Are you using a 13 sector boot floppy or a 16 sector boot floppy? The original Disk ][ (and controller card?) could only boot and use 13 sector disks. You might need/want to upgrade the PROMs (on the disk or the controller card, or both) to boot and use 16 sector disks.
The Apple II+ works fine without the disk interface card. When I plug the card in, the Apple does NOT beep, and screen is ful of garbage characters.
Looking at the address bus, it seems like the Apple runs a few instructions as normal, then reads/writes at sth like 0xFFFE, and then freezes at 0xFFFF. BTW anyone knows what happens when the 6502 reaches execution att 0xFFFF? Does it roll-over to 0, or halts? It would help to know.
The Apple II+ seems to have the autostart ROM. Normally I get the "]" prompt (actually, it is a swedish character set, so I get "å" for prompt - quite funny).
Part of the problem could be that I had to replace the (broken) 8T28 data bus driver at H10 with an 74HC245. The '245 might not be able to supply all the current that the disk interface card sinks through the data bus when not active. As I said, the card seems to sink quite some current thorugh the data bus even when not active, since one of the PROMs does _not_ leave its pins in a high-impedance state when turned off.
Okay, it doesn't sound like the problem is the 13 sector thing I posted.
Here is a little test program to test the "roll-over" behaviour of your 6502, you need the 16K RAM card installed...
]HOME:CALL-151 ;clear screen and go into the monitor
*C081 C081 ;write enable RAM card, read from ROM
*D000<D000.FFFFM ;copy ROM to RAM
*C083 C083 ;write enable RAM card, read from RAM
*FFFE:A9 0 ; FFFE- LDA #$0
*0:8D 0 4 ; 0000- STA $400
*3:60 ; 0003- RTS
*FFFEG ;run this program, see what happens
If your 6502 "rolls over", this program puts an inverted @ in the lefthand corner and returns to the monitor gracefully.