A2SCSI-A03 - Apple II SCSI card clone

Submitted by GG on Fri, 07/22/2022 - 01:59
Project Status

A2SCSI FrontThe Apple II SCSI card from 1986 was one of the first interface cards to attach a Hard Disk Drive to the Apple II line of computers. A2SCSI is a clone of the last revision (rev.C) of the card with a few enhancements to improve compatibility with modern SCSI drives.


The schematics has been reverse engineered from our original Apple II SCSI card revision C. Some of the schematics documentation around pseudo DMA and IIgs mode is still incomplete. Our disassembly of the on board SCSI ROM has been published on github.

The differences from the previous A2SCSI-A02 are:
1- Removal of the 74LS240 chip used for the card SCSI ID. The ID has been fixed to 7 in the patched SCSI ROM and patched GSOS driver.
2- The discrete logic has been refactored to replace the hard to find open collector 74LS09 with a much more common 74LS08
3- Replaced PTH RPACKs with discrete SMD resistors

The card maps the 53C80 registers in the first 8 bytes of the slot I/O range ($C0x0-$C0xF where x=slot#+9). The remaining 8 addresses are used for the card control.

I/O Addr R/W Function
$C0x0 R Current SCSI Data Register
$C0x0 R Output Data Register
$C0x1 R/W Initiator Command Register
$C0x2 R/W Mode Select Register
$C0x3 R/W Target Command Register
$C0x4 R SCSI Bus Status
$C0x4 W Select Enable Register
$C0x5 R Bus and Status Register
$C0x6 R Input Data Register
$C0x7 R Reset Parity/Interrupts
$C0x9 - Not used
$C0xA W Memory Bank Select Register
$C0xB W Reset 5380 SCSI Chip
$C0xC - Not Used
$C0xD W PDMA Mode Enable
$C0xE R Read DRQ Status through bit 7
$C0xF - Not used

The card is also equipped with 16K of ROM and 8K of RAM. These are mapped in the $C800-$CFFF card memory using a banking scheme. The $C0xA bank register selects the which bank of RAM and ROM are mapped. RAM is always at $C800-$CBFF and ROM is at $CC00-$CFFF. The boot code in the first 256 bytes of ROM bank 0 is also mapped in the IOSEL space ($Cn00-$CnFF).

A2SCSI Driver for GSOS
ProDOS SCSI Utility Disk
Reference Documentation
Z53C80 Datasheet
Apple II SCSI Card Technical Reference Manual
Filed under