Apple II Technical Notes Developer Technical Support
Revised by: Matt Deatherage November 1988
Written by: Mike Askins May 1985
This Technical Note describes the signature bytes of the UniDisk 3.5.
The signature bytes for the UniDisk 3.5 are the same as those for any SmartPort device:
$Cn01 = $20
$Cn03 = $00 ProDOS Block Device
$Cn05 = $03
$Cn07 = $00 SmartPort Interface
where n is the slot number of the device.
When searching the slots for a UniDisk 3.5 it is very important to check all the signature bytes, since there are other peripherals with similar ID bytes. Once you find a SmartPort card (or port), you should do a SmartPort STATUS call to determine which devices are connected to it. Any number of different devices could match the SmartPort ID bytes, so trying to identify a device without making a SmartPort STATUS call is very likely to produce inaccurate results.
Why the UniDisk 3.5 Does Not Auto-Boot on Older Machines
If you look carefully, you will notice that the older (][, ][+ and unenhanced IIe) Autostart Monitor will not boot any SmartPort device because the ID byte at $Cn07 = $00 instead of $3C (like the old Disk II). If Apple had left the ID bytes the same as the Disk II, then older versions of Apple II Pascal (1.2 and earlier) would assume that the drive was a Disk II.
Where This Leaves You
The enhanced IIe ROMs, as well as the UniDisk 3.5 IIc ROMs and later (which you have if you are using a UniDisk 3.5 on a IIc) check only the first three ID bytes. This check means that they will not only auto-boot the UniDisk 3.5, but any SmartPort or ProDOS block device. On an older machine, you can boot one of these devices by typing PR#n from AppleSoft or Cn00G from the Monitor.
Further Reference