I haven't found any documents clarifying the drive select pins role with 3.5" drives. I have to disassemble a drive and see how the pins are routed on the daisy chain board.
Looking at the Mac Plus schematics, it looks like pin 16 is head select and its controlled by the 6522 VIA. Besides the change in what chip is controlling it, it should use the same logic across both machines.
On Macs, the Drive Select 1 signal from the IWM is wired to the internal floppy header and Drive Select 2 is wired to pin 17 of the external floppy connector. On the IIgs, Drive Select 1 signal from the IWM is wired to pin 17 and Drive Select 2 is wired to pin 9.
I highly suspect that the 3.5 Disk Enable pin has logic on the floppy drive end. If the pin goes high, the Apple 3.5" drive responds to the machine, if its low it ignores it since the instructions are bound for 5.25" drives. What makes matters confusing is how does the Apple 3.5" drive work on a Mac if this pin is grounded and unused?
Looking at the Mac Plus schematics, it looks like pin 16 is head select and its controlled by the 6522 VIA. Besides the change in what chip is controlling it, it should use the same logic across both machines.
On Macs, the Drive Select 1 signal from the IWM is wired to the internal floppy header and Drive Select 2 is wired to pin 17 of the external floppy connector. On the IIgs, Drive Select 1 signal from the IWM is wired to pin 17 and Drive Select 2 is wired to pin 9.
I highly suspect that the 3.5 Disk Enable pin has logic on the floppy drive end. If the pin goes high, the Apple 3.5" drive responds to the machine, if its low it ignores it since the instructions are bound for 5.25" drives. What makes matters confusing is how does the Apple 3.5" drive work on a Mac if this pin is grounded and unused?