Greetings fellow nerds!
I am working on an FPGA-based peripheral card for Apple II (currently tested in a IIe). This board in its current state works as expected but I wanted to add some I/O control to turn on and off features, so I decided that the DEVICE SELECT signal was the best choice. However it seems like I cannot activate the DEVICE SELECT signal!
As I understand, the DEVICE SELECT signal is activated when referencing an address between C0n0 and C0nF, where n is the slot number + 8. My card is in slot 3, so that would mean address C0B0 to C0BF. I tried jumping into monitor (CALL -151) and then referencing C0B0, C0A0, C0C0, etc. Nothing seems to activate the DEVICE SELECT signal. I even modified the FPGA firmware to react to any falling edge on the DEVICE SELECT signal (i.e. regardless of the Apple clock signal), but that doesn't work either. Why?!
Also noted that the I/O SELECT signal has a similar behavior. The disk drive card seems to use both signals: http://www.applelogic.org/files/APPLEDIISCH.pdf
Any reason why? What is the difference?