Added double-sided support to Quay 900 floppy formatter

I’ve added support for formatting double-sided eight-inch floppy disks to the Quay 900 floppy formatter program I wrote as part of my RetroChallenge 2014/07 project. The source code is still provided under the GPL v3.0 license.

I still haven’t worked out how to use interrupts and DMA, so the code is still polled.

I tried to have it sense whether the media is single-sided or double-sided, which is possible with 8-inch floppies because the index position is different, and double-sided drives usually report detecting a double-sided disk by driving the TWOSIDED/ line of the interface (pin 10 on Shugart-compatible drives) active.  The NEC uPD765, Intel 8272, and equivalent FDCs sense that signal during seek commands and provide it in status word 3.  However, I’m finding that the bit reads as zero regardless of whether I use a single-sided or double-sided medium.  Some drives try to disallow access to the second side of a single-sided medium, but the particular CDC 9406 drives in this Quay 900 do not.

I stuck a logic probe on the TWOSIDED/ signal, and found that with these particular drives it is always either inactive, or toggling at a little under 1 kHz. I have no idea what that’s all about, but it clearly means that autodetection of media sidedness is not going to work.

Update: the toggling has nothing to do with the drive, but is caused by the uPD765/8272 feature of automatically scanning four disk drives to monitor their status. I’ll have to trace out the signal on the Quay board to figure out why it apparently isn’t getting to the FDC correctly.

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