The
single-sided and double-sided disks have the index hole (in the
jacket) in a different place. The single-sideed (single head) drive had
one indexc sensor, to line up with the hole in the single-sided disk. The
double-head drive (SA850, etc) has 2 index sensors, one to line up with
each hole. Of course when a particualr disk is inserted in a double-head
drive, only one index sensor will be active (the other will be blocked by
the opaque disk jacket), and by detecting which sensor is active, the
drive can signal to the conttoller whether it's a single or double sided
disk.
Q: Did ALL DS 8" drives also support SS?
I was once told that SOME DS drives had the DS index sensor, but no SS
index sensor!
Did ANY manufacturer try to cut corners THAT much?
Since the only system I ever used an 8" DSDD floppy was the PDP-11
using a DSD 880/30 with the (emulated) RX03 drive, I don't have sufficient
knowledge of what any other manufacturers did.
But, at least the floppy drive in the DSD 880/30 definitely had two sensors
with two detection circuits, as mentioned by Tony.
Jerome Fine