Fred Cisin wrote:
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. However, at one point,
I did notice another similar system by another manufacturer which also
supported the (emulated) RX03 (there MUST have been two sensors in
that drive as well) along with the ability to DMA to all 4 MBytes of
PDP-11 memory. The DSD 880/30 could DMA only to 256 KBytes
since it was limited, just like the DEC RX02, to 18-bit addresses.
But, at least that floppy drive in the DSD 880/30 definitely had two sensors
with two detection circuits, as mentioned by Tony. By placing a DPDT switch
in the two circuits which routed the signals normally on one position and
swapped the signals in the other position, it was possible to deceive the
floppy drive into assuming that a single sided floppy media (with the index
hole close to the center of the jacket) was to be used as a double sided
floppy media.
THAT one-time alteration to the drive supported using single sided media
as double sided media, although it was always necessary to perform an
LLF on both sides of the media (the DSD 880/30 supported the off-line
ability to perform an LLF, but it was all or nothing, of course) prior to
actual use. In addition, almost every manufacturer of single sided floppy
media could be expected to have used double sided rated media. The
only consistent exception that I found were DEC 8" floppies that had
been used for the distribution of software. In my experience, somehow
DEC managed to find a vendor who had used floppy media with the
second side degraded enough that it would not accept an LLF as a
double sided media, only as a single sided media. It was not a big
issue since by that time I had more 8" floppy media than I could ever
use, just strange that the 8" floppy media from DEC were the only ones
aside from the very few that had obviously been damaged in some manner.
Jerome Fine
