13 Aug
2015
13 Aug
'15
2:22 a.m.
Chuck Guzis wrote:
On 08/12/2015 11:12 AM, Jerome H. Fine wrote:
If I remember correctly, the DSD 880/30 provided the DY(X).SYS
device driver in RT-11 with a bit that specified if the index hole was
for a SS or a DS floppy media. If the media was SS, DY(X).SYS
would select ONLY side 1 for any operation. If the media was DS,
then side 2 was also allowed. So the index hole for DS media was
used to determine if sectors on side 2 should be allowed to be read
or written. The signal for DS media did not actually change what
happened except to support (or not support) the use of side 2.
One rather interesting aspect was that the DEC code did not alternate
between side 1 and side 2 as the head advanced. All of side 1 was
processed first followed by all of side 2. As a result, when a floppy
media with a SS index hole was written on both side 1 and side 2, a
DEC RX02 drive could still read all of side 1.
On other hardware which also supported the same electrical and LLF
setup, the DS 8" floppy media device driver alternated between side 1
and side 2. If a floppy media from that system was using in a DEC
RX02, only the first few sectors on side 1 could be successfully
read.
If anyone actually wants to know, I can look up the code. It must
have been more than 15 years ago when the device driver was
enhanced.
Jerome Fine
I forgot to mention one item. Since I had VERY
few Double Sided
media with the index hole offset that extra 1/2" from the Single Sided
index hole - and I have many RX02 compatible floppy media which
are Single Sided with the index hole in the wrong place for being a
Double Sided media - I added a DPDT switch to the sense circuit
of the DSD 880/30 drive. In the normal position, the Single Sided
floppy media are detected as Single Sided floppy media. In the
opposite position, a Single Sided floppy media is detected as Double
Sided and the DSD 880/30 is then able to read / write both sides
of the floppy media. Since the DSD 880/30 is also able to perform
a LLF (Low Level Format), I am able to use all of the RX02 media
as Double Sided WITHOUT the inconvenience of having to punch
the extra index hole.
My drives are configured to take either index signal--so it doesn't
matter if the media is coded as SS or DS. Many floppy drives can do this.