RX01 Floppies

Jerome H. Fine jhfinedp3k at compsys.to
Fri Nov 21 21:48:02 CST 2014

 >Michael Thompson wrote:

>Where can I get RX01 SSSD 8" floppies?
>Used are fine.
I should have realized that the situation needs to be clarified:

(a)  As far as I know, (almost?) all soft sectored 8" floppy media
are formatted as either single-density (which is the IBM standard -
also known as the DEC RX01 option) or double-density.(which
is the additional DEC RX02 option).  In the latter case, the format
is essentially the same, but only the data portion of the sector is
double-density.  In addition, while a DEC RX01 drive can use only
single-density media, a DEC RX02 drive can use BOTH single-
density (SSSD) AND double-density (SSDD) media which means
that the DEC RX02 drive can read / write media from a DEC RX01
drive - but can boot an RX01 media only if the boot block is first
changed from DX to DY.

(b)  The IBM standard, the DEC RX01 and the DEC RX02 all
use ONLY single-sided media. The DY.MAC V04.00 device file
(the device driver for the DY.SYS file in RT-11) included (probably
never tested) code to support double-sided media which would have
used the DEC RX03 drive.  At least a few of the third party drives
and controllers for the Qbus PDP-11 which could read / write 8"
floppy media actually supported both single-density and double-density
media that were BOTH single-sided (SSSD, SSDD) and double-sided
(DSSD, DSDD).  However, at the moment, I can recall the name only
for the DSD 880/30 drive (Data Systems Design).

(c)  The standard for single-sided media was to place the index hole
very close to the center of the jacket for the floppy media.  For
double-sided media, the index hole was offset by about one inch.
HOWEVER, the DEC RX02 drive is always able to read / write
the first side of a double-sided media if there is an index hole in the
expected position for single-sided media.  The DSD 880/30 is able
to always read both sides of a double-sided 8" media regardless of
which position is used for the index hole, BUT normally the DY: device
driver will detect the position of the index hole and make only the
first side of the media available if the hole is in the single-sided 
When the index hole is in the double-sided position, the DY: device
driver will detect the position of the index hole and make both sides
of the media available.  HOWEVER, it is possible to modify the
function of the detection circuit (using a DPDT switch) and reverse
the functions of the index hole position - in that case, the RX03 drive
will support the use of both sides of the 8" media when the index hole
is in the single-sided 
(Rather handy to flip the switch on the RX03 drive rather than punch
a new pair of holes on each single-sided media!!!!!!!!!!!!!!!!!!!!!!!)

(d)  At least for the PDP-11 (and probably for all DEC hardware), it
was NOT possible to perform a Low Level Format (LLF) on 8"
floppy media using either the RX01 or the RX02 drive.  What was
possible with the RX02 drive was to switch the media from single-
density to double-density and from double-density to single-density
AFTER the initial LLF was done on the 8" floppy media.  Under RT-11,
the FORMAT command SOMETIMES seemed the be able to "rescue"
8" media with bad sectors using the FORMAT command when those
specific sectors on the media could no longer could be read or written.
I don't remember if that operation could also be done with the RX01
drive.  HOWEVER, the 8" media would have had to already been
through an LLF first (SSSD).

(e)  At least for the Data Systems Design 8" floppy drives, it was
possible to perform an LLF.  For the DSD 880/30 hardware which
was composed of 3 * RL02 and a single RX03, it was possible to
take the unit off-line and perform any of the 4 possible LLF operations:

Have I left out any information?  Are there still any questions?

Jerome Fine

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