31 Aug
2001
31 Aug
'01
8:52 a.m.
On Fri, 31 Aug 2001, Richard Erlacher wrote:
Well, if the written spec's can be believed, you
can't SEE the difference
between DD and SD media. That doesn't change the fact that they have different
spec's. Further, DS media are certified on BOTH sides, while the emulsion on
one side may be defective.
It's true that DS media have their index hole located differently than SS media.
The index timing is different, according to some drive makers, albeit not VERY
different. At least one mfg spec's the DS drives with 160 microsecond +/- 50
from the start of /Index to the data burst, while SS media are at 200. I've
never given much thought to this and how it affects things, but I am not
comfortable with the notion of switching, mechanically, between the two sensors.
Dick
Probably of little consequence, Dick, since his DSDD disks will not be
readable by anyone else unless the have performed the same modification
to a single-sided drive.
- don
----- Original Message -----
From: "Jerome Fine" <jhfine(a)idirect.com>
To: <classiccmp(a)classiccmp.org>
Sent: Friday, August 31, 2001 7:42 AM
Subject: Re: 8" Floppies
Absurdly
Obtuse wrote:
For those who asked, I need DSDD 8"
Soft-Sectored Disks.
Jerome Fine replies:
I don't know if this will help, but here goes.
I was using the DSD 880/30 - an RL02/RX02 compatible. It
holds a single 8" floppy drive which can use both SSDD and DSDD
diskettes (soft-sectored). I found that since I had a small number of
DSDD diskettes and the media seemed to be identical, the ONLY
difference that I could observe was that the index hole was in a
different location.
I wired a DPDT switch into the index hole detection circuit. In the left
hand position, it was wired to duplicate the existing circuit. When I
wanted to REVERSE the operation, I used the right hand position of
the switch. In the second position, the index hole for a SSDD diskette
caused the drive to route the signal from the sensor to the portion of
the circuit which normally detected the signal from the DSDD index
hole. Thus, the drive and the software "THOUGHT" that a DSDD
media was present and I was able to use a SSDD diskette as a DSDD
diskette without having to punch a second set of index holes. The
use of a DSDD diskette as a SSDD diskette is also possible if that
is useful - I never found it to be needed.
Naturally, this required that I do a LLF (Low Level Format) on both
sides of the diskette. This the DSD 888/30 is able to do all by itself
and that solved the problem. As for using the DSDD diskette with
DEC software, V4.00 of RT-11 had some code which showed how
to access the "second" side and use the floppy drive as an RX03.
I used that code as a starting point since I also wanted to allow the
DYX.SYS device driver in a 22-bit environment with V5.03 of
RT-11. Note that the DSD 880/30 and the DEC RX02 with an
M8029 can use only the first 1/4 MByte (18 bit addresses) when
a transfer is made to/from the DMA silo from/into a buffer in the
memory of a PDP-11. Software in the DYX.SYS device driver
is able to use all 4 MBytes of the PDP-11 physical address space
in a manner totally transparent to the user of the device driver.
I realize that Sridhar is not using RT-11, but I just thought I might
mention the above for those of you that do.
Sincerely yours,
Jerome Fine