4 Mar
2007
4 Mar
'07
1:36 p.m.
On Sun, 4 Mar 2007, Dave Dunfield wrote:
10x512byte sectors can be twitchy on some PC setups.
It
pushes the limits of the drive and formatting more than
the standard PC formats do. Most systems which use 10x512
have a more flexible FDC than the PC does.
. . .
- Marginal disk format.
The PC uses a fairly inflexible floppy disk controller chip, which
cannot handle data as close to the index pulse as the controllers
in some non-PC systems. These system may pack extra data onto the
disk (for example, 10x512byte sectors resulting in 400k disks).
Some PC controllers can handle this better than others.
You may also be able to create the disk by slowing the drive
down by 5-10rpm, allowing extra time for the data to be written
before the next index pulse occurs. See the ImageDisk notes.
For dealing with such marginal formats, (not necessarily with ImageDisk):
When reading or writing preformatted disks, (BUT NOT FORMATING - It will
NOT work for formatting, don't even ask), some of the sectors that "can't
be found" may be findable by "masking off" the index pulse.
As Dave said, the 765 type FDC used in the PC performs a "partial reset"
whenever it sees the index pulse, and is not ready to read or write after
the index pulse as soon as a WD 179x controller is.
If you "mask off" the index pulse, then the FDC never sees the index
pulse, and can read sectors that are "right up against" the index.
By "mask off", I mean any method of intercepting and eliminating that
pulse reaching the FDC. That can include cutting that wire of the cable
and splicing in a switch. Or even covering the index hole of the disk
with opague tape. Do NOT use tape on the disk unless you are completely
willing and able to open the drive and fish out the tape if it falls off!
But, be aware that there are some drives, including some of the Teac 55's
that rely on the index pulse reaching the drive circuitry to know whether
the drive is ready. Tandon TM100's, and Shugart/Panasonic 4x5 drives
don't have that problem.
In addition, the software that comes with the Central Point option board
relies on the index pulse to work.
Some of the functions of Dave's software (such as checking drive speed)
will also probably need the pulse.
Therefore, your method for masking off the pulse must be temporary and/or
easily reversible.
Once more: the index pulse "band-aid" is only for reading and writing disk
sectors. It is not ever for use during formating.
--
Grumpy Ol' Fred cisin at xenosoft.com