From: Jules Richardson <julesrichardsonuk at
yahoo.co.uk>
Philip Pemberton wrote:
Strange that the SA800 only provides an index
pulse once per revolution -
I guess it's intended for soft-sector discs.
I believe that's right; the 801 is the hard-sectored version.
Hi
No, the difference between the 800 and 801's was the clock
separator. It had nothing to do with hard sectored.
As I stated before, there was a - option that told if there
was an index/sector separator. In any case, this circuit can
be bypassed to get the raw index signal.
It would be best to bypass any sector/index separator and
just use the raw signal. Reading and writing would be easier
then.
As for the location of the first sector, that could vary, even
for hard sectored. Most often, each sector had a header with
the sector number. It was quite common to interleave sectors
to speed disk activity. In these cases, the order of the sectors
need not follow any particular order from the index mark.
To my knowledge, the Polymorphic, Heathkit H8/89 and the
NorthStar were of this type. The index mark was only used
during formatting and as a rotational indicator.
This is not always true. My Nicolet splits the 32 hard sectors
into two 16 sector marked chunks. Only the first of these
two vertual sectors has any header and that is a track
indicator value. The rest is just data.
Dwight
The 850 is a double-sided version of the 800, and the 851 is a double-sided
version of the 801.
The 85x docs talk about MFM data though, whereas I don't think that the 80x
docs do. I'm not sure if there's any good technical reason that the 80x
drives can't handle an MFM data stream, though. (I think there might be
some on-board "helper" circuitry for separating FM clock/data bits on at
least some of the 80x flavours - but I think this can be jumpered out so
that the controller does the stream encoding/decoding instead).
Given that there's no
indexing on the hub like there is on 3.5" drives, the SA800 would probably
output one index pulse every time an index hole was detected - if you used
soft-sector discs that would be one pulse per rev, or 32+1 pulses if you
used 32-sector hard-sector discs.
Aren't hard-sectored 8" floppies available in more than just 32
sectors/track flavours? It's no big deal if your circuitry is analysing
timing relationships to detect index pulse, but it obviously is if the
circuitry's relying on timing windows as the timing will be different for a
32-sector disk versus one with n sectors...
(I've only got the SA400 manual here, but the index/sector detection
circuitry uses different timing depending on the media used - I presume
because 5.25" hard sectored media was also available with different numbers
of sectors per track)
I have 32 sectored 8 inch disk. This was the most common but I believe
there was a 16 and a 10 sectored 8 inch as well.
To my knowledge, the 5-1/4 only came in 10 sectored. This was used
by Polymophic and NorthStar, as well as a few others.
As for using a 800 or 801 as a MFM, one could do it but the filters where
real close to the required frequency edge on the read. There was a - option
for this as well. If one can find the documents, there was several
capacitors
and resistors that one could modify to get the desire frequency responce.
I made this modification my self for a friend back in the 80's but that
is all I remember.
Dwight
cheers
Jules
--
"What progress. It's almost as good as taping it... on tapes which self
destruct in seven days."
- Bill Bailey on the BBC's "watch again" service
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