7 May
2007
7 May
'07
7:32 a.m.
Jules Richardson wrote:
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).
IIRC, the SA800 provides:
/RDDATA: Raw data from the head - one pulse if there's a flux transition
/SEPDATA: Separated data - no clock pulses
/SEPCLK: Separated clock - no data pulses
So you'd just connect RD DATA up to the controller and let it do its own
thing. If you have a weird drive that only has SEPDATA and SEPCLK, you AND
them together to get the data and clock pulses together (they're active-low
remember - 1&1=1, everything else=0). Then just feed the AND gate output into
the RDDATA input on the controller.
Aren't hard-sectored 8" floppies available in
more than just 32
sectors/track flavours?
Yes, that's why I said "for 32-sector discs". IIRC you can get 8, 16 and
32-sector discs.
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...
Yes, and the timing window is variable. Usually you'd have it set to
0.75Tindex (where Tindex = the time between two normal index pulses).
I think I've posted the pseudocode before - but basically, the pulse detector
looks for two index pulses that came less than Tthreshold after the previous
pulse. Set the threshold to 0.75Tindex and you can detect the "half way"
pulse, even if it's up to 0.25Tindex late, or if the next sector index pulse
is up to 0.25Tindex early.
This is one of those things that's easier to explain with a couple of pictures
(or a whiteboard)...
(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)
On the SA800 you can change a few jumpers (more specifically, you cut a track
and bridge one of two others) to get 8, 16 or 32 sectors per track. The
manual's on Bitsavers if you're interested...
--
Phil. | (\_/) This is Bunny. Copy and paste Bunny
classiccmp at philpem.me.uk | (='.'=) into your signature to help him gain
http://www.philpem.me.uk/ | (")_(") world domination.