8" drive support on PC's - test-drb@ccmp.vtda.org

17 Mar 2005

On Mar 17 2005, 16:04, Randy McLaughlin wrote:

...
  There is confusion over pin 2 use at least in my head.
 The first use was
...
  with the SA800 interface for reduced write current. 
Today on the 34 pin
...
  connector that is what it is now used for even for
3.5" drives.
 Traditionally the write current line was used completely differently from
...
  how it is used today.  Traditionally it changed
according to where on the
...
  disk the data is being written, for today's drives
it is for the entire
...
  media. 
Yes, that's right.

...
  That is a side issue to my question about 765 TG43
controls.

 On a real 765 it generates the reduced write signal and outputs it on the
...
  same controller pin as the direction signal.

 Today the 765 is emulated via VHDL code and I am curious as to whether the
...
  TG43 logic was kept.  I can and will experiment on my
systems but I would
...
  like to know if others are familiar with the issue.

 Obviously all PC's that use a real 765/8272 does generate the signal and
...
  does bring it out to the 34 pin connector, it just
happened to be hidden on
...
  pin 18.  I question if it is still there? 
Well, I looked at the data sheet for the WD37C65, which used to be
fairly common, and the answer is "sort of".  Pin 39 is the one that
you'd be interested in.  It's an output labelled "/RWC,/RPM" and when
it's low, and the controller is in Base Mode, it says it causes a
reduced write current when bit density is increased towards the inner
tracks, being active when track > 28 (I think that refers to 40-track
drives, which would be 56 on an 80-track drive).  This is very similar
to to the TG43 signal, except for being relative to a different track,
but it's not on the same pin as /DIRECTION (Pin 31).  I can't help
wondering if "28" is a misprint for "23" or something like that, but
I
checked two separate printings of the data sheet and both said "28".

It also says this function changes when in AT/EISA mode (it then
becomes completely under program control, and appears to be intended to
be used to set the density and/or speed for the whole disk) :-(

However, if you have control over the FDC rather than letting the BIOS
or DOS do it for you, you could make it do what you wanted.

I also looked at the Intel 82078 FDC, which is a successor to the 8272,
in either a 64-pin QFP package or a 44-pin PLCC, either 5V or 3.3V,.
and it was an antecedent of many of the multi-I/O chips.  Unfortunately
it doesn't give enough detail.

Lastly, I looked at the NatSemi DP8473.  It has /DIRECTION on pin 4
(PLCC version), and /LC (low current) on pin 51.  Pin 51 just goes high
for HD media and low for SD/DD media (actually it goes low when 250kbps
or 300kbps data rates are set, instead of 500kbps).  So it doesn't look
like that does what you'd want, either.

-- 
Pete						Peter Turnbull
						Network Manager
						University of York