Date   : Thu, 14 Feb 1991 06:24:24 PST
From   : sprague.wbst311@xerox.com
Subject: 3.5inch HD disk formats: Why not 11*1024

It was suggested that I post the the "one chip solution" for generating Motor
On to a 5.25" HD drive, when making it act like an 8" drive.  Note that I have
*NOT* yet tried my one chip solution .... I will as soon as the NEW drives I
ordered arrive.  Grumble be VERY careful that you don't plug things in
backwards!!!  If something is wrong with this, PLEASE let me know!  I have been
known to make mistakes in the past.  :-)

Since most 8" drive motors are AC and *USUALLY* always spinning (though there
are hardware solutions for this), there is almost never a Motor On signal in
the 8" drive interface.  Some 5.25" drives allow you to set a jumper in order
to use the Drive Select signal for Motor On.  Other drives (Epson for example)
do not seem to do so.

In addition, there is only one drive select line for a 5.25" interface.  While
I am not sure on this, I believe IBM's twisted cable allows them to use one of
the unused Drive Select lines for a Motor On signal, thus they have a Drive
Select *AND* a Motor On signal for each drive.  Let's hope that the high
density drive manufacturers are not making drives specifically for IBM's
twisted cable .... that is, I am banking on the fact that you can still power
*BOTH* drives Motor On signal at the same time like you can for most 360k
drives.  If you can't, this is a serious hiccup in the one chip solution.

Last note that the drives use reverse logic, that is, active low signals.  For
drive select to be active, it must be low.  For the motor to turn on, Motor On
must be low.

A truth table for a two drive system:

 DS0 | DS1 || Motor On
-----+-----++---------
  0  |  0  ||    X     This should never happen.
  0  |  1  ||    0     Drive "A:" has been selected, Motor On.
  1  |  0  ||    0     Drive "B:" has been selected, Motor On.
  1  |  1  ||    1     Neither drive is active, Motor Off.

So if you make the "X" condition a logical "0", you have the truth table for an
AND gate (7408).  Of course, you can also use a NAND gate (7400) and take the
output into both inputs of another NAND gate (4 of them on a 7400), which will
make it act like an inverter.

For a system with more than two drives, use a 7411 for three drives or a 7421
for 4 drives.  Again, by using one of the spare gates in the chip as an
inverter, you could also use a 7410 or a 7420.

Remember, it is a good idea to put a capacitor between Vcc and ground.  A .1uF
ceramic disk should do.

In my case, on a Xerox 820-II, I have a 5 Volt power line in the interface
cable.  I have a small wire wrap perfboard cable adapter which ties the correct
lines in the 820s drive interface cable to the cable that goes to the 5.25"
drives.  This chip will be mounted in the center of the perfboard.  While power
is not a concern, I will probably use LS TTL chips, because I have them.

The CompuPro S-100 will not be so easy, assuming I need to do this.  For it, I
will have to pull 5 Volts from the floppy drive power supply itself.

                               ~ Mike  (Sprague.Wbst311@Xerox.Com)