8" drive support on PC's

Dwight K. Elvey dwight.elvey at amd.com
Thu Mar 17 14:06:08 CST 2005

>From: "Randy McLaughlin" <cctalk at randy482.com>
>On the Nec 765 (Intel 8272) the direction pin is also use for reduced write 
>current (TG43).
>When the step line pulses the direction pin controls the direction.  All 
>other times it is high if the track counter is 43 or less, low if the track 
>counter is greater than 43.
>The current disk controllers are based on the 765.
>Does anyone know if the direction pin does the same on the newer clone 
>chips?  I have started looking at the datasheets but so far I have not found 
>any reference to that function on "modern" parts.
>If it's true then the 34 to 50 pin cable I use (and others) can be made to 
>include TG43 without using an adapter like FDADAP.
>It may turn out some do and some don't but since everyone is basically using 
>the same licensed core I would expect all newer chips to act the same.  The 
>question is was TG43 support kept from the original 765?

Hi Randy
 See web page:

 You'll note that it mentions that a normal PC does not
supply the TG43 signal. Their adapter includes what looks like
a uP to keep track of the current track to generate this
signal for writing to 8 inch disk.
 If you were doing software to write 8 inch disk, I would guess that
you could double up this function on the step direction wire.
There is otherwise no signal to do this for you.
 Don Maslin's connection was primarily to be used for reading
9 inch disk. It may also be assuming that the drive has a built
in track 43 sensor.
 I think this is were the confusion over pin 2 comes in. This
is usually uses as a speed control on PC's but historically
it was a write current control.

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