TG43 signal and the Nec 765

[Allison]

>
>Subject: TG43 signal and the Nec 765
>   From: Dave Dunfield <dave04a at dunfield.com>
>   Date: Thu, 28 Jul 2005 06:32:58 -0400
>     To: cctalk at classiccmp.org
>
>Hi Guys,
>
>I'm using a relatively modern 1/2 height 8" drive for my PC
>interface, which does not seem to care about TG43 - I left
>it unconnected, and it appears to be writing images just fine.
>
>It would however seem to be a good idea to support it for
>older drives and drives that need it...
>
>Question:
>
>According to the NEC 765 datasheets, pin 38 is LC/DIR, it
>indicates "Low Current" (TG43) during writes to inner tracks,
>and "direction" during STEP operations.
>
>It would seem that you might be able to simply stuff this
>signal into both pins 34 (Direction) and 2 (TG43) of the 8"
>interface, since the drive should not care about DIRECTION
>when STEP is not active, and it should not care about TG43
>when WRITE  is not active, and the two operations are mutually
>exclusive. However none of the information and cable diagrams
>I've seen indicate such a connection, suggesting that this
>might not work.


Some drives don't like that.

>
>Q: Can you simply drive both signals from the one source, or
>   do TG43 and STEP need to be externally demultiplexed?
>

The standard has to bee demultiplex it.  The need and use of 
TG43 is for writing.

>As an alternative, I think I will also provide an option to
>generate TG43 via a parallel port output.
>
>Q: Should TG43 be asserted any time the drive is positioned
>   to a cylinder >43, or should it be asserted only during
>   WRITE operations when at a cylinder >43?

TG43 is mostly a relic signal of older 8" drives.  Not all
of them used it.  Also it's supposed to be meaningless on read.

I may add that more than a few designers also used demuxed TG43
to switch write precompensation in DD modes on 8" drives for 
better data reliability. 


Is this for a Cpro Disk-1(A) or something from the ground up?


Allison