28 Jul
2005
28 Jul
'05
12:27 p.m.
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