NCR PC'S

[uhclem at trsvax.UUCP]

B>My question is: What is the difference between ST412 and St506?
B>Do these people know what they are talking about?

I recently came across an important difference between the ST506
and drives claiming to be ST412's.  I got a ST4096, a ST412 drive
and tried to use it with a ST506 controller for a non-clone system.
Would work great until it reached cylinder 512, then it would fault.

Problem was, in the ST506 drives pin 2 (believe that is right) was 
for Reduced Write Current (RWC).  The ST412 drives are smart enough
to either not need RWC at all or know internally when to do it themselves.
(Usually the host simply asserted that signal at the halfway mark.)
This freed up a line in the interface that is now used for Head Select 3
(zero based), allowing up to 16 heads.

On my system, only 8 heads were allowed anyway and I was willing to toss the
9th head because I got a 1024x9 drive cheaper than a 1024x8 of similar
performance.  Anyway, what would happen is that the host controller
would signal RWC at the halfway mark (cylinder 512) and the drive would
take that to mean, Select upper 8 heads.  Very quickly after that things
went a bit crazy.  Solution, bit of tape over pin two on the ST412 connector
and things are are running fine.

<My opinion, and not that of my employer, who stopped reading this note
 when the phrase "non-clone" appeared and started burning the phospher.>

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