RD51 reduced write current signal?

[Paul Koning]

> On Feb 1, 2021, at 1:36 PM, Warner Losh <imp at bsdimp.com> wrote:
> 
> 
> 
> On Mon, Feb 1, 2021 at 10:54 AM Paul Koning via cctalk <cctalk at classiccmp.org> wrote:
> Looking at the DEC Pro documentation there's some ambiguity I'm trying to figure out.
> 
> The hard drive documentation talks about the "reduced write current" signal.  In one place it's explicitly described as relevant to the RD50 only.  But later on in the RD50/RD51 chapter the signal is described generally, without any indication that RD51 ignores it.
> 
> Does anyone know which is correct?  If RD51 also uses it, how does the right value get set?  What IS the right value, anyway?
> 
> The RD51 was the ST-412  while the RD50 was the ST-506. There's a combined OEM manual for these drives. http://www.bitsavers.org/pdf/seagate/ST412_OEMmanual_Apr82.pdf which has the answer. It's used in the ST406, but ignored in the ST-412. See Page 16 for the notes.
> 
> Based on that, I think it's pretty safe to say the RD50 needed it, but the RD51 and newer did not.

Great information, thanks!

I've now tracked down analogous manuals for all Pro drives except for the RD53, I don't know what kind of drive that is.

Learned some other interesting things.  The Pro technical manual says the write precomp register should be set to do precomp starting at cylinder 256 for the RD52.  That's correct for the Quantum model (8 head). It's wrong for the Atasi, which is the other "RD52" variant mentioned in the manual; its documentation says it should be cylinder 320.  And the CDC RD52, mentioned in internal documents (I don't know if it shipped) is a CDC Wren, which has a strange rule: precomp "recommended for all cylinders, required for cylinder > 128".

	paul