RD51 reduced write current signal?

Warner Losh imp at bsdimp.com
Mon Feb 1 12:36:58 CST 2021

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.


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