Hanging a 2nd disk off a 3b2-310?

[Stephen J. Friedl]

In article <806 at ttrde.UUCP>, pfales at ttrde.UUCP (Peter Fales) writes:
> 
> The earliest models of the 3B2/300 (possibly 310?) did only support disks
> with eight heads or less.  The following simple modification will
> allow an early model 3B2/300 to use more than eight heads:
>
> [simple mod deleted]

     I can provide some additional information on the 3B2 hard
disk system that might help with some understanding here...

     The disk controller chip on the 3B2/{300,310,400} is the NEC
7261, and it has two drive-select lines plus three head-select
lines.  Fully decoded, this provides four drives of eight heads
each. The CDC Wren II drives that AT&T uses have *nine* heads, so
AT&T -- in the 310, at least -- decided to use one of the drive
select lines as a head select.  So, the controller can now talk
to two drives, each having sixteen heads.  You may imagine that
the driver software has to do some odd stuff to make this happen,
and you're not too far off.

 >>>>> the following is *total* speculation on my part. <<<<<

     I would guess that Peter's mentioned modification is doing
what is described above -- routing the controller's second
drive-select line to the interface's fourth head-select.  When
they came out with the /310 and the /400, this modification was
part of the standard system.  I would also speculate that this
mod won't work on an original /300 with original software,
because the drivers would not have known how to do the diddling
with the lines -- everything since (at least) SVR2.0.4 has it
standard in the IDISK driver.

     Steve


-- 
Stephen J. Friedl        3B2-kind-of-guy            friedl at vsi.com
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