Re: IIGS hardware questions

[dempson@actrix.gen.nz (David Empson)]

In article <CrFDr8.8zz@crash.cts.com>,
Randy Shackelford <shack@crash.cts.com> wrote:
> In article <2tl97u$s22@nyx10.cs.du.edu>,
> Chris Deschu <cdeschu@nyx10.cs.du.edu> wrote:
> 
> >	While I was looking over my logic board for the above
> >described problem, I saw something I'd never noticed before.
> >Underneath the power supply, about an inch and a half behind the
> >VGC chip, at coordinates C-4 on the logic board, is a pair of pins
> >sticking out, like for a jumper block. They are labeled "W1".
[snip]
> >Anyone have any idea what they are for?
> 
> My ROM 01's have those as well. I always wondered what they do but no
> one in my circles knew.

It is a jumper pair which is intended to be shorted to enable an
option.  Whether or not this is a good idea is another matter.

It controls the polarity of the 7M clock signal being fed into the
Sound GLU and Ensoniq.  If the jumper is disconnected, the chips are
clocked using an inverted copy of 7M.  If the jumper is connected, 7M
is passed through noninverted.

I have no idea why you might want to change this, or what effect it
would have.

The circuit diagram calls it "SOUNDJUMP", which doesn't tell us much.
It isn't mentioned in the main body of the hardware reference.


Now for the really puzzling part: the 1MB IIgs (ROM 3) doesn't have
this jumper.  7M is connected directly to the Sound GLU and Ensoniq.
This is the opposite polarity to the default setting on the ROM 1
(jumper not connected).
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand