Re: Revision of VGC Chip

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

Bryan Parkoff <nospam@nospam.com> wrote:

>     I notice that there are revision of two or more VGC chip for Apple IIgs.
> Newer VGC chip corrects DHGR problem on older VGC chip.  What is this
> correction for?  I am curious.

My first IIgs was a relatively early ROM 00 with a faulty VGC. The VGC
got replaced, so I no longer have one.

I believe the fault had somewhat variable symptoms between different
machines, so mine might have been better or worse than another one.

My recollection is that there were two major display issues on mine:

1. In monochrome double hi-res mode, grey regions on the screen (e.g.
rows with alternating black and white pixels, such as the MouseDesk
desktop background) appeared pink instead. I think it might have been
some kind of alternating line thing: one line was grey, the next line
was pink. There were also pixels which flickered betweeen grey and pink.

2. In 80-column text mode, with certain combinations of foreground and
background colour (I specifically remember it happening with white text
on black background), some white pixels would also flicker to pink.
There were similar effects with different colour combinations.

I don't recall seeing any problems in 40-column mode, standard hi-res or
super hi-res modes. I didn't use lo-res, double lo-res or colour double
hi-res modes enough to pay attention to them.

-- 
David Empson
dempson@actrix.gen.nz