On 12/10/2010 03:28 PM, Tony Duell wrote:
> Now that's interesting. The knob which I
assume is the contrast (just below
> the brighness one) appears to do absolutelyt nothing?
>
That's supposed to be horizontal hold or something, I've never used it
either. In most cases if you have video issues, the pots on the video
board are far more useful for corrections.
I don;t think it relates to a physical knob... There
is a DAC on the I.O
board (see sheet 5 of the schemaitc I think) that is fed from the same
VIA port as the data lines to the Widget (the latter are buffered by a
'245 chip) -- I think the CIA and buffer are on sheet 2. I've not traced
the output of the DAC in detail, and some bits of schematic don't seem to
exist on the site I was looking at, like the backplane, but I thoguht the
schemaitc implied that DAC was some kind of software-controlled contrast
facility.
That's correct, the contrast is controlled via software. This allows
the Lisa to dim it's display all the way down to off when it's not in use.
Instead of just a hard off, it used to slowly dim its display down, so
it wouldn't create a harsh surprise for the user. :-)
The guys that designed this machine knew how to make it human friendly.
I don't know of any other machine of that era that did this, or even had
a software controlled power supply.