compucolor

[Tony Duell]

> The compucolor also another failure mode; it is a rare example of how a 
> software bug can cause physical damage.  The compucolor logic is stuffed 
> inside a normal TV case (there are even empty holes for 
> tint/brightness/contrast along the bottom and a convenient carrying 
> handle on the top) with the board driving baseband video to the normal 
> TV logic.  The CRT timing is generated by a chip that has timing 
> registers loaded from an on-chip ROM.  However, the CPU is free to write 
> new values to the timing registers, and if the wrong value is entered, 
> it causes something dire to happen in the TV section's logic (eg, if the 
> horizontal frequency is too low then the flyback transformer's impedence 
> drops and draws a lot of current and smokes).

This is not as rare as you might think. Several machines -- the IBM PC 
MDA card + 5151 monitor, for exampe -- suffer from this. There is no 
horizontal oscillator chip in the monitor circuit, the line output stage 
(horizontal output stage) is driven from the signal produced by a 
software-contorlled IC (in the case of the MDA card, it's a 6845). 
Mis-programming that does just what you said....

-tony