How easy is it to damage an IBM CGA display?

der Mouse mouse at Rodents.Montreal.QC.CA
Thu Nov 29 17:26:21 CST 2007

> With regards to the 'killer poke' somebofy mentioned, it's impossible
> to bring the drive frequency down to 0, the signal is capcitor and
> transofrmer coupled.

Well, I suppose this depends on whether you consider the absence of a
signal to be a frequency of 0. :-)

> But it's certainly possible to reprogram the 6845 on the MDA board to
> drive the monitor way off-frequency, at which point the HOT is likely
> to fail.

Sticking a 0 in the register doesn't necessarily mean setting the
frequency to 0; I would expect that the register is, loosely, the
reciprocal of the frequency (much the way BRGs in UARTs often require
the baud rate to be set by specifying a value by which some clock is
divided to get the baud clock).  Setting it to zero would likely give
you as high a frequency as the electronics can generate, under which
circumstances it's no surprise the magic smoke gets let out given what
you said about the horizontal stage of the MDA monitor.

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