5150/5151 Video Card??

tony duell ard at p850ug1.demon.co.uk
Mon Aug 15 12:26:01 CDT 2016

> Does THIS particular board handle BOTH CGA and MDA frequencies?
> If not, telling him to connect his 5151 or "hercules" monitor, is going to
> cost him a monitor.
> If so, telling him to connect his 5151 or "hercules" monitor without the
> right switch settings, is going to cost him a monitor.
> 5151 and "hercules" (MDA) monitors do not like wrong frequencies.
> SERIOUSLY: connecting a 5151 or "hercules" monitor to the wrong signals
> can damage it.

That depends on the MDA/hercules monitor.

The IBM5151 does not have a horizontal oscillator. It will attempt to run
at whatever frequency you give it. Give it the wrong horizontal frequency
and the horizontak output stage will not like it very much.

Most other MDA type monitors were more conventional with an intenral
horizontal oscillator that attempts to lock to the incoming sync signal. In
any reasonable one, if the incoming frequency in way off, the oscilllator
will not lock to it, it will remain running at a frequncy close enough to 
normal that there will be no damage. The common Zenith MDA monitor,
the one with that well-known power supply [1] is like that,

[1] The one that combines the efficiency of a linear with the reliability
of a switcher. And which malfunctions if you replace the green power-on
LED with a red one. Ask me if you have seriously never come across 

In any case it rarely costs you the monitor if you get it wrong. A horizontal
output transistor, sure. But they are not that expensive.


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