Late-model Hercules card

tony duell ard at
Mon Apr 20 07:50:27 CDT 2015

> display? Given the date I just want to check that it's not one of their CGA
> or EGA compatibles.

You don't need to trace much to figure out what it is (assuming it's standard [1]). 

First check if pins 1 and 2 of the DE9 are solidly grounded. If pin 1 isn't, it's something
strange. If pin 2 isn't (but pin 1 is), it's likely to be EGA [2]. If they are both grounded, then
it's likely to be MDA (hercules) or CGA.

Now check if pins 3,4,5 are connected or floating. The diode check range on a DMM is 
useful here, try it both ways round between the pins and ground. If all are floating, it's MDA, 
if they are all driven, it's CGA.

Now pin 7 is an oddity. On the original IBM CGA card it was not connected. On a lot of clones
it's a composite video output,

[1] I have a hercules-compatible card here that drives a monitor other than at MDA frequencies.
There is a PAL between the data bus and the 6845 that intercepts writes to some of the 6845 registers
and changes them so that the actual values written to the chip are not what you'd expect. Made by AES
and used in one of their 80286-based machines IIRC 

[2] The original IBM EGA card had a jumper link to either ground pin 2 (if you were using it with 
an MDA or CGA monitor) or connect it to the appropriate colour output, Some clones have this,
others don't, but it is easy to spot.


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