If a modern display lacks the Monitor Sense pin needed to enable 31kHz operation, MCGA comes up in 15kHz, something many displays do not support.
https://ardent-tool.com/docs/pdf/PS2_Model_25_Technical_Reference_Jun87.pdf#page=49
Page 65 physical
Display Sense Register, Index 12: This register contains the sensed
levels of the monitor sense 1 and 0 signals at pins 13 and 14 of the
display connector [HDD15]. This information is used by BIOS to properly initialize all video registers to match the display.
Figure 1-41 Monitor Sense Bits
MS1 MS0 Type of Display Attached
0 0 Reserved
0 1 Analog Monochrome Display
1 0 Analog Color Display
1 1 No Display Attached
0 = Shunt
1 = Open
Also, the internal 14 pin header does not match up with the HDD15.
From the Model 30 schematics,
Funct "14 pin" HDD15 72X8300 Funct
MS1 12 13 44 V Ext
MS0 11 14 58 H Ext
So, if my years of advanced hedonism haven't totally destroyed too many
brain cells, we leave MS1 open and tie MS0 to GND. This should provide
the MCGA controller with the correct MS pins to enable the system to
come up expecting an analog color display.
You could do the shunt inside the case, permanently setting your system
to analog color. No need to open your display or add a dongle...
YMMV.
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