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According to the information I found scattered around the web the following bits control the enhanced features of the Motorola MC6847T1 / XC80652P VDG chip .

Bit 4: switch to lowercase char set
Bit 5: switch to inverted screen
Bit 6: switch to light border

When I set bit 6 of $FF22 on my real CoCo 2 or the VCC CoCo 3 emulator the border does not seem to change. The other two bits which control lower case and the inverted screen work correctly.

Does anyone have a PDF copy of the data sheet for the MC6847T1 / XC80652P VDG chip?

I have discovered that setting bit 6 to switch to a light border doesn't work if you have inverted the screen. I was previously trying to switch to a light border after I had inverted the screen.

Setting bit 6 of $FF22 still doesn't switch to a light border on the VCC CoCo 3 emulator. Unfortunately I have never been lucky enough to be offered a suitable PAL CoCo 3 machine for sale, so I can't test this on real hardware. Is this a bug, or does this happen on a real CoCo 3?