[Coco] [ColorComputer] VDG Detection

[Michael Pittsley]

I ran on Coco3 stock and CoCo 3 with Gime-x both returned 00263
Then I ran on CoCo 1 then CoCo 2 without T1, then with T1 and then with CoCoVGA all returned 00262.


Michael Pittsley
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From: ColorComputer at groups.io <ColorComputer at groups.io> on behalf of Alex Evans via groups.io <varmfskii=gmail.com at groups.io>
Sent: Sunday, December 21, 2025 10:05 AM
To: colorcomputer at groups.io <colorcomputer at groups.io>; CoCoList for Color Computer Enthusiasts <coco at maltedmedia.com>
Subject: [ColorComputer] VDG Detection

In the ROM Diagnostics thread, there is the question of whether or not it is possible to detect through software whether an MC6847 or an MC6847T1 was present. An idea was presented that there may be a difference in the bumper of lines per frame. I came up with some code to test this possibility, but I only have access to a machine with an MC6847. I have managed to get a null result using xroar (in both cases 262 for NTSC and 312 for PAL). If there's anyone is willing to post results from actual hardware to confirm this null result is accurate, it would be greatly appreciated.

Attached you will find the source code for the test as well as DECB and DragonDOS binaries.

Thank you.
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