vwestlife
Veteran Member
I just installed a Sound Blaster Pro card in my Tandy 1000SL and I was dismayed to discover that the GLX MOD file player doesn't support CGA. I could hear the music playing just fine, but all I got was a blank screen. But before I installed a VGA card, I wondered what would happen if I switched the Tandy's onboard video into MDA/Hercules mode by pressing Ctrl-Alt-Shift-V, while still using my Tandy CM-11 CGA RGB monitor. Mind you, it's not supposed to work, because MDA outputs its video on pin 7, while CGA monitors ignore this pin and use pins 3, 4, and 5 for Red, Green, and Blue, respectively.
But to my surprise, after adjusting the monitor's controls, I got a steady, clear image! The top line and left side were cut off a bit, but the majority of the screen was perfectly readable. It was even showing on the CM-11 in the obligatory green phosphor color! (click for hi-res photos):
The odd thing is that some of the vertical bar characters appeared to be gray, instead of green. It didn't photograph well, but here's a close-up:
And here's the kicker: the CM-11 monitor doesn't even have pin 7 connected to anything -- it's missing from its DE9 connector. So, clearly the Tandy is outputing MDA/Hercules video through the RGB pins... and perhaps could even be emulating the early IBM MDA cards with the "color MDA" RGB output? I know it's also outputting video on pin 7, because I've connected a real MDA monitor to it and it worked fine.
Just to prove there was no trickery involved, here's the same CM-11 monitor showing proper color CGA video -- it's squished down to the center of the screen because that's how far off I had to adjust the monitor's controls to get the MDA video to display properly (or at least as properly as it could manage):
I had the CM-11 running with the MDA signal for nearly an hour with no apparent ill effect.
But to my surprise, after adjusting the monitor's controls, I got a steady, clear image! The top line and left side were cut off a bit, but the majority of the screen was perfectly readable. It was even showing on the CM-11 in the obligatory green phosphor color! (click for hi-res photos):
The odd thing is that some of the vertical bar characters appeared to be gray, instead of green. It didn't photograph well, but here's a close-up:
And here's the kicker: the CM-11 monitor doesn't even have pin 7 connected to anything -- it's missing from its DE9 connector. So, clearly the Tandy is outputing MDA/Hercules video through the RGB pins... and perhaps could even be emulating the early IBM MDA cards with the "color MDA" RGB output? I know it's also outputting video on pin 7, because I've connected a real MDA monitor to it and it worked fine.
Just to prove there was no trickery involved, here's the same CM-11 monitor showing proper color CGA video -- it's squished down to the center of the screen because that's how far off I had to adjust the monitor's controls to get the MDA video to display properly (or at least as properly as it could manage):
I had the CM-11 running with the MDA signal for nearly an hour with no apparent ill effect.