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IBM PC Convertible CGA Compatibility

G

Great Hierophant

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I know that the unexpanded IBM PC Convertible isn't any marvel of CGA compatibility. However, you can install the optional CRT Display Adapter for connecting to a CGA monitor or composite color monitor. The technical reference indicates that the Display Adapter offers full CGA compatibility, including a fully programmable MC6845, But can anyone tell me how compatible it really is with CGA? Does it pass all the CGA Compatibility Tester tests? Can the system (when expanded to 640KB) run 8088MPH? How close is the composite artifact color output to CGA? I know the default font varies from CGA, but the fonts are redefinable. Does the CRT Display Adapter display snow?
 
I dont have composite video adapter for my IBM PC Convertible, but i can tell that IBM 5140 is most IBM PC incompatible machine. Even IBM PC/XT clones are more CGA compatible than IBM PC Convertible. I tried to run 8088mph but it freeze on start.
Most games also cant run properly...
 
I don't know about the CGA questions specifically, but the fact that the PC Convertible uses static RAM probably means that 8088 MPH won't run correctly on it (as the timings will be different).
 
8088MPH may not be perfect, but I suspect that it may mostly run in the Convertible that is equipped with the CRT Display Adapter. It doesn't get very far in my Tandy 1400 LT, but that machine uses a V20 and has poor 6845 CRTC compatibility. Can someone prove me right or wrong?
 
A bit necro, but --- I've just tried 8088mph on my fully expanded PC Convertible. tl;dr: almost, but no banana.

You need the CGA addon. Without it, it just hangs, presumably as it waits for a CGA refresh which never happens, or something. You also need to have done MODE CO80 or it behaves weird and then crashes. And you need to be running 8088mph in drive A:.

Once then, it'll go through several of the scenes quite happily, looking damn good on a baby LCD composite monitor I have. Then the music glitches out, then it twitches oddly and skips the entire where-we're-going-we-don't-need-sprites scene, then the music starts up again before glitching out again, then the Kefrens Bars scene produces complete hash, then the music starts up again for the last scene, and then it hangs completely when it tries to play the end credit music.

Given that the 5140 has static RAM and no DRAM refresh, this would throw out all the cycle counting (it actually warns you of this on startup), so I'd expect anything timing-critical to behave oddly. The failing scenes are all the ones which use precise counting to determine where the electron beam is.

Maybe I'll video it. It's quite interesting.
 
Awesome, a long-burning question of mine about the 5140 has been put to rest.

I am interested to know how close the 5140's artifact composite colors are to a standard IBM CGA card. It sounds like the 5140's colors are in the ballpark.
 
Seconded - also, whether the CRT Adapter's composite signal is more like the 'old' or 'new' revision of the IBM CGA (if the colors are indeed in the ballpark, then the final version of 8088MPH should tell you that on launch).
 
Heh, nice. So basically it's a 'new-style' CGA, but with the hues offset quite a bit (possibly in a non-trivial way).

I wasn't aware that so many cards would have their own unique ways of mangling the graphics, but that could've had adverse effects on my motivation so perhaps ignorance IS bliss. ;)
 
There's a lot of colour distortion in the video --- it's recorded with a cell phone, and it didn't like the little LCD much. The colours were less weird in real life. (But still not that similar to the sample picture.)

I tried fiddling with the settings, but they didn't affect the colour any.
 
hjalfi said:
My cell phone tripod mount finally arrived. I need a microphone, but here's the video.

https://www.youtube.com/watch?v=OtGFsAjnG9s

It kept crashing, and I had to run it about six times before it finally reached the end (not including the credits sequence).
Click to expand...

I never saw the video until now, but it essentially answers the burning question of whether the PC Convertible CRT Display Adapter produces composite artifact colors similar to an IBM CGA card. It does produce composite artifact colors and behaves similarly to a new-style IBM CGA card. Excellent demonstration!
 
Thanks!

I've also tried the CGA composite patched Commander Keen 4, which works flawlessly and looks great --- sadly the machine's not up to running it at full speed and it's nigh unplayable. Interestingly, I tried SuperZaxxon and it claims it detected composite mode, although I don't know how; on my monitor it's definitely showing more than four colours at once. I found these comparison screenshots, both CGA, one composite and the other RGBI, so this seems to be a thing:

zaxxon.jpg
zaxxon-rgbi.jpg

That's from 1984!
 
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hjalfi said:
It kept crashing, and I had to run it about six times before it finally reached the end (not including the credits sequence).
Click to expand...

That is likely because we relaxed the DRAM refresh timing at various points in the demo to suit our nefarious purposes, which we found our desktop 5150s and 5160s could easily handle. Looks like the 5140 uses different DRAM that does not hold a charge as long (which I think makes sense, given that it's a laptop, but that might be unrelated).

hjalfi said:
Interestingly, I tried SuperZaxxon and it claims it detected composite mode
Click to expand...

There is no way to detect if a monitor is connected via the composite port or not; you were merely running the composite CGA version (there is an RGB version too, they were opposite sides of the same "flippy" disk IIRC).
 
The 5140 doesn't actually have DRAM --- it's SRAM all the way and a static CMOS 8088. This lets it suspend the system by simply stopping the clock. Apparently it did decent standby times, but while I do have the battery for mine, but I've left it unplugged for reasons of not catching fire, so I can't say for sure.

Re SuperZaxxon: yes, that makes complete sense. I don't have the original disk, naturally enough.
 
SRAM makes perfect sense, whoops. I was trying to explain why it worked sometimes and not others, but if DRAM refresh isn't actually a thing on the 5140, then I have no idea why it was operating randomly.
 
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