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5150 Keyboard "noise"

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bettablue

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Hy all!

Well, you all know Alice; my IBM 5150 PC system. Well recently, the computer has begun having this little issue when I use certain programs. One of them is Microsoft Works, and another is XTree Gold. When typing while using these programs, the video image seems to kind of "flicker" like the keyboard is introducing some kind of noise. This happens whether or not I'm using the expansion unit, and not on every program, but primarily programs that use color. In Xtree, it occurs when I press the arrow keys to navigate between menus or programs. In Works, it happens whenever using any of the data entry programs including word processor and spreadsheet.

My 5150 is a 3rd gen system, with 256Kb of RAM on the mother board. Of course, I'll provide whatever other information you need to help in troubleshooting if I can. I would include it here, but I honestly don't know where to begin, other than what I see on minuszerodegrees.net. I've had the system open many times, and haven't seen anything "visibly" wrong, so burnt areas, no missing cap bodies etc. Could it have anything to do with a faulty capacitor in the power supply, or another of the power supply components?

Since I'm essentially still new to troubleshooting with these computers, I was wondering too, if it may be due to some kind of line interference, or possibly a series of blown tantalum capacitors that I haven't seen?

Where do I need to begin looking? What could cause this. What would be a worst case scenario?

Thanks in advance.
 
This sounds kinda like "CGA Snow". But if that were the case You probably would have noticed it earlier. Can you think of any recent hardware changes? Any cards that you've moved around near or including the video card?
 
ibmapc said:
This sounds kinda like "CGA Snow". But if that were the case You probably would have noticed it earlier. Can you think of any recent hardware changes? Any cards that you've moved around near or including the video card?
Click to expand...
CGA snow still sounds like a very reasonable explanation, by default XtreeGold will do this on an IBM CGA each time you press a key (because it updates the screen almost every key press).

Have you set "Flicker Free Display" to "No" in XtreeGold?
 
Never heard of CGA snow. But that does make sense. No, I haven't set the flicker free. I haven't found where most of the options are just yet. I was just primarily messing around with a few different menu programs when I noticed it. To be completely honest, I had seen this before with MS Works. But it's usually not too bad. It's not all that bad here either. Now that I have something to go on, I'll check it out and let y'all know tomorrow.

Thanks much. At least I know it's nothing serious. Where can I find more into on CGA snow? Wikipedia and a couple of other sites try to explain it, but they don't go into why this happens on IBM CGA adapters, and not others.

SpidersWeb said:
CGA snow still sounds like a very reasonable explanation, by default XtreeGold will do this on an IBM CGA each time you press a key (because it updates the screen almost every key press).

Have you set "Flicker Free Display" to "No" in XtreeGold?
Click to expand...
 
bettablue said:
Where can I find more into on CGA snow?
Click to expand...
I remember lots of programs had settings for snow back in those days. I didn't understand it then because I had a dislike for CGA and wouldn't use it. lol Anyway, I seem to recall that there's been some particularly useful exchanges here on the forum regarding this matter. I can't figure out how to search the forums efficiently but perhaps you can. Trixter had some particularly pithy information if I remember correctly.
 
"Snow" happens because IBM's CGA cards used single-ported RAM chips, which only allow one read/write cycle at a time. It only occurs in 80-column color text mode, which needs twice as much bandwidth as any other CGA mode. The end result is, when your program is busy directly accessing video RAM, the CRT controller can't read it -- resulting in "garbage" data being sent to the screen, hence the snow.

All other CGA modes have a low enough bandwidth to circumvent the problem (reads/writes are staggered across alternate cycles). Non-IBM CGA clones typically used dual-ported RAM to eliminate snow in 80-column text mode.
Fixing the snow problem in software requires accurate timing: any direct access of video RAM is restricted to the vertical retrace period, which ensures that the CRTC's output is not visible until the next frame is displayed. Essentially, the "bottleneck" is being hit only when you can't see it.
 
Thanks for the info. I haven't been able to locate Trixters explanation but knowing this actually does make a lot of sense now that it's been laid out. Now if I were to exchange the CGA video controller with a different brand, will that eliminate the snow? The reason I ask, is because there are a couple of programs that exhibit this behavior. I'm sure I'll run into more as I go through my collection too. It would be nice to not have that issue popping up whenever I'm typing a document or something.

Thanks again.

el_VR said:
"Snow" happens because IBM's CGA cards used single-ported RAM chips, which only allow one read/write cycle at a time. It only occurs in 80-column color text mode, which needs twice as much bandwidth as any other CGA mode. The end result is, when your program is busy directly accessing video RAM, the CRT controller can't read it -- resulting in "garbage" data being sent to the screen, hence the snow.

All other CGA modes have a low enough bandwidth to circumvent the problem (reads/writes are staggered across alternate cycles). Non-IBM CGA clones typically used dual-ported RAM to eliminate snow in 80-column text mode.
Fixing the snow problem in software requires accurate timing: any direct access of video RAM is restricted to the vertical retrace period, which ensures that the CRTC's output is not visible until the next frame is displayed. Essentially, the "bottleneck" is being hit only when you can't see it.
Click to expand...
 
Some CGA controllers didn't suffer from snow; most EGA controllers with CGA backwards compatibility don't either.

Unfortunately, access during the vertical retrace period really puts a crimp in performance. ISTR that when the BIOS CGA code needed to scroll in text mode, it would disable video output so that it could perform the memory-intensive operation without snow. This produces a somewhat odd flicker however.
 
So basically, what you're saying is that it's probably best to just leave it alone? I'm OK with that, but still would like to find a reasonable solution that doesn't involve changing the monitor. Exchanging the GA controller will be simple, and relatively inexpensive. Is there on controller that you would recommend over the others?



Chuck(G) said:
Some CGA controllers didn't suffer from snow; most EGA controllers with CGA backwards compatibility don't either.

Unfortunately, access during the vertical retrace period really puts a crimp in performance. ISTR that when the BIOS CGA code needed to scroll in text mode, it would disable video output so that it could perform the memory-intensive operation without snow. This produces a somewhat odd flicker however.
Click to expand...
 
No, you can replace the CGA with one that doesn't exhibit snow--you'll just have to do some looking. If the program that you're using has an "anti-snow" setting, use it.
 
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