IBM 5155 CGA Issue (not snow)

[ghoul]

I recently obtained an 5155 and it seems okay, picture is pretty clear but when text is highlighted it's almost illegible. Furthermore games/graphics look fine until they don't..

As you can see in the images, the first level in tapper looks pretty normal, whereas in the second one it looks completely broken and has some form of jailbars. Black levels are also completely destroyed as you can clearly see if you compare the letterboxed areas of the screen. In Lotus 1-2-3, the text is legible everywhere except for the highlighted part which is almost impossible to read in real life.

I was wondering whether this was some kind of limitation with the card or monitor, but it would be strange for a text-heavy business machine to have such illegible text. Anyway, I'm not very knowledgable in this area. What is causing these seemingly broken graphic modes? The CGA card, monitor, or RAM? Or am I misunderstanding?


Level 1; normal

Tapper level 2

Close up of level 2

Lotus 1 2 3, highlight very hard to read in real life. Parts of the highlight background seem missing as well; see upper left corner.

 

[mbbrutman]

I think this is working as designed.

The CGA card is generating 16 colors, all of which are being rendered on the tube as amber. It's just never going to look good with certain color combinations.

Monochrome color tubes really should use a monochrome-friendly color palette. When I write software for this type of display I give the user the option to limit the color scheme to normal, bright, reverse normal, and reverse bright.

 

[jafir]

I wonder if it’s got the originally equipped CGA card or if it was replaced with an older version at some point. I seem to recall some talk that the later CGA cards have a slightly different composite output than the earlier card to make them work better on the 5155 monochrome display.

But with regards to what Mike said, I seem to recall lots of software back in the day had an option where you could force a mono mode, even the DOS edit command could do that, if I’m remembering correctly.

 

[MicroCoreLabs]

One would think level 1 and level 2 would look similar.... Hmm.. I wonder if there is an issue with your CGA's video memory. Maybe the memory at page 1 is ok, but page 2 has a stuck bit in the 'I' (intensity) field.

If you run the IBM PC Diagnostics I believe it tests each video page independently to help you confirm. Diagnostics

 

[ghoul]

Appears as if brutman is right. In part my confusion comes from the fact that I'm using a monochrome display - when I mentioned the black levels disappearing, I forgot to ask myself whether the next scene should even be black in the background. It's probably a daytime setting with a new palette that looks horrible on the 5155.

My diagnostic results were the same as the person in MicroCoreLabs's link to diagnostics as far as they tested. When doing option 11 video test, I could not read the text line on maybe half of them. Also one of them was just a black screen and I saw nothing. Not sure if this is maybe expected on monochrome.

There is one more thing though, why is part of the highlighting background missing in that Lotus 1-2-3 picture I supplied? It does it on all of the options to different degrees. Even if this is again normal it would be interesting to know why that happens.

 

[modem7]

I once owned a 5155, and I remember the video output of certain software appearing poorly. As a result, I added the page at [here] to minuszerodegrees.net

 

[vwestlife]

It's because the 5155's internal monitor has no provision to filter out the NTSC color subcarrier (a.k.a. color burst). So when the CGA card is sending out a composite color signal, the monitor displays the color information as dots and lines, which can often render the video illegible.

In DOS you can fix this by typing MODE BW80 (or BW40) to set the CGA card to black & white mode and disable the color burst. Better-behaved programs will abide by this setting; but many games may force on composite color mode unless they have an option to manually switch to black & white mode.

 

[resman]

This may help: https://forum.vcfed.org/index.php?threads/fixing-cga-composite-monochrome.54881/

 

[vwestlife]

You could also replace the CGA card with an aftermarket one whose composite output is purely monochrome (or hardware-switchable between mono and color); some have the pin header for the 5155's internal monitor (originally intended for connecting to an RF modulator).

 

[VileR]

IIRC, the disk for Tapper (like some other Sega conversions for the IBM PC) boots from both sides... one is meant for RGB CGA and one for composite.

From your photos I'm pretty sure you're running the RGB version. Stage 1 looks fine, because it uses the "mode 5" alternate CGA palette (which has the side effect of BW output on composite). Stage 2 however uses one of the standard palettes, which still produce color (= interference on BW composite), plus it sets a bright background which doesn't help either.

What you want is the composite version. It's still meant to be seen in color, but it uses pure artifacting, which should look less obnoxious on the 5155.

If you have a DOS version rather than the two-sided boot disk, you simply need the correct DOS version which uses the composite mode. Otherwise the TSR by resman may help, but TSRs have to rely on hooking BIOS calls, which won't work if the game bypasses the BIOS video interface and writes directly to the CGA.