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Old vs new style composite out in a 5155

Kanpai

Kanpai

Member
Joined
May 23, 2019
Messages
10
Hi everyone,

I recently build a CGA Redux by Hkz with an old style composite out configuration. I tested the card in an IBM 5155 connecting the internal CRT to the card. When I turn the machine on I notice a bright horizontal line (probably 1 cm heigh) on the CRT for a second or two (out of sync or horizontal collapse?). Afterwards, I get a normal image. RGBI output on an 5153 and external composite out over a composite to VGA adapter look normal.

Furthermore, from a blog entry by Reenigne I gather that old style composite will not provide the best image on a monochrome CRT like the one in the 5155 to beginn with. In short, old style composite generates less shades of grey.

My plan for the card is to added a daughterboard which will provide new style composite out over an internal header, while the card itself will provide old style composite out over the RCA jack. In theory this should provide a good grey scaled image on the internal CRT and old style composite externally for games that were optimised for old style composite:

cga-redux.jpg

Note: the position of the RCA jack on the PCB will interfer with the chassis of a 5150/5160. If you would like to build your own card you may want to move the jack further down.

Has anyone tried a CGA card with an old style composite out in a 5155 before and can confirm my observations? Did IBM actually ship 5155's with old style composite out or only with CGA cards in the new style configuration? I am not sure if old style composite in general or my card configuration (use of standard TTL IC's instead of LS) might cause this bright horizontal line on start up and eventually damage the CRT in the long term.
 
Facinating post. Makes me wonder what I have in my 5155.

In a nutshell is your daughterboard designed to alleviate the bright horizontal bar and save the tube?

Did you replace the composite out jack with a gold one in the hopes you would get better picture?
 
Yeah that is basicly the idea of the daughterboard, while still providing external old style composite out which does have a different kind of color scheme.

I did not want to stress the CRT of my IBM 5155 more than necessary, therefore I did not look for other difference between old and new composite out on the CRT itself. However, I think the XT-IDE boot menu was hard to read with the CGA Redux and its old style composite configuration, which would be in line with Reengine's statements about the limited levels of grey shades with an old style configuration. With the stock IBM CGA card and new composite output the XT-IDE boot menu is nice and crisp on the CRT.

The gold jack was what I found at a local seller, that's all there is to it.
 
Does anyone have any indication of when early and late cards came out?

The 5155 is an “early” machine when compared to the entire range of XT class machine, but it might be considered late when compared to PC class machines.

Maybe the 5155 was why they changed the behavior of the composite output?
 
The IBM 5155 was exactly why IBM introduced the "new-style" cards.

See the photo in PC Magazine's article on the 5155 on the top left at https://archive.org/details/PC-Mag-1984-05-29/page/n115/mode/2up: "The modified color graphics card turns the color ar into an even amber scale."

And on page 117, bottom right: "The Portable PC is available only with the color graphics card. The display monitor uses the composite output of the card in order to produce a monochrome image instead of the color image. To this end. the card has been modified to produce a better gray scale (or amber scale in this case). The balancing resistors that blend the red, blue, and green color signals into the composite signals are optimized for the portable machine."
 
Thanks VileR.
I will order the proposed daughterbaoard for the CGA Redux next year and test it with my 5155. If it works as intended (new style internally, old style externally), I will add it to my github repro. However, this daugherboard will likely fit only on the CGA Redux Rev. 2 and not on an original old stlye CGA cards by IBM due to component placement on the raster.
 
VileR said:
The IBM 5155 was exactly why IBM introduced the "new-style" cards.

See the photo in PC Magazine's article on the 5155 on the top left at https://archive.org/details/PC-Mag-1984-05-29/page/n115/mode/2up: "The modified color graphics card turns the color ar into an even amber scale."

And on page 117, bottom right: "The Portable PC is available only with the color graphics card. The display monitor uses the composite output of the card in order to produce a monochrome image instead of the color image. To this end. the card has been modified to produce a better gray scale (or amber scale in this case). The balancing resistors that blend the red, blue, and green color signals into the composite signals are optimized for the portable machine."
Click to expand...

Whoa -- never knew that. Nice to have a definitive answer. When did we discover this?
 
Trixter said:
Whoa -- never knew that. Nice to have a definitive answer. When did we discover this?
Click to expand...
Can't recall! Must have been quite a while ago... I'm pretty sure I've brought it up before.

Interestingly, there was at least one photo posted on this forum showing a Portable PC with an old-style CGA board (all the non-gray color bars having the same brightness). Although I assume that wasn't what it originally came with.
 
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