How Does the LC575 / Color Classic Resolution Mod Work?

[MacintoshLibrarian]

I was wondering if anyone knew how exactly the Color Classic 575 logic board resistor swap trick works?
This allows the 575 board work on a 512x384 stock Color Classic Monitor.

Method in question:

Mystic | powercc.org

If you plan to upgrade your Apple Color Classic we show you what is possible by just switch to an other compatible board.

powercc.org

Next, solder a 4.7k-ohm resistor on location number R109 (near the R116). 4.7k-ohm resister is showing "472". It means 47 x 10 x 10. If you can't get the resistor, you can use the 0 ohm resistor which you removed from R116.

Finally, check R110. If any resistor is at the location, please remove. If there is no resistor....it's OK.

Are we letting the 575 board know that we are connected to a different video source based on the 'jumper' resistors we swapped around?
Are we modifying the Sense Line codes that the logic board sees? https://support.apple.com/kb/TA21618?locale=en_US
I am just trying to get some background for a future video I am working on
I appreciate the help!

-Kate

 

[retr01]

Hi @MacintoshLibrarian!

Are we letting the 575 board know that we are connected to a different video source based on the 'jumper' resistors we swapped around?

Yes. By changing the connections, such as ground for off (zero) and not connected for on (one), we are letting the Mac know which screen resolution to display. The binary codes with those connections conduct sense lining.

Are we modifying the Sense Line codes that the logic board sees? https://support.apple.com/kb/TA21618?locale=en_US

Yes.

I am just trying to get some background for a future video I am working on

That will be a great resource!

I am continuing my research on the "how" of the sense lines. I can see the pattern of changing logic gates by swapping around the connections to tell it to display a particular resolution. I'd wait and see if others can pitch in their experiences and how they perceive the function of sense lining for video in the Color Classic.

 

[jajan547]

I also advise you take a look at the VGA mod to get a better resolution out of the board swap.

 

[Paolo B]

This is the reference website:

Welcome to powercc.org!

powercc.org is the ressource about the Apple Color Classic and gives you answers and nearly lost content about this incredible computer.

powercc.org

here’s also this rather excellent video from @JDW

It’s possible to run a 575 motherboard without any further hardware mod by using software hacks (ResEdit stuff). They are only available for limited choice of OS, though.
I have it like that on my Mystic.

 

[retr01]

Yeah. We are talking about the "how" sense lining works in theory, not the physical modifications.

Sense lining is done by binary coding by the connections to have the Color Classic display at a particular resolution. We know that when we connect to ground, that is zero. When we do not connect, that is one.

There are three sense lines for video signals passing through that area of the mobo that include S1, S2, and S3. For the Hi-Res mode, the S0 line is connected to ground while S1 and S2 are not connected thus the binary code is 1 1 0. VGA mode means S0 and S1 connected to each other while S2 is not connected, so the binary code is 1 1 1.

By doing binary coding, the sense lines are activated or not to specify the video mode. Therefore, it is reasonable to perceive that changing the resistors and connections are part of the work to adjust the sense lining by changing the binary codes. This is nothing new. Binary coding by wiring been around since 1940s and probably earlier.