My question is what exactly do a laptop lcd video cable do? Apart from taking data from board to lcd and back, does it also act as an identification mechanism?

Sony vaio laptop, m/b and graphics card supposedly support 3 different lcds, each has its own video cable according to Sony, however they look identical to the naked eye.

How does the motherboard know which lcd is connected to it? Is that the function of the video cable?

I removed the lcd panel and replaced it with one with a different resolution but the motherboard did not detect it correctly. (See older thread. Link) Haven't yet changed the video cable - waiting for it to be delivered.

You're doing some hairy stuff.

To the best of my knowledge, the identification echanism on a laptop is in the motherboard interface circuitry - but I'm not sure.

You need a cable that matches the display in order to carry signal properly although I dare say that there are standards applicable to each display type. I'll leave that aspect to you.

Once you've sorted that, I'd expect your lash-up to work wiuth the display being initially driven from the BIOS and then from Windows in 640x480 mode. Then you can sort out drivers.

For future reference, I found that the SXGA cable and the XGA cable have different number of wires. In particular pins 1/4 are connected together in one of them and in the other it is pins 2/4. This is at the end of the cable that plugs into the video circuitry on the motherboard.

So does it all work now?

Yes!