Help on Hardware ( Correction )

I'm currently trying to make an external input device. I'm using the COM1 of an
IBM PC. It is the 9 pin one and having the address 3F8h. The software should be
able to detect whether it is on or off and the difference in the amount of
current coming through. I'm reding data from port[$3F8] that is evoked by an
Interrupt. I'm currently able to detect if the Blue and White wires are
connected (on).

Now for my problems. Answer as many as you can , please.

1) I opened up my old mouse to learn as much as I can. There are 4 wires only.
Red ( or Orange. I'm not sure), Blue, Green and White. What are each of these
wires for? Can anyone help?
{ Red- Pin 1, Blue- Pin 2, Green- Pin 3, White- Pin 4 }
2) I'm only currently able to detect on/off with the Blue and White wires. Is
there a way for me to detect the amount of current coming into the device
through one of the wires? ( any one of the four ) Like when I use different
resistors or a variable resistor. If so, how? Which wires to use? What port to
check ?
3) If it is possible to detect the amount of current coming through the device,
How much resistance should I use if any?.
4) Does a capasitor play a part? If so what part?

Any help would be appreciated. I'm working on a school project and I need to
know how to communicate with an input device which I can make myself. I've tried
looking for information in books but most books only tell you the software part
and not how the hardware works. I hope that I would not need to use an IC but if
I do please tell me exactly which one to use. One that is easily available would
be preferred.

If you can't help me, can you please point me towards the right direction by
telling me where I can go to get the information I need.

Thanx Alot !

Tanx Vely Mach !
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