Making a USB interface

Question

Something I always wanted to do is to be able to design some hardware that would send information into a computer and communicate with some software that the user could interact with. Here are some examples: I have different hardware with USB inputs such as a biofeedback device, a sound level meter, etc. Or, if I designed a circuit that sent and received information to a USB output which could be plugged into a computer. It would be great to be able to write a program that gets and sends information to these devices. But, I just do not know where to start. I was wondering if one of you could point me in the direction of understanding the process. Does it involve writing a driver? If so, what is involved? etc.
Here is concretely what I am looking to do now.
I have an exercise bike that has an eight pin input from the bike into a circuit board which connects to a screen. I would like to disconnect the screen and communicate with the bike through my laptop running Mac OS X. I figure it must be possible to have the eight pin input connect to a USB computer input that would connect to a computer. I imagine, if you understood how Mac OS X works at a fundamental level, you could then write a program that would then use the input from the bike (through the USB input).
Somebody must know how to do this because every device that hooks into a computer must do this. I would just like to know the basics of how to get this to work at the most simple level. Any help would be greatly appreciated.

Answer 1

Here is concretely what I am looking to do now.
I have an exercise bike that has an eight pin input
from the bike into a circuit board which connects to
a screen. I would like to disconnect the screen and
communicate with the bike through my laptop running
Mac OS X.
Forget about USB and OSX and programming etc for now.. the first thing you will need to do is work out what each of those wires are used for. Are they connected to sensors or another circuit? Do they drive the screen, or does the screen drive them? Do they need power? What voltage? etc. If you can't do this.. then there isn't much point continuing any further until you can. If you aren't already handy with a multimeter and a soldering iron, then you might want to try interfacing the computer with something a little less complicated to start with, or find someone who is experienced to help you.
How to talk to/program for a custom USB device is going to be the last thing you want to deal with, not to mention requires many years of system level programming experience. Something I am assuming you don't have by the way you asked the question (no offence intended).
That's not to say don't attempt it.. on the contrary, it sounds like a cool idea, however it's not going to be easy. For the computer interface part I would probably be looking for an already supported, cheap, input device that can be repurposed to suit, such as a USB gamepad controller, which have numerous digital and analogue inputs, not to mention readily available example source code from apple on how to talk to it.
Seeya...Q