USB CABLE

Guest · Posted: Tue Nov 07, 2006 5:50 am

Hi,

If I want to connect my PC to a PIC18F4550 using full-speed USB at 12Mbps. Could I build a crossover cable or a straight cable ?

thks

rberek · Joined: 10 Jan 2005 Posts: 207 Location: Ottawa, Canada

I don't think USB has the concept of a crossover cable. There are only 4 signals in the cable, +5V, GND and two differential data lines. There's nothing to cross over.

Guest · Posted: Tue Nov 07, 2006 6:20 am

yes, my doubt is about crooss or not D+ and D-

thks

rberek · Joined: 10 Jan 2005 Posts: 207 Location: Ottawa, Canada

I'm no expert, but you use a crossover cable in RS232 because you have separate Rx and Tx lines, and because you need to cross them over when you have similar devices talking to each other (i.e. DCE to DCE).

With differential signaling, crossing the signals over doesn't really accomplish anything other than inverting the signal (correct me if I'm wrong. I'm thinking on the fly here).

In USB, who is talking and who is receiving is determined by the USB protocol, not by the physical layer (i.e. the cable wiring).

But as I said, I'm no USB expert. In short, you wouldn't have to cross anything over.

Ttelmah · Guest

Dead right.
This is why USB, has the two different plugs. Basically, the type A 'flat' plug, always goes towards the master device. The cable is always straight through. There is no such thing as a USB crossover cable. The units that allow two masters to connect to one another, actually have two slave units inside them, and the 'crossover', is done between these, not on the USB interface itself.

Best Wishes