Depends on the signalling standard. Ground isn't needed if isolated interfaces are used - such as with ethernet, where the twisted pair goes into a transformer so group loop issues are avoided. In simple systems the pair has a voltage potential forced across it. Make the + wire 5V higher in potential than the - wire, and that might signal one state. Make the - wire 5V higher than the + wire and that might signal another state. This is similar to RS-485 and CAN, though RS-485 is not isolated and without a ground you will be more susceptable to signal issues, noise, and ground loop currents. Other methods use the twisted pair just as the phone system normally works - a current loop where data is transmitted as audio or near-RF signals that draw more or less current. I think this is confusing to many beginners because we are taught voltage first, and voltage is used for signalling significantly more frequently than current signalling designs. However, voltage and current are simply two sides of the same coin, and one can 'convert' one to the other when needed . -Adam On Fri, Sep 11, 2009 at 11:32 AM, solarwind wrote: > On Fri, Sep 11, 2009 at 6:28 AM, peter green wrote: >> It is generally a twisted pair but typically not a particulally tightly >> twisted one. Some older phone wiring and a lot of cheap extentions are >> not twisted > > I thought that in a twisted pair line, the pair would carry the signal > and a third wire is required for ground. How does twisted pair > signalling work with only two wires? > -- > http://www.piclist.com PIC/SX FAQ & list archive > View/change your membership options at > http://mailman.mit.edu/mailman/listinfo/piclist > -- http://www.piclist.com PIC/SX FAQ & list archive View/change your membership options at http://mailman.mit.edu/mailman/listinfo/piclist