understand the negative signal in case of differential signalling

[m4rtin]

In case of single-ended signals the signal is referenced against common ground. In addition, as I have understood, ground and signal path form the closed circuit. Now differential signals do not have a ground. Does this mean that electrical circuit is formed between the signal+ and signal-, e.g. between Tx+ and Tx- in case of 100BASE-TX?

[Richard Crowley]

Yes, exactly. That kind of balanced/differential signaling has been used for everything from old-school analog telephone lines to MIDI and most recently as LVDS (low-voltage differential signaling) used in HDMI and SATA and the internal connection to flat-panel displays, etc. etc.

http://en.wikipedia.org/wiki/Low-voltage_differential_signaling#Applications

[T3sl4co1l]

Note that there is still an implicit ground, whether you provide one or not: common mode current, interference and so on.  Differential traces on a PCB almost always exhibit more "lone trace" character than "differential" character, as does shielded twisted pair.

On the rare occasions when a twisted pair is specifically designed to behave as such (like Ethernet), it's almost always with some horrible catch, like how Ethernet is transformer (AC) coupled only.  So your encoding scheme has to take that into account.

You can't simply shove, say, RS-422 down CAT-5 and expect it to work anywhere near as well as Ethernet does!

Tim