For example: There is Din+ connected to pin a1 and Din- to pin a2. Suppose I am sending "0101". How does it get sent?
If there was only Din connected to Pin a1, which has a voltage supply to the bank of 2.5 V, I understand that a '0' would be 0 V and a '1' would be 2.5 V. But how is it sent when Din+ and Din- are there?
An 'unbalanced' signal is composed of a signal and ground. The problem with unbalanced transmission lines is that they are more susceptible to noise induced in the line, because it appears on the signal line. Having two signal wires typically means its a 'balanced' signal. Din+ is always opposite in voltage from Din-. (so if Din+ is 0101, then Din- is 1010). A balanced transmission line is immune to common noise (i.e both signal lines get the same noise) in the line. It makes a difference if there is a decent amount of signal line distance. For very short runs its less important. Also note that the receiver must have a linear front end with fairly high dynamic range so that it can truly measure the difference between the two inputs even when there is notable 'common mode' noise. Otherwise a lot of even unbalanced (common mode) noise might saturate both inputs.
