Events, transaction, propagation delays and concurrence.

Lets start our discussion with an example of half-adder circuit. Half-adder circuit shown below has 3 inputs (x, y and carry_in) and 2 outputs (sum and carry_out). This circuit computes sum of two bits and reports carry.

Event - Whenever there is a change at input there is a corresponding change at the output. This change is know as an “event” in digital circuit design and analysis. 

Transaction - Its not necessary that a change at input will result in an output change. This scenario is know as transaction. 

Before we go any forward, lets discuss more about physical layout of digital circuits. -
The physical design primarily consists of devices like transistors, wire and noise canceling circuits. The transistors can itself be laid out in various structures to create logic gates , flip-flops etc. 

Propagation Delay - Whenever a change is applied at the input of a circuit there is an associated delay or resistance in the path. This period of time is known as Propagation delay. These delays are also know as inertial delays and their values are directly proportional to the length of wire.