Decoders and multiplexers are termed, medium scale integration (MSI) devices; this term implies that the device is complex in construction and usually its operation as well. Below is shown a block diagram of a 2 -> 4 decoder    

The decoder can be thought of as a simple base 10 converter. The binary input's A, B select the base 10 equivalent output, namely O₃-> O₀. It should be noted that usually the decoder is active low outputs i.e. logic '0' for on and logic '1' for off. Various size decoders are available 2->4, 3->8, 4->16, and 4->10. The operation is made easy by considering the truth table for a 2->4 decoder: -   

               
Within logic we also have sequential logic circuits, the building blocks of these MSI devices are flip flops. There are four basic flip flops namely:
 
            SR Flip flop
            JK Flip flop
            D-type
            T-type
 
Note the following truth tables shown below are only valid if the clock is active (usually active low).The operation of these devices are simple in that the devices either stores the last value (i.e. doesn't change), resets, sets, or toggles (i.e. output inverts itself).

It should be noted that the CMOS and TTL logic families have sequential logic within their families and they usually have asynchronous preset (PR) and clear (CL). Let us consider some simple applications of sequential logic.