Discussion

A multiplexer (also called a data selector) is a logic block that steers selected input data to its output. This logic function has multiple data inputs from which to choose, but only one of the inputs will appera on the output. Control signals called selct lines are used to determined which data input will be routed to the output.

Figure1: Functional diagram of a digital multiplexer (MUX)
 
MSI multipelxer chips such as the 74LS151 are also available. As shown in Figure 2. this chip is a one-of-eight or 8 channel multiplexer. Three select lines are necessary to choose one of the eight possible inputs for the data output.

For added flexibility, this chip has both a normal data output and an inverted data output, as well as a strobe or enable control input. A more detail description of the 74LS151 IC is available in the manufacturer's data book.

 

 
Figure 2: 74LS151 Multiplexer
 
Demultiplexer

A demultiplexer is also called a data distributor since it has several possible destinations for the input data to be sent to. the single line that will receive tha data is controlled by the specific code applied to the demultiplexer. the large arrows for inputs and outputs in Figure 3 represent on or more lines.
 

 
 
 Figure 3: General Demultiplexer (DMUX)
 
 

The 74LS138 is an example of MSI demultiplexer chip. It is a 1 line to 8 lines Demultiplexer. One bit of inputdata may be sent to any one of eight possible output destinations. Data that is input to one of the G2 enables will not inverted by the DMUX chip. The other two enable pins may be utilized as enable controls. A more detail description of the 74LS138 is available in the manufacturer's data book.
 

 
LS138, SN54138, SN74S138A
Function Table
                                       H = high level, L = low level, X = irrelevant
 
Figure 4: 74LS138 demultiplexer
 

Figure 5 is a security monitoring system that is to be implemented in the laboratory experiment. The system is to monitor the open/closed status of 8 access doors in an industrial plant which are door 0 to door 7. The status of each door is displayed on the LEDs that are mounted on a remote monitoring panel at the security guard's station.
 
 

 
Figure 5: Security Monitoring System
 
If a door is open, its LED will turn on only during the time interval that the counter is at the appropriate count (which corresponds to that particular door) and it will be off at all other counts. Thus, the LED will be flashing on and off if its door is open. The flashing rate can be adjusted by changing the frequency of the clock.