1. Digital Logic Design
Week 7
Encoders, Decoders, Multiplexers, Demuxes

2. MSI (Medium Scale Integration) Circuits (chips)
MSI chips have complete circuits, built from multiple gates, on a single chip.
Two classes of such integrated chips (ICs) are:
Encoders and Decoders
Multiplexers and Demultiplexers
Are designed and based on binary coded input.

3. simple binary codes 2–bit codes: 3–bit codes:

4. 10 key numeric keypad data encoded into 4-bit binary code.
Encoders
Encoders typically have 2N inputs and N outputs.
These are called 2N–to–N encoders.
Typical examples include 4–to–2 encoders (probably not used much)
8–to–3 encoders
16–to–4 encoders
It is assumed that only one input at a time is active.
10 key numeric keypad data encoded into 4-bit binary code.
Due to the decimal arithmetic, we also have 10–to–4 encoders. (Why not 10:3?)

5. Exercise
Write the truth table for the 10:4 decimal keypad encoder and boolean expressions of each output. Draw the circuit diagram.
The Circuit Diagram
for the 10–4 Encoder
Input (10 bits) code
Output code
D9 through  D0 Y3 Y2 Y1 Y0 1
In the above encoder, one should note that the input D0 is not connected to any output. An output of 0000 always implies that button 0 is pushed.
Y3 = D8 + D9, Y2=… , Y1=… , Y0=…

6. Decoders
Decoders are the opposite of encoders; they are N–to–2N devices.
Typical examples include 2–to–4 decoders
3–to–8 decoders
4–to–16 decoders
optionally, they may have an enable line
active-high or active-low (the selected output goes to logic 0)
used for to convert n bit binary information to 2n unique outputs

7. 2-to-4 Decoder

8. 74LS138 (3-to-8 active-low decoder)

9. Exercise
Define the equations for each output of the previous given 3:8 decoder IC (74LS138) truth table. Draw a circuit for equations of the outputs.

10. 74LS139 (2-to-4 duo-decoder IC)

11. BCD-to-7 Segment Display Decoder

12. Decoder
for
7-segment
display

13. Multiplexer
RS D-Type Serial Port Data Multiplexer

14. A 2-input multiplexer
Quadruple 2-Line To 1-Line Data Selectors/Multiplexers
Definition: An n-input multiplexer (called a MUX) is an n-way digital switch that switches/directs/routes one of ‘n’ inputs to the output line.
controlled by the control lines ‘S’ to select which input line to be “connected” to the output.
means that the logical value of the output will be the same as the logical value of the selected input.

15. 4:1Mux
Fout= I3S0’S1’ + ……

16. Exercise
The logic circuit above is a design of 4 to 1 multiplexer with strobe input using NAND gates. Show the truth table for the mux and write the expression of the output Y.

17. Exercise
Here we see an 8:1 multiplexer made of two 4:1 and one 2:1 multiplexer.
Try to design a 4:1 mux using three 2:1 muxes?
CD74ACT151: 8-LINE TO 1-LINE
DATA SELECTORS/MULTIPLEXER

18. Figure for 16:4 multiplexer
(or Quad 4:1 mux)

19. Exercise
Define the function (job) of the logic design shown in figure?

20. Demultiplexer
74HCT238 3-to-8 line decoder/demultiplexer