I know how a multiplexer and demultiplexer work theoretically, I would like some help to connect the theoretical knowledge with these particular circuits (see attached pictures). To be more specific I want to understand exactly what the inputs and outputs on them are. Here's how they look: Demultiplexer 74HC139 ___________ Multiplexer 74HC153 This is how they actually look at the circuit board, so this are the connections I need to understand. I have looked at the data sheets for these components and written down the names of all the connections (as they are given above), and what I think they mean. Please correct me if I have made some misstake: Demultiplexer: 1Y3 - output Y3 1Y2 - output Y2 1Y1 - output Y1 1Y0 - output Y0 SEL1B - control signal B SEL1A - control signal A EN1G - enable the demux (1 disable, 0 enable) Then the same kind of connections are repeated, but with a "1" instead of a "2": EN2G SEL2A SEL2B 2Y0 2Y1 2Y2 2Y3 So first of all, here there seems to be two of each connection type. Also when looking at the circuit diagram in the data sheet (page two) there seems to be two independent circuits. Does this mean this is actually two demultiplexers in one component? So if I want to only use one demux, it's enough that I use the connections that has a "1" in them, and ignore all connections with "2" in them? Multiplexer: OUT1Y - output Y 1CO - input C0 1C1 - input C1 1C2 - input C2 1C3 - input C3 SELB - control signal B ST1G - enable the mux (1 disable, 0 enable) ST2G - enable the second mux? SELA - control signal A and repeated: 2C3 2C2 2C1 2C0 OUT2Y Unlike the demux however, in the circuit diagram for the multiplexer every part is connected to the same circuit, so there doesn't seem to be two separate circuits. However there are still connections named "1" and "2" which gives the impression there are two parts. Looking at the circuit diagram in the data sheet (page 2) it looks like we have the same control signals for both parts. How should this be interpreted? What does the "1" and the "2" represent here? Is this in fact one multiplexer or two? I appreciate if someone can answer these questions in order to help me understand.