I want to implement this truth table (in Figure 1.1) into a circuit using an 8-1 multiplexer. I did the circuit schematic (in Figure 1.2) in Xilinx ISE Design Suite and it does work, but when I try to make the wiring diagram (in Figure 1.3) with a program the circuit does not work. Can you help me identify the error?


                                                         Truth Table
                                                      Truth Table (Figure 1.1)

Wiring Diagram
                                                      Circuit Schematic (Figure 1.2)

Circuit Schematic
                                                       Wiring Diagram (Figure 1.3)

  • 4
    \$\begingroup\$ try putting chip pin numbers on your schematic \$\endgroup\$
    – Andy aka
    Commented Oct 28, 2020 at 14:05
  • \$\begingroup\$ Good idea, do you know any good program or way to do this? \$\endgroup\$
    – Ivan.2j
    Commented Oct 28, 2020 at 16:49
  • 1
    \$\begingroup\$ FWIW, and maybe not much, I'd connect D0 and D1 directly to Vcc and GND. That is, if I got it right that they're going to get known values when used, since C controls both them and S0. \$\endgroup\$
    – ilkkachu
    Commented Oct 29, 2020 at 10:28

1 Answer 1


The cathode of the LED must be connected to the ground not Vcc

  • 1
    \$\begingroup\$ Wow, good catch! \$\endgroup\$
    – Justin
    Commented Oct 28, 2020 at 14:14
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    \$\begingroup\$ I feel so stupid right now, thank you so much, you saved me from hours of pain! \$\endgroup\$
    – Ivan.2j
    Commented Oct 28, 2020 at 16:49
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    \$\begingroup\$ Spending many hours chasing what turns out to be a trivial screwup is par for the course....... \$\endgroup\$ Commented Oct 28, 2020 at 19:08
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    \$\begingroup\$ Trivial stuff is the easiest to overlook since you usually assume its correct. Thats why those errors hide unless someone else looks over. \$\endgroup\$
    – masterX244
    Commented Oct 29, 2020 at 11:40
  • 2
    \$\begingroup\$ "When a progammer has a hard time finding a bug it's because he's looking in the wrong place". Guess it applies to hardware as well. \$\endgroup\$ Commented Oct 29, 2020 at 12:31

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