Multiple-Input Multiple-Output Exclusive-OR Circuit

07/11/2001


This circuit is interesting for several reasons:

4-Input 4-Output EOR PDF File  (565Kb)

  • It is called an "Exclusive-OR" Circuit because it will become active if ANY of the inputs go active high, but ONLY one at a time. A conventional OR with inputs "A" and "B" will go active if "A" OR "B" OR Both! An Exclusive-OR (EOR, XOR) with inputs "A" and "B" will go active if "A" OR "B" go active, but NOT Both.
  • A close examination of this particular circuit will reveal that when input "A" goes high it not only presents a request at the input of U2A, but it also presents a disable to U1A and U4A. This also causes a "Cascading Series of Disables" to U1B, U4B, and U2B, which would prevent "X" or "Y" or "Z" from becoming active. Providing that there are no high inputs at "B" or "C" or "D", U2A is enabled to be driven low at the output.
  • If you further examine what happens with a high only at "C", you will find that it presents a request at the input of U4B, but also presents a series of disabling controls at U1D and U1B. These will then cause a "Cascading Series of Disables" to the left of the "C" input (from U1D), and also to the right of the "C" input (U1B), which would prevent "W" or "X" or "Z" from becoming active.
  • This simple principle of "Disabling" all to the left or to the right of the selected input, is what provides for the "Exclusive-OR" action, in that ANY one, but ONLY one will activate the outputs. Providing more than one active input will disable each other.
  • This circuit is easy to troubleshoot, once these basic principles are understood. It makes a good lab for teaching troubleshooting digital circuits, and you will find some worthwhile "bugs" at the end of this document that I used in the classroom for many years.Another interesting feature of this circuit, is how it can be simply extended to "Many-Inputs" and their respective outputs. In fact, this circuit is the digital implementation of an age-old relay-logic-circuit, that was used to allow the selection of any number of single-inputs, providing that it was ONLY ONE selected. It prevented the possible confusion caused by simultaneous selections.
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