Xor-gate-up-down-counter

  
One can transform an ordinary binary counter, such as a 7493, into an up/down counter with mode control by adding XOR gates 7486 to the counter`s outputs. The circuit counts up when the DN/UP line is low and down when the DN/UP line is high. To use the 7493 counter to count out its maximum count length of 0 -15, connect the QA output to the BIN input and apply clock pulses to the A IN input.
Xor-gate-up-down-counter - schematic

The reset input, when high, inhibits the count inputs and simultaneously returns the outputs Ao through Do to low in the up-count mode or 15 in the down-count mode. For normal counting, the reset input must be low. One can easily cascade this counter by feeding the Qv line to the clock input of a succeeding counter.




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