variable logic gate

6-30 — elektor june 1980 variable logic gate For those readers who are not familiar with digital technology and thinking, a variable logic gate provides an excellent opportunity to "get into" it. To make things easier, a truth table of all the logic functions is given in table 1. All the sym- bols representing the various logic func- tions are also included. The noughts and ones indicate the logic level. A "0" means 0 volts and a "1" means there is a voltage (for TTL it is +5 V). The truth tables indicate how the output (Q) behaves when various logic states are fed to the input (A) or inputs (A and B). variable logic gate the output. The truth tables show this clearly. The equation for this function is: Q=A • B. The OR gate does something quite dif- ferent. It produces a logic "1" on the out- put if input A or input B has a "1" fed to it. It also has a "1" on the output if both the inputs are "1". Expressed in algebraic terms, this is: Q= A + B where "+" is to be read as "or"....