Resistor
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A resistor limits the current in a circuit. With ohms law one can calculate current and resistor values, being that the values are unaffected by forced voltage or current. The resistor value in ohms is defined as the ratio of the voltage over the resistor, and the current trough it.
I = V / R I = 12V / 1,000ohms I = 0.012A (12mA)
Pull-up
A pull-up resistor pulls the idle state positive, if a negative signal is connected this will overrun the pull-up and the output goes low. This gives positive N.O and negative N.C.
Pull-down
A pull-down resistor pulls the idle state negative, if a positive signal is connected this will overrun the pull-down and the output goes high. This gives positive N.C and negative N.O.
