A biased control loop for setting the impedance of an output driver includes a dummy driver having a variable output impedance, a sample and compare circuit to compare the output impedance of dummy output driver to a reference, and an up/down counter to modify the impedance. When the loop is locked, an error signal alternates positive and negative about a reference value. A digital filter produces a filtered version of the error signal with an apparent error value that does not alternate. The digital filter has a biased lock circuit that guarantees that the apparent error does not alternate. A simultaneous bidirectional port includes an output driver and the biased control loop to set the output driver impedance. When the output driver drives a bidirectional line and serves as a termination impedance for another driver, the reduced apparent error variation provides improved impedance matching.