The apparatus includes a frame to accommodate more than two instrumentation modules; a multiplexed bus associated with the frame and including more than two connectors; and a sense instrumentation module and a source instrumentation module mounted in the frame, electrically connected to respective ones of the connectors, and collectively including a communication link, and a circuit path that includes, in series between a sense input and a source output, a sense circuit to generate a sense output signal representing a sensed parameter dependent on a source signal; a feedback controller to generate a control signal in response to the sense output signal; and a source circuit to generate the source signal in response to the control signal. The communication link is in the circuit path, and includes a bus transmitter and a bus receiver each configured to select a channel of the multiplexed bus via which to communicate.