an rf switch employing a pin diode as the switching element includes a first bias circuit providing forward dc current through the diode to enable rf transmission therethrough , and a second bias circuit providing reverse dc voltage across the diode to disable rf transmission through the switch . status circuitry monitors the switch for failure modes of the diode , and control circuitry selectively enables the bias circuits . a short circuit failure of the pin diode is indicated by the flow of reverse current in the second bias circuit in excess of the diode leakage current . under this condition , the reverse current is latched by a thyristor in the second bias circuit and the status circuitry generates an alarm signal to the control circuitry inhibiting further selection of the first bias circuit . optical couplers provide electrical isolation of the first and second bias circuit from the control and status circuitry .