Patent ID: 7614717

Claim:
A pen fault check circuit for an ink jet printer, comprising: a comparator comprising a pen signal input and a reference signal input; a switch array coupled to the pen signal input, wherein the switch array comprises a high voltage diode; a first constant current source coupled to the pen signal input and to a plurality of ink jet nozzle switches in the switch array; and a second constant current source coupled to the reference signal input and to a single reference switch in the switch array, wherein the first and second constant current sources have substantially equal current outputs, the high voltage diode of the switch array is coupled to a pulse node of an ink jet nozzle that is one of a plurality of ink jet nozzles in an ink jet nozzle array, and the switch array is coupled to a register that selects one of the plurality of ink jet nozzles in the ink jet nozzle array for a fault check.