US Patent 7471897 Electrically looped back, fault emulating transceiver module

A transceiver module having transmit data lines looped back to receive data lines that emulates faults without requiring optical subassemblies inside the module or optical test equipment external to the module. That data lines can be buffered for transparent probing of signals. One or more values directing emulation of a fault are received, such as over a two wire serial interface, and the fault is emulated. The values can be values for electronic pins of the transceiver module, values for operation status of the transceiver module or values for characteristics of signals within the transceiver module.