102 www.xilinx.com UG024 (v1.5) October 16, 20021-800-255-7778 RocketIO™ Transceiver User GuideChapter 5: Simulation and ImplementationRDiagnostic SignalsOften a diagnostic check is needed upon power-up. RocketIO transceivers have severalinputs and outputs to run these checks.LOOPBACKLOOPBACK allows the user to send the data that is being transmitted directly to thereceiver of the transceiver. Table 5-4 shows the three modes for loopback.Table 5-4: LOOPBACK ModesInputValue Mode Description00 Normal Mode The normal mode is selected during normal operation.The transmitted data is sent out the differentialtransmit ports (TXN, TXP) and are sent to anothertransceiver without being sent to its own receiver logic.During normal operation, the LOOPBACK should beset to “00”.01 External Serial Mode The external serial mode is used to check that the entiretransceiver is working properly. This includes testing ofthe 8B/10B encoding/decoding. This emulates whatanother transceiver would receive as data from thisspecific transceiver design.10 Internal Parallel Mode For testing of interfacing logic, the Internal ParallelMode allows the use of linking the transmit and receiveinterface logic without having to go to anothertransceiver in the cases of 8B/10B bypassed or toreduce data latency from TXDATA to RXDATA.
