Troubleshooting and verifying 8b/10b encoded signals with a real-time oscilloscope

Debugging protocol errors

Issues with the PHY layer will often cause intermittent faults. Usually PHY verification and protocol testing are done with different test equipment and under different conditions using an oscilloscope.

To ensure best signal fidelity and highest timing resolution, the engineer should evaluate the link at the compliance test point with an oscilloscope and convert the acquired “analog” waveform into binary values or even characters and commands. The protocol trigger and decode software can be used to convert the waveform data into a binary format by recovering the clock first and comparing the voltages with a user-defined threshold and some hysteresis. A block diagram of how this software works is shown in figure 3, with results shown in figure 4.



As shown, there are two tables that list the characters and the protocol. The protocol is correlated to characters and to 0s and 1s in the acquired waveform. This makes is easy to track errors in the protocol down to the physical layer. The displayed waveform in figure 5 can be useful to understand why wrong 0s and 1s have been possibly misinterpreted by the receiver. Cursors and the actual zoom window can be synchronized with the scope waveform display and can helpful in locating the cause of the protocol error.