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Troubleshoot, verify 8b/10b encoded signals with real-time scope

Posted: 05 Oct 2012 ?? ?Print Version ?Bookmark and Share

Keywords:8b/10b coding? transformers? optical channels?

Most digital oscilloscopes provide a large portfolio of triggering capabilities. Traditionally, troubleshooting is related to time and level qualified triggering. With the advanced trigger modes available, triggering on glitches, transitions, runts and so forth is much easier than in the past.

For protocol errors, it can be useful to trigger on commands, characters, or bit sequences. Unfortunately a serial trigger circuit designed for NRZ patterns cannot find those faults because most high speed serial data signals are 8b/10b coded and require a dedicated hardware solution.

A standard NRZ trigger cannot trigger on words of 8b/10b coded data streams because of two reasons. First, the coding of the 8 bit word to 10b symbol will change at the speed of the data rate and would require an adjustment of the symbol rate in the trigger memory to the same speed.

Second, triggering on the right 8b/10b symbol requires synchronization or alignment of the 8b/10b codes to the data stream. For real time triggering the hardware must be capable to synchronize to one of the "comma symbols" (K.28.1, K.28.5, and K.28.7) which are unique and cannot be found in the data stream at any bit position in the code.

The synchronization character can be somewhere in the data stream and might be very infrequent or appear only once. One example for a synchronization character is the comma symbol, K28.5 (011110101). Once the alignment symbol has been found, the decoding of the subsequent symbol values can proceed.

Software "triggering" solutions actually perform a search through the acquired data and therefore have long dead times that cause very large gaps between the acquisitions and increase the chances of missing the character in question.

Many higher end oscilloscopes are equipped with a dedicated trigger chip for triggering on 8b/10b data patterns in high speed serial signals up to 6.25 Gb/s. This enables the instrument to find rare events since it is now able to trigger on 8b/10b characters.

Characters are acronyms for a pattern of 10 bits of the 8b/10b code, i.e. D31.6 or K28.5. A second option related to a high speed serial standard's protocol is triggering on 8b/10b words (commands), where words consist of multiple characters (commonly 4 words or 40 bits). It should be noted that every standard has its own word definitions.

A powerful debugging tool is the ability to trigger on 8b/10b code errors. No serial trigger would be able to trigger on all possible character errors, disparity errors or losses of byte synchronization, but it is usually possible to trigger on common errors such as disparity or character errors.

Network element delay measurements
Triggering on 8b/10b serial patterns can be used for measuring the time delay of an active network element. One might think this an easy task to solve even without special triggering on 8b/10b. But it can be challenging when it's necessary to measure the time delay under real conditions. The setup for this measurement is shown in figure 7.

Figure 7: Time delay measurement setup.

The input signal of the network element is connected to Channel 1 and the output data stream is connected to channel 2 of the oscilloscope. A data generator will provide the required data stream to the input of the network element (DUT).

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