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Fig. 4: Protocol view of characters, protocol and waveform. 6: Dead time between acquisitions. network element is connected to channel 1 and the output data stream is connected to channel 2 of the oscilloscope. In the set up shown in figure 7, a bidirectional link con- a data generator will provide the required data stream to the nects the two network elements. network element a acts as a input of the network element (DUt). a unique pattern inside data source to ensure that network element B (DUt) works in the data stream will perform as a timing reference. this timing a desired operating mode. the communication link has to be reference needs to be a very rare pattern to prevent confusion established by proprietary commands and the data flow of this with another timing position. If the pattern has been defined commands needs to be maintained during the measurements you can search for that pattern in the acquired signal of chan- to keep the DUt in the desired operation mode. therefore a nel 1 and channel 2 and then measure the time between the static data pattern provided by any data generator will not two locations. work. A software decoding function could help to find the se- similar to the previous example a unique pattern is required quence in the data stream. Because of its infrequent occur- for a timing reference (marker) in the data stream. since the rence the search for the pattern can be very difficult. Given os- pattern is very infrequent and there is no trigger signal avail- cilloscope acquisition memory limitations, the chance of finding able from the source network element a, an 8b/10b trigger is the pattern for the timing reference is low. Unlike searching, required to find the pattern in the input and the output signal of 8b/10b triggering makes it much to find the sequence. This the DUt. is because triggering ensures that the pattern will always be the optimum way to see and measure the delay between inside of the acquisition window. the input and output of the network element is to use two zoom windows. The first zoom window is placed at the beginning without 8b/10b triggering the delay measurement would of the pattern sequence at channel 1 and the second zoom require a trigger signal from the data generator to the oscillo- window at the beginning of the pattern sequence at channel 2. scope. That would be the only way to synchronize the starting For the delay measurement the acquisition time window of the point of the 8b/10b timing reference at the output of the data oscilloscope should be equal to or greater than the delay time. generator with the acquisition window of the oscilloscope. however, this method fails to replicate real world conditions. Fig. 5: Tracking a protocol failure to a glitch in the waveform. Fig. 7: Time delay measurement setup. www.electronics-eetimes.com Electronic Engineering Times Europe January 2013 17


EETE JANUARY 2013
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