EtherCAT Period, Jitter, and Packet Capture

Real-Time Optimization and Debugging

7.4 Measuring Period and Jitter

If the target period is T, record the actual wake-up time for each cycle and calculate:

period  = current_start - previous_start
latency = current_start - target_wakeup
jitter  = maximum_period - minimum_period

Record the following during testing:

• Test period and duration.
• Maximum, minimum, and percentile latency, rather than only the average.
• Number of slaves, PDO size, and DC configuration.
• CPU binding, IRQ binding, frequency governor, and system load.
• Whether a dedicated network driver, TSN, or hardware timestamps are used.

First use cyclictest to evaluate system scheduling latency, then run the actual EtherCAT application to evaluate the end-to-end cycle.

7.5 Packet Capture

Use Wireshark or tcpdump to inspect EtherCAT frames:

sudo tcpdump -i eth0 -e -n ether proto 0x88a4

Packet capture adds load and should be used only for diagnostics, not run continuously on a real-time system. When a dedicated driver is used, a standard network capture interface may not observe every frame.