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Ethernet is now becoming a popular physical layer for industrial communications due to its reliability, performance, and widespread use. Its actual cutting edge is a minimum cycle time on the order of 100 mus and a jitter of 1 mus. Usually, high-cost laboratory devices are used to log network traffic, but they are still not suitable for an industrial environment. In this paper, a field-programmable-gate-array-based instrument is proposed to log 100-Mb/s Ethernet frames, assigning them a high-resolution time stamp. Full-duplex full-rate network traffic is duplicated and sent through two auxiliary Ethernet ports to a personal-computer-based monitor station for frame analysis. A prototype has been realized to verify feasibility and performance. Results show that the proposed instrument introduces a maximum delay of about 500 ns. The measurement capabilities are comparable with commercial instruments: The time-stamp assignment has a jitter of 150 ns measured on a test frame set.