Transparent clock improves network timing
To get the most out of IEEE 1588 in an instrumentation network, you need Ethernet switches that support the standard.
Martin Rowe, Senior Technical Editor -- Test & Measurement World, 5/1/2010 12:00:00 AM
When Ethernet-based measurement systems call for precise timing, you can use instruments and network switches that comply with the IEEE 1588–2008 Precision Time Protocol to synchronize clocks (Ref 1). To get the most out of IEEE 1588 in an instrumentation network, you need Ethernet switches that support the standard. These switches use a transparent clock that minimizes latency and delays in the network by providing a local clock for network nodes rather that letting instruments rely on the IEEE master clock.
Researchers at the School of Electrical Engineering and Computer Science at Seoul National University in Korea set out to prove how a transparent clock can minimize timing with errors as low as 30 ns—a result that goes beyond the 100-ns error tolerance specified in IEEE 1588. Their paper is available from a link in the online version of this article (Ref. 2).
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The IEEE 1588 master clock and Ethernet switches use an FPGA (field-programmable gate array) developed by the researchers to implement the transparent clock. The researchers compared the timing errors between identical network configurations using switches with and without the transparent clock. Using a four-channel oscilloscope, the researchers found maximum errors of about 30 ns with the transparent clock (figure). When using standard Ethernet switches, errors increased by 1000X to 30 µs for slave 3. The network used Gigabit Ethernet running at 50% capacity. Without traffic, timing errors for slave 3 were about 500 ns.
The FPGA consists of the Ethernet switch fabric, transmitter, receiver, clock, and MAC (media access control). A GMII (gigabit media-independent interface) connects the functional blocks to a PHY (physical-layer interface) IC. The paper's authors report that the network latency varies depending on the PHY manufacturer, which adds a few cycles of timing uncertainty to the measurements.
IEEE 1588 is closely aligned with the LXI Standard, which requires that all Class A and B instruments conform with the timing protocol. But a network need not be based on LXI to reap the benefits IEEE 1588 offers.
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