Large Time Offset Between Master and Slave in IEEE 1588

Issue Details

When configuring IEEE 1588 (PTP) synchronization using NI PXI timing modules (for example, PXI-6682 as the Master and PXI-6683 as the Slave), you may notice a seemingly large time discrepancy between the two devices when referring to the “Current Time (local)” parameter.
As illustrated in the example:

If we calculate the difference between these timestamps:

• Master (PXI-6682H, Dev2): 4:02:22.783951696 PM 5/25/2026
• Slave (PXI-6683H, Dev3): 4:02:22.751024466 PM 5/25/2026

This results in an approximate time difference of:

Difference=22.783951696−22.751024466=0.03292723 seconds

Final result ≈ 0.03293 seconds = 32.93 ms

At first glance, this suggests that the slave device is approximately 32.93 ms behind the master, which appears significantly outside the PXI-6683 synchronization accuracy specification of ±25 ns.

Solution

Based on this observation alone, one might conclude that the system is not properly synchronized. However, it is important to note that this interpretation is misleading, as it relies on the direct comparison of the “Current Time (local)” values, which are influenced by software processing and display latency rather than reflecting the true hardware-level synchronization accuracy.

The correct way to identify the offset, by referring to the "Mean Path Delay" if we refer via NI MAX test panel but for LAbVIEW you can refer to "Offset from TR" . For the conclusion, the PXI-6683 slave is properly synchronized with the master. The large delay observed is a misinterpretation caused by Software latency and Manual timestamp comparison. Best practice is always to use Offset from TR for accurate synchronization verification.

To further validate this observation, you may also perform verification using LabVIEW.
In this case, testing was conducted using the NI shipping example VI: “Start 1588 and Wait for Quality.vi”.

Based on the results obtained from this example, the accurate time offset between the master and slave devices should be evaluated using the “Offset from TR (Time Reference)” parameter. This parameter provides a precise, hardware-based measurement of synchronization accuracy.

On the other hand, if the “Current Time” outputs are used and compared directly between the master and slave, it may indicate a significantly larger offset. This is because the displayed timestamps are subject to software processing and display latency, which introduces additional delay and does not reflect the true synchronization performance.

Referring to the test results above, manually comparing the displayed timestamps can lead to a substantial overestimation of the time offset. For instance, the observed difference may appear to be approximately 663.49 µs when using the “Current Time” values, whereas the actual offset measured using the correct method (Offset from TR) is approximately 1 ns, which is well within the expected specification.