Spectracom - Essential Ingenuity

Troubleshooting Holdover

Question

Why is the time server going into Holdover mode and how do I troubleshoot it?

Answer

Generally Holdover mode occurs when the unit has been synchronized since it last powered-up but it no longer has any valid references to maintain synchronization. Another situation that will indicate a holdover condition is if the time figure of merit (TFOM) exceeds the user-configurable MaxTFOM value even if references are connected. Check to see if the MaxTFOM has been configured to anything other than the default value of 15. See About MaxTFOM for more information.

Let’s consider the typical case of synchronizing the unit to GPS (but the behavior is the same if synchronizing to another reference such as IRIG).

Upon the loss of GPS signal, the unit will enter Holdover for a period of time when the time will be held very accurate and useable by all devices given by the performance of the internal 10 MHz oscillator. Holdover is the result of the GPS receiver tracking 0 satellites, even for one second. If GPS reception is restored (declared valid because the receiver is once again tracking at least one satellite) before the Holdover period expires, the System Time will be corrected as needed and the unit will be back to the fully Synchronized state. Otherwise, if GPS reception is not restored before the Holdover period expires the unit will go out of sync. In this state, it is likely connected devices will ignore any timing signal from the unit.

Many factors, including site-related issues (such as antenna cable issues or the GPS antenna not having a clear view of the sky), a faulty antenna or receiver will prevent tracking at least one GPS satellite at all times. Therefore, we recommend some basic troubleshooting to determine the source of the problem. See these documents to diagnose unexpected antenna problems such as a short in the cable:

Troubleshooting GPS for SecureSync
Troubleshooting GPS for NetClock 9483 and 9489

When the unit reestablishes connection with the synchronization signal, several minutes later, the front panel Sync LED will turn green. A few minutes after that the Fault LED should turn off, indicating all alarm conditions have cleared.

Please note that after the initial installation (including locating its antenna outdoors with a good view of the sky), it could take about 45 minutes for synchronization to occur and the Sync LED to turn green. If the unit is subsequently rebooted/power cycled, it should resync in just a few minutes.

There are 2 other cases that can cause a holdover condition:
  1. The unit switches between input references (GPS and an IRIG generator are set as references). If the references aren’t precisely synced to each other, then the Time/1PPS difference will inherently cause the TFOM value to change. If the TFOM value exceeds than the MaxTFOM value then the Holdover condition occurs.
  2. There is a problem with the internal 10MHz oscillator.

Data contained in various log files can help troubleshooting synchronization issues. See Using Log Files to Troubleshoot Synchronization for more information.

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