Effects of Considerable Server Time Changes

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Time-stamping is critical to the successful operation of ClearSCADA, and the software is not designed to handle sudden, large jumps in the PC clock. Typically, sudden PC clock changes cause the following types of problem:

Unexpected Pauses in Operation

ClearSCADA uses the PC clock for the timing of internal house-keeping activities. If the time of the clock suddenly moves backwards, the amount of time by which the clock has changed will cause a delay of equal time on some of the timed activities. For example, if the PC clock changes to three minutes earlier, a delay of three minutes will be added to the timed activities, causing an unexpected delay in system operation.

High load

Moving the time of the PC clock forward causes some timed activities to expire prematurely, which can cause an avalanche of activity. For example, if a system is set to poll every minute, it is quite common to have the polls staggered so that they do not place a high load on the system. If the PC clock moves forward, the timing of the staggered polls can be disrupted and may lead to several polls occurring at the same time (placing a high load on the system).

Out Of Sequence Data Timestamps

When a driver reads current data from a device, the driver time-stamps that data with the time of the PC clock. If the time of the PC clock is moved backwards, a new current data update will be stamped with a time that is earlier than the time of the previous update.

Recommendation

To avoid problems with PC clock jumps, we recommend using a time synchronization program such as NTP. This will synchronize the PC clock by adding or subtracting small increments so that the PC clock drifts to the correct time. By drifting the clock in small increments, there is no sudden change in the PC clock time and so the problems associated with PC clock jumps are avoided. NTP can synchronize the PC clock to an Internet time source or a GPS time source.

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