Time synchronization for data acquisition

All I/O Servers that support the SuiteLink protocol add a timestamp and quality stamp to plant data as the data is acquired.

It is important to understand how synchronization is handled between the timestamps for I/O Server, the computer clock for the IDAS(s), and the computer clock for the AVEVA Historian(s).

Note: This Historian-controlled time synchronization method is not recommended on slow networks. If you are running AVEVA Historian on a slow network, please use a tool suited for your network configuration to synchronize your clocks.

How time synchronization works

1. If you have multiple historians on your network, you should synchronize all computer clocks to a single universal time server using standard Windows functionality.

2. Periodically, a Historian automatically synchronizes the computer clock of any remote IDASs to its own computer clock. The IDAS synchronization is enabled by means of the TimeSyncIODrivers system parameter.

3. Every hour, an IDAS automatically synchronizes the timestamping mechanism of any associated I/O Servers with its own computer clock. This does not actually change the system clocks of any I/O Server computers. Instead, the difference in the system clocks on the two computers (I/O server and Historian) are determined, and a bias is calculated that is then applied to all values from that I/O server computer. For example, if the Historian clock is seven seconds ahead of the I/O Server computer's clock, SuiteLink adds seven seconds to every timestamp from the I/O Server. If a topic is disconnected/reconnected due to a topic time-out or other communications failure, the I/O Server timestamping is not updated until the time synchronization interval has passed. You can change the frequency of the synchronization using the SuiteLinkTimeSyncInterval system parameter.

   The SuiteLink protocol also does some time adjustments to keep timestamps consistent across nodes. SuiteLink bases this adjustment on the time difference detected at startup and each hour. For example, NodeA and NodeB have a time difference of 17 seconds. The I/O Server is on NodeA, and the IDAS is on NodeB (either local to the Historian or a remote IDAS for a Historian on another NodeC). When the I/O Server on NodeA timestamps a value at 12:00:00.000, it is transmitted to NodeB with an adjusted timestamp of 12:00:17.000. If the Historian is configured to timestamp at the source, this value is stored with a timestamp of 12:00:17.000. If, instead, the Historian is configured to timestamp at the server, and there is a two-second communications latency, then the value is stored with a timestamp of 12:00:19.000.

   For normal operations on systems with synchronized clocks, there is no adjustment made by SuiteLink and everything operates as expected. However, when either the systems are out of sync, or even were out of sync when SuiteLink communications between the nodes started, the timestamps will be adjusted. Because of the way SuiteLink adjusts timestamps, it is easy to produce misleading results if system tests involve adjusting system clocks on the systems, because SuiteLink does not immediately update its time skew.

   Note: Time synchronization does not apply to I/O Servers that use DDE because these servers do not perform timestamping. The time of the IDAS computer is always used for data coming from DDE from I/O Servers.

For more information on setting system parameters, see Edit system parameters.

The following diagram shows an example of how computers can be synchronized to a single time:

For an MDAS-enabled or HCAL-enabled client application, you can use the net time command (for the Windows operating system) to synchronize the client computer's clock to your primary Historian.