Where do OPC timestamps come from

OPC Data Access servers provide three attributes for every point: a value, a quality and a timestamp.  OPC specifies that a timestamp must be provided for each point, but it does not specify where the timestamp must come from. It may be supplied by the OPC server, or it may be passed up from the underlying system.  Sometimes a timestamp is not available from the device or protocol interface. For example, Modbus does not provide a timestamp from the PLC. In this situation, the OPC Server provides its own timestamp. But some device vendors do provide a timestamp with each value. So when the OPC Server receives a reading, it also receives the timestamp from the PLC and passes it on.

 

An OPC Server may be designed to ignore a timestamp, even if one is available. Since OPC does not specify where the timestamp must come from, sometimes OPC Server vendors choose to ignore the device timestamp because this takes less development effort.  It some OPC Servers where the timestamp comes from may be a configurable option.  Although the general convention is, ‘if the device supplies a timestamp use it’, there is no guarantee that is always the case.   I highly recommend asking your vendor where their timestamp comes from.  In OPC UA each point can supply: a value, a status quality, a source timestamp and a server timestamp.  The rule is a source timestamp is always passed from the underlying source, and is never overwritten by the server.

 

For more on the basics of OPC refer to the OPC Tutorials section.