OPC Servers always send OPC Client applications information on the Value, Quality, and Timestamp of an item. But some control systems do not provide a timestamp. In this case, the OPC Server sends the PC timestamp, which is the only timestamp it has available.
When the OPC Server connects to control systems that have information on the timestamp, the OPC Server has a choice. It can either use the timestamp from the control system, or it can use the timestamp from the PC. Some OPC Servers even enable the integrator to make this choice.
The ability to use the timestamp from the control system and even to select the source of the timestamp is a differentiating feature amongst OPC Servers. The OPC Training Institute recommends that System Architects investigate the timestamp requirements. For example, a high-speed data acquisition to record a Sequence of Events (SOE) typically requires the timestamp to come from the control system. However, if the OPC Server is only used in conjunction with an HMI (Human Machine Interface), the control system timestamp may not be required.