14.3.1.6        Communication Mode

OPC Servers can be “polled” by the clients (synchronous communication).   Most OPC servers also can “advise” the clients when a value has changed (asynchronous communication). Use “Sync and Async” if possible; this will reduce network load (why read data that has not changed?).

When Sync Only is chosen, the WebAccess SCADA node will use the Scan Time to read data from the OPC Server. If Sync and Async is chosen, the SCAN time is ignored and WebAccess waits for the OPC Server to “advise” the SCADA Node of changes in the values of tags.

Almost all OPC Severs support “synchronous” polling by clients. If your OPC Sever does not support Asynchronous Mode (report by exception), then you should use Synch Only. Otherwise, the OPC Server may not understand the Advise request for Asynchronous mode and hang.

The * (8000) error in Point Info Dialog box is typical of communications to an OPC Server that does not support the Asynch mode. Try “Synch Only” if you get the *(8000) error.

Sync Only

Almost all OPC Severs support “synchronous” polling by clients.  The OPC Server responds only if the Client asks for the data (polls). When Sync Only is chosen, the WebAccess SCADA node will use the Scan Time to read data from the OPC Server.

If your OPC Sever does not support Asynchronous Mode (report by exception), then you should use Synch Only.

KMC Controls is an example of an OPC Server that uses Synch Only (synchronous only) mode.

Sync and Async

Many newer OPC Servers support asynchronous  communications mode where the OPC Server “advises” the Client when a value has changed.  WebAccess will also poll periodically to ensure communications exists. Asynchronous reporting by exception can reduce communication load on the server and networks.

If Sync and Async is chosen, the SCAN Time is ignored and WebAccess waits for the OPC Server to “advise” the SCADA Node of changes in the values of tags.

The * (8000) error in Point Info Dialog box is typical of communications to an OPC Server that does not support the Asynch mode.

Cimetrics Bacnet OPC Server, Kepware and LANSTAR II are examples of OPC Servers that support Asynch & Sync Modes.