GeoSCADA communicates with Alley-Bradley PLC

[kangbin]

When GeoSCADA communicates with Alley-Bradley Compact Logix PLC through its integrated OPC server, GeoSCADA POLLs data from PLC. The POLL is synchronous or asynchronous?

[AndrewScott]

You not said which OPC DA client driver you're using (advanced or simple).

For the advanced driver the Read Type and Write Type settings on the OPC DA server define whether the synchronous or asynchronous interface is used, see Configure OPC DA Server Properties 

For the legacy simple driver the Read Mode and Synchronous Write settings on the OPC DA server define whether the synchronous or asynchronous interface is used, see Configuring the OPC Server Properties 

[BevanWeiss]

As Andrew mentioned...
But also you didn't mention what context you mean 'synchronous' / 'asynchronous' for, so Andrew has assumed you mean in the OPC context.

If you instead meant in regards to the PLC scan context.   Then this is a Rockwell specific aspect.  For the Logix platform (ControlLogix/CompactLogix etc) then HMI access is potentially asynchronous to PLC scan cycles.  It depends on which tags are being read/written, and how they are referenced from within the PLC logic.  But if you do your SCADA / PLC logic as though they not synchronous you will always be safe.