Read access

Determines whether read access to process variables in this group of the communication component should be synchronous or asynchronous.

Setting	Description of the
Synchronisation	The variable server forwards a read request to the communication component and waits until the process variable value is returned. Synchronous reading increases data consistency in the variable server, as there is always a wait after the request until the process variable value is returned. A disadvantage, however, is that the entire system may have to wait for the process variable value to be returned if the communication component does not return the data quickly enough.
Asynchronous	The variable server sends a request to read a process variable value to the communication component. After processing the read request, the communication component sends a message containing the process variable value to the variable server. The advantage is that the variable server continues to work between the request and the message.

For process variables from VisiWin drivers, this setting only has a function if the Enable synchronous communication parameter is activated. Otherwise, the variable requests are executed asynchronously.

Parameters for module:

• Variables

• Group