Communication between OPC server and OPC client

Most of OPC clients support both the synchronous and asynchronous communication with OPC server.

Synchronous communication

It is based on the periodic queries (so-called polling) of OPC client to a data change on the OPC server. If data has been changed since last query, they are sent to the OPC client. The synchronous communication causes a high load of transmission channel than the asynchronous one. In addition, it does not allow a fast monitoring of value changes - if the query period is for example 1 second and the data are changed more than once within a second, then the OPC client gets only the last value within a second.

Asynchronous communication

Using the type of communication, the OPC server will enforce a transmission of data, when they have been changed. The parameter Update rate of OPC client defines the transmission period for sending data from the server to the client. It is defined in milliseconds and it defines the minimal time interval for sending data to client. If the data are changed more quickly than the the interval, the OPC server sends only last value from given interval. If Update rate = 0, then OPC serve sends each value change to the client.