4.2.5.1 DAIS::DataAccess::IO Overview

   These are definitions for transmitting item values to clients. Interfaces are defined for server side read and write operations and client side callback operations. Clients shall implement the Callback object for the server to use at transfer of data. A client may have any number of callback objects. The client shall connect each callback object to a server object implementing ConnectionPoint.

   The IO interfaces support three different ways to read data and two different ways to write data.

• Synchronous read where the data is received at return from the sync_read() method.

• Asynchronous read where the data is returned at the Callback object.

• Subscription where data is sent spontaneously by the server at the callback object.

• Synchronous write returning to the client once all the written data has reached the devices.

• Asynchronous write returning when the data is received by the DAIS server. A callback on Callback is sent by the server once the written data has reached the devices.

   Each item value is transmitted in a struct with a timestamp and quality indication. A sequence of this struct is either sent via the callback object or directly in read or write calls.

   In OPC the IO interface corresponds to the OPC interfaces IOPCSyncIO, IOPCAsyncIO, and IOPCDataCallback.

   Figure 4-12 DAIS data access IO IDL in UML

   The DAISConnectionPoint callback() methods correspond to a get or set method for the callback attribute.