OPC UA communications information

Communication between the probe and the OPC Unified Architecture (UA) server consists of a sequence of messages. The probe issues a request, and the server returns a response. Each message has two parts:

  • A header that contains control and security information
  • A body that contains the actual request or response

The request header includes a number of parameters that are common to all service requests. The probe maintains these parameters as properties:

Table 1. Properties used in request headers

Property

Usage

RequestHeaderRequestHandle

Specifies the requestHandle assigned to one or more requests. The requestHandle can be used to cancel the requests. All outstanding requests with the matching requestHandle will be cancelled.

RequestHeaderReturnDiagnostics

An integer value that represents a bit mask that defines the diagnostic information the server is to return in the response. This value appears in the returnDiagnostics field of the header.

The value of this property can consist of 0 (zero) or a bit value available in the bit mask defined by the OPC-UA specification. The default value of 64 (0x0000 0040) indicates that the diagnostics to return is "OperationalLevel / LocalizedText". Specifying no value indicates that diagnostics are not to be returned.

The content of the diagnostic information is vendor-specific. Refer to the documentation for the OPC UA device your site uses for information about return diagnostic information.

RequestHeaderTimeoutHint

Specifies the timeout period (in milliseconds) for all HTTP requests sent to the OPC UA server. This value appears in the timeoutHint field of the header.

For a server this timeout is only a hint and can be used to cancel long running operations to free resources. If the server detects a timeout, it can cancel the operation by sending the Service result Bad_Timeout. The server should wait for the specified timeout period after it received the request before cancelling the operation. Specifying a value of 0 indicates no timeout period.

Ensure that all these properties have values that are suitable for the needs of your site.