Value/status points

The InstrumentTag is used to specify the variation of the DNP Object type. The variation describes the type of data being requested. For example, the user may be requesting 32-bit or 16-bit, with or without a parameter. A blank InstrumentTag informs the interface to include a variation type of 0 in the request for data. A variation value of 0 is a request for the default variation the remote device supports. If a variation is found in the InstrumentTag and if the point is not an event type point belonging to an event scan class, it will be included in the request to the DNP slave device. The DNP slave device is not required to respond with the exact variation requested. However, if a specific variation is required, a variation that contains a parameter for example, then it can be requested.

The following is an excerpt from the DNP V3.00 Application Layer Protocol Description regarding the DNP data type variation:

• The object field is two octets in length. The first octet specifies the general type of data (analog inputs) and the second octet specifies the variation of the data type (16-bit analog inputs or 32-bit analog inputs). In the request direction, if the object variation is specified as 0, this indicates all object variations belonging to the same group (all or any analog inputs types). In the response direction however, variation 0 cannot be used to specify the objects. The specific variation has to be specified.

• Consider the example where the request message specifies counter objects in the first octet and the variation is 0. Given that the Outstation supports only 16-bit counters, it will respond with an object header with the variation field set to indicate that the counter objects are 16-bit counters.

• With the same request message directed to another station, the returned object header may indicate a 32-bit count field in the counter objects it returns. By requesting data with the variation set to 0, it is not necessary for the master to know what variations the Outstation supports. The master must however be able to interpret the object headers and have some knowledge of the structure of each variation.

Heartbeat points

• Set the InstrumentTag = HEARTBEAT

Reset link counter point

• Set the InstrumentTag = RESETLINK

Counter freeze request point

• When the freezing of the object 20 counters is required, set the InstrumentTag = FREEZE

Octet string point

• This value must be set to a value of 1-255.

  It is recommended to use a variation of 255 when creating PI points to read an Octet String in order to read the maximum size of an Octet String. If the size of the Octet String received from the RTU is less than 255, the length of the Octet String stored in the PI Data Archive will reflect the smaller size.