wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

DNP3 and IEC 60870-5 58-5
• Optional modes of operation where the field device can report change data without being polled
by the master station.
• Test procedures to verify the correct operation of a protocol implementation in a device.
• To support SOE functionality, devices incorporate event data buffers. The protocols define mechanisms
to verify the correct reporting of the data from these buffers: This data can be repeated if necessary
to ensure that it is correctly reported. Once verified, it can be cleared from the event buffers.
• T104 and DNP3 have standardized XML file definitions for mapping between their standard data
objects and data attributes of the Logical Node objects defined in the substation automation standard
IEC 61850. T101 can also make use of the T104 mapping process.
• DNP3 has a secure authentication procedure that is based on IEC 62351-5. The equivalent functionality
for T101 and T104 is under development at the time of writing.
Any device that implements these protocols can choose to implement only those parts of the protocol
(data objects and functions) needed to support the operation of that device. Each protocol has some
mandatory “housekeeping” functionality that must be implemented.
58.3 Differentiation between IEC 60870-5 and DNP3
Operating Philosophy, Message Formatting,
Efficiency, TCP/IP Transport
While T101, T104, and DNP3 share many common operational characteristics such as the use of RBE,
there are some detail differences in operating philosophy between the IEC 60870-5 protocols and DNP3.
These differences are evident in the data object and message formats and in the command sequences
used to manipulate them.
In some cases, the IEC 60870 and DNP3 protocol descriptions employ differing terminology to
describe the same thing and similar terminology to describe different things. This easily leads to
confusion, especially for people conversant with the details of one protocol who have relatively superficial
contact with the other. In such cases, the existing familiarity with one protocol readily leads to
misunderstanding or incorrect implementation of the other. Careful reading of both sets of specifications
can be necessary in order to clearly understand the similarities and distinctions of the protocols.
Sometimes, the end users also need to be aware of these details in order to correctly select configuration
options, etc.
An example of differing terminology: The master station equipment typically found in a SCADA
control center is called a “controlling station” in IEC 60870 and a “master” in DNP3; the substation
equipment such as a remote terminal unit (RTU) that reports substation data to the control centre is
called a “controlled station” in IEC 60870 and an “outstation” in DNP3.
An example of the same terms meaning somewhat different things exists in the use of data classes. In
IEC 60870-5, there are two data classes used for polling data when using the unbalanced data link procedures,
loosely corresponding to high priority (typically event data) in Class 1 and low priority (typically
cyclic analog measurements) in Class 2. In DNP3, there are four classes: Class 0 for reporting “Static Data,”
meaning the current value of all kinds of data; and Classes 1, 2, and 3 that are three separate priority
groups for event data. The mechanisms for deciding which class to read are different in the two protocols.
The principal tenet of the IEC 60870-5 series protocols is that the controlled station in the substation
should determine what data is to be sent to the controlling station (master station). In the balanced mode
of operation, the controlled station simply transmits whatever data it wishes at whatever time it wishes. The
controlling station simply acknowledges receipt of the data and issues control commands when required. In
the unbalanced mode of operation, the controlled station acts in a similar manner, but must wait for a poll
request (effectively an “invitation to transmit”) from the controlling station, to which it responds by sending
a single message. The controlling station has little control over what the controlled station will transmit,
other than being able to request “Class 1” or “Class 2” data. Normally the controlling station requests Class 2
© 2011 by Taylor and Francis Group, LLC