Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

214 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
object addresses, as for two-octet addressing. The information object address of zero is
reserved for the case when the address is not relevant. The information object address is
shown in Figure 8.20.
Data Unit
Identifier
Type ID
Variable Structure Qualifier
Cause of Transmission
Common Address of ASDU
Information Object Address
Information Information Elements
Object 1
Time Tag
Bit 7 6 5 0
Octet 1
Octet 2
Octet 3
Figure 8.20
Information object address
On a system basis, specific data is uniquely identified by the combination of the
common address and the information object address.
An example of how this might work in practice is where there are a number of identical
intelligent electronic devices IEDs connected to a sub-master RTU, which is in turn
connected to a master station. These could be re-closers on a distribution system. Each
IED will have an identical data structure internally, as determined by the device manufacturer.
Most likely this will be some tens of information elements or data points, which
could be addressed by a one-octet information object address. However, as the system
includes RTUs that have many more data points, two-octet addressing is used across the
system. At a system configuration level, there will be a single model for that type of
re-closer, and configuration of each device into the system database will be a matter of
using the standard point-mapping for that device type and adding the station number to
form unique point references.
8.5.4 Message addressing and routing
Control and monitor directions
An important concept in understanding addressing under IEC 60870-5 is the difference
between control and monitor directions. It is an assumption that the overall system has a
hierarchical structure involving centralized control. Under the protocol, every station is
either a controlling station or a controlled station. The communications network structure
will normally be aligned with this, and for unbalanced communications links the controlling
stations will be primaries, and the controlled stations will be secondaries at the link
level. This follows naturally from the fact that a hierarchical structure involves multiple
controlled stations, controlled by one or at least few controlling stations.
In such a system, control messages such as commands or interrogations are transmitted
by the controlling station, and these result in actions and return information transmitted
by the controlled station.
Addressing of ASDUs
Messages are addressed in the control direction by the common address field of the
ASDU. This address field is one or two octets, and defines the station (or logical station)