Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Fundamentals of IEC 60870-5 237
FBP
Fixed test bit pattern, two octet
7 6 5 4 3 2 1 0
1 0 1 0 1 0 1 0
0 1 0 1 0 1 0 1
8.7 Set of ASDUs
Key
Pattern
Fixed Value UI16[1..16]
This section presents the set of application service data units or ASDUs defined under IEC
60870-5-101. ASDUs are the application level units of data that are used to convey
SCADA information between controlling stations and controlled stations. Thus, there are
ASDUs for each type of information carrying, such as sending control parameters,
sending set-points, sending commands to set or clear particular status points, and for
sending data such as measured analog values or counter values.
In Figure 9.17, the overall structure of the ASDUs was presented. This showed that each
ASDU is made up of a data unit identifier followed by one or more information objects.
The reader will recall that there are two types of structures that can be used, and these
depend on the state of SQ bit of the variable structure qualifier field, which is in the data
unit identifier. If the SQ bit is zero, then multiple information objects are allowed,
between 1 and 127.
The two structures are shown in simplified form in Figures 8.24 and 8.25 following.
Data Unit Identifier
Information Object 1
Information Object 2
This is the structure
with SQ = 0, which
implies a sequence of
information objects.
Each object has its
own information object
address.
Information Object N
Figure 8.24
Simplified structure of ASDU with SQ=0