Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

204 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Variable
Length Frame
L
Start 0x68
L
L
Start 0x68
C
A
A
Link
user
data
Checksum
End 0x16
ASDU
Data Unit
Identifier
Information
Information Elements
Object 1
Time Tag
Information Object Address
Information
Object 2
Type ID
Variable Structure Qualifier
Cause of Transmission
Common Address of ASDU
Information Object Address
Information Elements
Time Tag
Figure 8.15
Message structure under IEC 60870-5-101
The only important point to note about the relationship between the ASDU and the link
layer frame is that a maximum of just one ASDU is allowed per frame. This sets an upper
limit for the size of the ASDU at 255 octets, minus 2-3 octets for the link control and
address fields.
8.5.2 ASDU structure
The structure of the ASDU is in two main sections. These are the data unit identifier, and
the data itself, made up of one or more information objects. The data unit identifier defines
the specific type of data, provides addressing to identify the specific identity of the data,
and it includes additional information in the cause of transmission field. The fields of the
ASDU are now discussed in turn.
Type identification
The type identification field is a single-octet unsigned integer field. Its content is interpreted
as a code in the ranges shown below.
Bit 7 0
Type Code
Code Range
Purpose
Standard type definitions
Reserved for message routing – private
For special use – private
Figure 8.16
Type identification field