Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

188 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
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Figure 8.5
Bit-sequence representation of FT1.2 Frame
From the bit-sequence representation above, it is possible to see that the maximum data
rate of the frame is approximately eight-elevenths of the bit transmission rate. This is
reduced further when the frame overheads such as addressing, start and stop characters,
checksum, and control information are accounted for. These overheads can be seen in
Figure 8.6 which shows the overall frame structure.
Variable
Length Frame
Start 0x68
L
L
Start 0x68
C
A
A
Fixed
Length Frame
Start 0x10
C
A
A
Checksum
End 0x16
Single
Control Character
0xE5
L
Link
user
data
Checksum
End 0x16
Key:
L = Length 0.255
C = Control Field
A = A ddress Field
Figure 8.6
FT1.2 frame options under IEC 60870-5-101
The following points are noted about the data link frames:
• Only the variable length frame can carry user data
• The variable length frame can carry up to 253 octets of link user data
• The length L is repeated twice, and the two values of L must be equal for the
frame to be accepted as valid
• The maximum frame length is 261 octets. However, a lower maximum frame
length may be specified by a manufacturer or by the system user as a system
parameter