Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

94 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
STATION
A
STATION
B
FCB=1
FCV=1
P3
Expected FCB = 1
Ind = Positive
S0
Ind = Data
Expected FCB = 0
FCB=0
FCV=1
P3
Ind = Positive
S0
Ind = Data
Expected FCB = 1
Function Code Key
P0 Reset Link
P1 Reset User Process
P2 Test Link
P3 User Data - Confirm Expected
P4 User Data - No Confirm
P9 Request Link Status
S0
S1
S11
S14
S15
Confirm - ACK
Confirm - NACK
Link Status
Non Functioning
Not Implemented
Figure 5.15
User data
The diagram shows the transmission of two consecutive user data frames by the
primary station. On the successful receipt of the first user data frame an ACK (confirm–
acknowledge) response is sent by the secondary station. The station also indicates the
receipt of user data and toggles the expected FCB. On receipt of the confirmation by the
primary, the FCB flag is toggled and another user data frame is sent.
In the event of an error in the received message, the frame is ignored and therefore
no confirmation is sent. In this case the primary will re-send the message after an appropriate
re-try delay time.
Unconfirmed send user data
This procedure is used for sending user data without requiring confirmation from the
secondary station. It may be used where the data is of low priority. The advantage is that
data can be transmitted at a greater rate, thus providing a better use of bandwidth.
However, the disadvantage is that errors in transmission will result in lost frames.
A frame found to have an error (by CRC checking) will be rejected by the secondary
station, but as no confirmation was expected by the primary there will be no way for the
primary to know that the message was unsuccessful.
Summary
• Used for low priority user data
• No confirmations are sent by secondary
• More efficient use of bandwidth