wilamowski-b-m-irwin-j-d-industrial-communication-systems-2011

58-8 Industrial Communication Systems
• DNP3 permits the use of “unsolicited reporting” on any channel where the devices are able to detect
channel activity in order to avoid collisions. In this mode of reporting, an outstation can send an
event notification to a master station without being polled. The approximately equivalent reporting
mode for T101 requires the use of the balanced data link rules and needs a dedicated point-to-point
full-duplex link between the master station and each outstation so that either device is permitted
to transmit at any time with no possibility of message collision. As will be noted in Figure 58.2, the
number of messages is the same in balanced and unbalanced modes, with the same application layer
data being transferred in both modes. The order and content of the “link layer” messages differs
slightly between the two modes. In DNP3, the use of unsolicited reporting removes the periodic
polling for events, reducing the number of messages as shown in Figure 58.3.
Due to the limitation of a single data type per message and a single COT per message, most command
sequences in T101 and T104 require many message transactions. In T101, each transaction requires that
a message be sent in each direction. For example, the two-pass control command on a T101 link shown
in Figure 58.2 typically involves 12 messages.
The equivalent transaction for DNP3 is shown in Figure 58.3, requiring that seven messages are
sent in systems where the master polls the outstations, or six messages if unsolicited reporting is used.
Similar distinctions between the protocols occur for most kinds of transactions.
When operating over TCP/IP, the difference between T104 and DNP3 is less pronounced and more
difficult to evaluate. For some transactions, T104 will be more efficient when it is able to send a stream
of information frames and receive a single confirmation for the set of frames (equivalent to removing
Unbalanced data link Balanced data link
Select control ACT
Select control ACT
ACK
ACK
Class poll
Select control ACTCON
Select control ACTCON
ACK
Execute control ACT
Execute control ACT
ACK
ACK
Class poll
Execute control ACTCON
Execute control ACTCON
ACK
Class poll
Data change
Data change
ACK
Class poll
Control ACTTERM
Control ACTTERM
ACK
FIGURE 58.2
T101 Two-pass command sequence.
Polled operation
Select request
Select response
Execute request
Execute response
Read events
Response with data update
Confirm
Unsolicited operation
Select request
Select response
Execute request
Execute response
Unsolicited data update
Confirm
FIGURE 58.3
DNP3 Two-pass command sequence.
© 2011 by Taylor and Francis Group, LLC