Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Advanced considerations of distributed network protocol 163
used to return very limited data. This would consume only a small bandwidth, but would
prove the continued operation of the communications system just as well as a larger data
poll would.
Sometimes polled static operation will be used for particular critical tasks, combined
with unsolicited report-by-exception for most data acquisition.
The minimum implementation mode supported is defined by the sub-set definition documents
to be polled report-by-exception. However, the documents recommend that one
of the reporting by exception modes should be available for any DNP3 implementation.
To this effect, the following specific recommendations are made:
• All slave devices should report event data
• All master devices should support a report-by-exception mode of operation
6.6 Time synchronization
6.6.1 General method for time synchronization
One of the important SCADA features of DNP3 is that it provides for time-stamping
of events. Time stamping in DNP3 provides resolution of events to one-millisecond.
For events to match up correctly across a system, it is essential that the clocks in all outstations
are synchronized with the master station clock.
Synchronizing of an outstation clock is done by sending a time and date object (object
50, variation 1) to the outstation. However, there is a finite delay in transmission from the
master to the outstation, and if this is not accounted for when setting the clock, it would be
set retarded by this transmission time. This delay time can be due to store-and-forward
delays introduced by a variety of devices along the transmission path, including:
• Time in modem buffers
• Time in data radio buffers
• Time in intermediate repeater store-and-forward buffers
In addition to these, the reader might also add the possibility of processing delays
between the application level and the lower levels, or ‘stack delays’ at both master and
outstation. These would indeed be significant, except that the DNP3 specification accounts
for these by requiring some of the time synchronization function to be performed at the
data link layer. The data link is required to record the time of transmission or receipt of the
first bit of any delay time message. When transmitting the message, the data link layer
triggers a freeze of the master station clock, which is stored as MasterSendTime. Similarly,
when a slave station receives a delay time message it must store a freeze of the local clock
as RtuReceiveTime.
DNP3 provides for measurement of the round trip delay time by Function Code 23
Delay Measurement. The procedure is shown below.
Delay measurement procedure:
• Master sends Function Code 23 Delay Measurement
• Master records start of transmission time as MasterSendTime
• Outstation records the receipt time as RtuReceiveTime
• Outstation commences to send a response and records time as RtuSendTime
• Outstation calculates its internal processing turnaround time RtuTurnAround
which is RtuSendTime – RtuReceiveTime