Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Advanced considerations of IEC 60870-5 293
Order of information transmission
When both event data and static data are transmitted from a station, it is important to
ensure that processing occurs in the correct sequence. For example, if a change event for
a point is generated after a static data read, but the receipt and processing of these at the
controlling station occur in reverse order, then the resulting image for that point in the
controlling station database will be incorrect. To prevent this occurring there is a defined
priority order of transmission that a controlled station must follow when transmitting data
to a controlling station.
This order is designed to ensure that the most topical data is delivered last. The specified
order is shown in Table 9.1. This priority order does not prevent the controlling station
from requesting data in any different order. For details of particular ASDU priorities
the reader should refer directly to the standard, which lists specific type numbers for each
priority.
Priority
Type of Information
Sequence
1 End of initialisation
2 Commands
Event reporting
3
Acquisition of transm ission delay
Clock synchronisation
Read command, Test procedure, reset process,
4
param eter loading
5 Station interrogation, transm ission of integrated totals
6 Cyclic data transmission and file transfer
Table 9.1
Order of transmission of information by controlled station
The reader may note an apparent conflict between the specified order of transmission,
and the fact that the link layer operation causes transmission of class 2 data prior to
commencing any class 1 data transmission from a controlled station. This is the case
because it is only after a frame is transmitted with the ACD bit set that the controlling
station becomes aware that there is class 1 data awaiting transmission.
It is probably for this reason that class 2 is generally used for cyclic analog data only,
or not at all. With analog data an error in processing sequence is unlikely to cause any
significant problem. All other data is typically treated as class 1, and is transmitted in the
specified priority order, with current static data last.
Double transmission
Double transmission refers to the transmission of both non-time-tagged and time-tagged
events arising from single status changes. This is allowed under IEC 60870-5 for all
information types which may be transmitted spontaneously, that is with the cause of transmission
COT code = . This reflects the different usage of the time-tagged data from
the untagged data. The time-tagged data is typically used to provide a sequence of event
record which may be used for the display of trends, or directly by human operators to
analyze event sequences. The non-time-tagged data however may be used directly in
logical control of the system. These two types of data may be transmitted with different
priority, with the non-time-tagged data being the higher priority.