Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

Advanced considerations of IEC 60870-5 297
Transmission of integrated totals
This refers to the integration, or counting, of values in counters. The counting process is
carried out over a specific and known time interval. If this interval is completed prior to
the transmission of the counter values, then the terminology ‘transmission of integrated
totals’ applies. Because of significant and indeterminate delays between acquisition and
transmission that are typical of SCADA systems, these are invariably the type of totals
that are transmitted. They are totals for time periods that have been completed.
There are two general forms of data transmission, totals and incremental values. If frozen
copies of the counters at particular times are transmitted, then the incremental values
in each period are calculated in the master station. If the outstation counters are reset after
each freeze, then the frozen values transmitted are in fact incremental counts already.
Both types of count are accommodated by the standard. These are indicated within
counter commands by the QCC qualifier of counter interrogation field.
Parameter loading
Parameters are values that change the way a system operates. The protocol provides for a
two-phase procedure for parameter loading. This accommodates the common requirement
to change a number of parameters simultaneously. After loading parameters, they are all
activated simultaneously by a parameter activation command.
Test procedure
The test procedure is simply a loop-back test from the application level of one station to
another, and then return. It causes no operational or state change. The test ASDU C_TS is
transmitted to the controlled station and then returned by it.
File transfer
File transfer functions accommodate the fact that files may be larger than the maximum
size of an ASDU, and therefore must be broken up for transmission. A file may be organized
in sections and these into segments. The communications procedures are different
for control and monitoring directions. Files generated in outstation devices are notified
to the controlling station by sending a directory ASDU. When the controlling station
uploads a file, the source file in the outstation is deleted, releasing buffer space for other
files. Files transferred in the control direction are primarily used for downloading parameters
or programs. This is controlled by the controlling station, and so directories are not
used or transmitted in this direction.
9.2 Interoperability
9.2.1 General considerations
Interoperability under IEC 60870-5 refers to the ability of different systems to operate
together to provide the telecontrol functions in accordance with the protocol. To ensure
this will be the case, it is necessary to ensure that the various selections that can be made
under the protocol are all compatible within a system.
Under the protocol there are a number of selections that can be made governing parameters
such as address sizes, and selections of sub-sets of ASDUs for example. For any
particular device, for example an intelligent electronic device, support of only certain