Practical_modern_SCADA_protocols_-_dnp3,_60870-5_and_Related_Systems

288 Practical Modern SCADA Protocols: DNP3, 60870.5 and Related Systems
Figure 9.1 shows the general sequence of initialization for the controlling station. After
the start of initialization, the link layer communications services will become established
by synchronizing across the link with controlled stations. This may occur before the
application level functions are fully initialized. When all application level functions are
available general interrogation and other functions are commenced. In the example
the controlling station commences a general interrogation following initialization from
‘cold’. In this case it has no controlled station data and so it must obtain the state of
all process data.
Controlling Station
Functions
Reset rem ote
station request
C_RP
Controlled Station
Functions
Start of Initialisation
Communications
R eady
Controlled Station
Initialised
M_EI
Station
Ready
Figure 9.2
General sequence for controlled station initialization
Figure 9.2 shows the initialization sequence for a controlled station. In this case a reset
process command ASDU is transmitted by the controlling station to the remote controlled
station. This resets the local station. Only after the link communications and all application
level functions are available is the end of initialization M_EI ASDU transmitted to the
controlling station.
An end of initialization ASDU must be transmitted by each controlled station following
its own initialization. In the case of multiple logical remote units in one physical device,
each must transmit a separate end of initialization ASDU. In this case there is one link
layer address, but more than one common address of ASDU for the device.
Link layer function
The link layer function has been presented elsewhere in this text and so only a brief review
is noted here. The link layer services are responsible for establishing the link, and they
proceed to do this immediately after being powered up or reset. On an unbalanced link,
the primary station repeatedly sends ‘request status of link’ until it gets a ‘status of link’
response. It then sends a ‘reset of remote link’ which synchronizes the frame count
bit (FCB) across the link. Once this sequence is completed the link is available for
communication services to the application layer. Balanced communications are basically
two unbalanced links operating in opposite directions, and so the synchronization process
occurs independently for each direction.