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17.5 Election of Priority Controller
Since the LAN logic requires that the various units that are part of the same group work in a coordinated way, it is necessary to
have a unit that plays the role of Priority and that coordinates all the others.
Bearing in mind that the composition of the group may change, both due to voluntary actions (e.g. power failure of one of the
units for maintenance) and involuntary actions (failure of the controller of one of the units), and that such actions could also
occur on the Priority unit, a logic is implemented whose purpose is to dynamically elect one of the units of the group to the role
of Priority unit.
The address of the unit that will assume the role of Priority is established in two stages:
1.
in the first stage the addresses of the units that can take on the role of Priority units are selected;
2.
in the second stage, the address of the unit that will play this role is determined, starting from those selected in the first
stage.
The addresses of the first stage are chosen by making the following checks:
1.
all units with a pLAN address between 1 and P26.01 are considered;
2.
among these units, only those indicated as belonging to the same group by means of parameter P27.25 are considered;
3.
among these units, only those that communicate with one another are considered.
Then, in the second stage, the lowest of the following is chosen as the Priority unit address:
1.
those of the units designated as Forced Priority units (P26.27), if set;
2.
those of the units designated as Priority units (P26.26) on which the S601 signal is not active, if set;
3.
those chosen in the first stage on which the S601 signal is not active.
The operations described are carried out at each program cycle and consequently a variation in the result of the first stage, for
example due to the disconnection of a unit, may lead to the election of a new Priority unit.
The switching of a unit from Priority to Secondary takes place with a delay of 5 seconds from the designation of a new Priority
unit. This is to prevent the units from losing the values of the parameters exchanged through the LAN network due to a
momentary unavailability of the new Priority, for example following the restoration of the units after a blackout.
The 5 second delay allows the last designated Priority to send the parameter values through the LAN network to the Secondary
units before becoming Slave itself following the designation of a new Priority.
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