ProSoft Technology MVI56-AFC - Manual (Page 137)

MVI56-AFC ♦ ControlLogix Platform Events
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 137 of 316
February 25, 2011
9.5.7 Access by Multiple Hosts
The functionality specified in this document can permit complete event-log
retrieval by multiple hosts, provided that these conditions are satisfied:
1 As the session’s dynamic context is local to the accessed port, multiple hosts
may perform sessions simultaneously provided that they access separate
ports. The Session ID is part of the dynamic context, so separate-port
sessions may use the same Session ID without ambiguity.
2 For multiple hosts that access the same port (using Modbus Master
arbitration or a similar scheme), all must perform their sessions at times
sufficiently separated so that one host does not interfere by disturbing the
dynamic context of another host’s session in an unpredictable manner. The
Session ID can provide significant protection against inadvertent infringement
of this condition.
3 One host must be the Active host, performing the Completion phase that
updates the AFC’s event log state (download pointer). All other hosts must
be Passive, failing to Complete their sessions but instead Abandoning them.
If this condition is disregarded, so that multiple Active hosts perform
simultaneous sessions each ending with the Completion phase, the AFC’s
Event Log, which is global, manages any updating of the download pointer
and posting of the Download event in a globally consistent manner, but each
host cannot be sure that the Download event written upon Completion, if any,
is exactly what it expected.
4 Each host must, in one way or another, have access to its own long-term
download context, which is the number of the earliest event not yet
downloaded by that host. All Passive hosts must maintain this context locally.
The Active host may let the AFC maintain its long-term context, using the
download pointer in the event log header for this purpose; in such a case the
same host must always be the Active one. If, however, each host regardless
of role maintains its own long-term context, the role of Active host may be
passed around among hosts.
5 All hosts must perform download sessions sufficiently often so that events are
not lost by being overwritten by newer ones before those events have been
downloaded by that host.