ProSoft Technology MVI56-AFC - Manual (Page 126)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 126 of 316 ProSoft Technology, Inc.
February 25, 2011
Dynamic Context
In order for the AFC to manage the log-download session, it maintains a
"dynamic context" for the session, which includes in particular the number of the
earliest event to be retrieved by the next F&A cycle. The dynamic context exists
only while the session is open; the Setup phase opens the session and creates
the context, and the Completion phase closes the session and discards the
context. The dynamic context must not persist (the session remain open)
indefinitely. Therefore, in case that a session is abandoned before completion, or
otherwise fails due to problems such as loss of communication, a timeout is
implemented that automatically abandons the session (closes it without
completion) after a short period of inactivity; all Modbus transactions for the
session must occur at a rate faster than this timeout for the session to be
completed successfully.
One dynamic context is maintained for each port, permitting multiple hosts to
perform download sessions simultaneously; see Section 8 for more on this. The
backplane is deemed to be "port #0", so that a session may also be performed by
the PLC via the Modbus Gateway feature.
To ensure maximum possible security, strict constraints are placed on the
sequencing, addressing, and lengths of all Modbus transactions for the session.
Violation of these constraints causes the offending transaction, depending on its
nature, to be either rejected with a Modbus exception code or accepted but
ignored; in no case does an offending transaction cause change of state in the
AFC.