ProSoft Technology MVI56-AFC - Manual (Page 125)

MVI56-AFC ♦ ControlLogix Platform Events
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 125 of 316
February 25, 2011
9.5.1 Basic Principles of Implementation
A properly committed log-download session has three phases:
Phases
Setup Phase
This phase opens the session by a single Modbus transaction to the AFC that
specifies the first event to be downloaded in the session. This event may be
identified explicitly by event number, or it may be conveniently identified as the
earliest event not yet previously downloaded.
Download Phase
This phase transfers event data during an open session by a sequence of zero or
more fetch-and-acknowledge (F&A) cycles. A first Modbus transaction reads a
short block of consecutive chronologically-ordered events from the AFC, and a
second transaction writes sufficient data to inform the AFC that the events just
read have been received without error; this pair of Modbus transactions
constitutes a single F&A cycle. The starting event of the first F&A cycle is the
event identified in the Setup phase. Each successful F&A cycle causes the
subsequent cycle, if any, to begin with the earliest event not included in the
previous cycle. The block of data read from the AFC includes, besides the events
themselves, information that identifies the first event in the block, the number of
events in the block, and the number of not-yet-downloaded events that remain in
the AFC following those in the block. A failed F&A cycle does not cause the
starting event to change, so it may be repeated as many times as is necessary to
accomplish success. All F&A Modbus transactions access a block of registers
beginning at the same address.
Completion Phase
This phase closes the session by a single Modbus transaction that updates the
download point in the event log header with the earliest non-downloaded event
and posts the Download event.