ProSoft Technology MVI56-AFC - Manual (Page 134)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 134 of 316 ProSoft Technology, Inc.
February 25, 2011
Acknowledge Transaction
There are four methods of acknowledgement, three of which may be used at any
time, and a session need not use any one consistently, even when repeating an
acknowledgement that has apparently failed:
1 Collapsed method - This method embeds the acknowledgement of the
previous Fetch transaction into the next Fetch transaction, as described in
Section 5.A Collapsed Acknowledgement bit value of 1 acknowledges the
previous Fetch; a 0, if the previous Fetch has not been explicitly
acknowledged by one of the other methods, elicits a repeat of the previous
block of events. The Collapsed Acknowledgement bit of the first Fetch of a
session must be 0.
2 Brief method - Issue a Modbus write of a single register to offset 0 of the
LDW, specifying function "Acknowledge Fetch" (0) with the correct Session
ID. Use this method to conserve bandwidth when use of the Collapsed
method is not possible.
3 Verbose method - Issue a Modbus write of (4+n*8) registers to offset 0 of
the LDW, that echoes the complete data block read by the Fetch transaction
except for insertion of the correct Session ID. The AFC verifies that all
register values, except those at offsets 0 and 3 of the LDW header, are the
same as were transmitted. Use this method for greater confidence of
acknowledgement when bandwidth is less of a concern.
4 Implicit method - The final Fetch transaction of a session can be implicitly
acknowledged by the Completion phase (7.3, next). Because of the potential
for undetected data corruption with the LRC of ASCII mode, only the Verbose
method is recommended for an ASCII-mode Modbus channel.
A successful F&A cycle adjusts the session’s dynamic context as follows:
A The SDP is advanced by the number of events returned by the fetch transaction.
B If at any time the SDP reaches the FDP, the FDP becomes "locked" to the SDP, thereafter
tracking the SDP so that it keeps the same value, until the end of the session.
This ensures that any update of the download pointer in the event log header
during Completion (7.3, next) is done only when it is guaranteed that all newly
downloaded events have been retrieved by the host.