ProSoft Technology MVI56-AFC - Manual (Page 133)

MVI56-AFC ♦ ControlLogix Platform Events
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 133 of 316
February 25, 2011
Setup Phase
This phase, which opens a download session, may be accomplished in one of
two ways:
The Detailed Method
Issue a Modbus write of two registers to offset 0 of the LDW, specifying function
Open Session (1) and the desired Session ID followed by the number of the
desired starting event.
The Quick Method
Issue a Modbus write of a single register to offset 0 of the LDW, specifying
function Open Session (1) and the desired Session ID. This is equivalent to the
Detailed method in which the starting event is copied from the download pointer
in the event log header.
The dynamic context for the session includes two event numbers: one is the
Session Download Pointer (SDP), which is the number of the first event to be
retrieved by the next F&A cycle, and the other is the File Download Pointer
(FDP), which is the number of the earliest event on file that has not yet been
downloaded. The Setup phase specifies, explicitly or implicitly, the SDP, and
initializes the FDP from the download pointer in the event log header, so the SDP
and FDP need not be the same number.
Download Phase
This phase performs the actual retrieval of logged events as a sequence of F&A
cycles. All transactions must place the Session ID into the first register of data
(writes), or into the "number of registers" field of the Modbus command packet
(reads), as described in Section 5.
Fetch Transaction
Issue a Modbus holding-register read (function 3) addressed to offset 0 of the
LDW and with length calculated as (4+n*8), where n is the number of events to
be read and must lie between 0 and 14 inclusive; any other length constitutes an
error. Note that a read of 0 events is permitted, so that the LDW header can be
inspected without changing the session’s dynamic context; however, such a fetch
must still be acknowledged in the manner described next. The response is a
block of the requested length formatted as described in Section 6, whose second
register at offset 1 is the current value of the SDP.