ProSoft Technology MVI56-AFC - Manual (Page 131)

MVI56-AFC ♦ ControlLogix Platform Events
Liquid and Gas Flow Computer User Manual
ProSoft Technology, Inc. Page 131 of 316
February 25, 2011
9.5.5 The Log-Download Window (LDW)
The LDW, located at Primary Slave holding register 65400 and optionally
mapped to the Virtual Slave by register Ph00147 consists of a header of 4
registers followed by up to fourteen 8-register event records. Accordingly, the
size of the block may be as large as (4+14*8) registers, that is, 116. The block is
returned as the data in the Fetch transaction of a F&A cycle. Certain subsets of
the block may be read or written at other times.
Layout
The header has this layout:
1 LDW+000: Function and Session ID
When written, this register is interpreted as two bytes, where:
The low-order byte contains a function code:
0: Acknowledge Fetch
1: Open Session
2: Commit Session
3: Abandon Session
All other function codes are rejected with Modbus exception 3, "Illegal Data".
The high-order byte contains the Session ID. This ID is specified by the host
upon opening the session and it must be matched by all subsequent accesses
during the session.
When read, this register is always zero.
LDW+001: Starting Event Number
This value is initialized to the number of the event selected by the Setup phase.
After every successful F&A cycle, it is advanced by the number of events fetched
by that cycle.
LDW+002: Number of Events This Block
This is the number of events returned by the Fetch transaction of a F&A cycle. It
never exceeds the number of events that can fit inside the size of the data block
requested, but it may be smaller, such as when fewer events are available than
the number requested. In the latter case, the extra unreturned event records are
all zero.
LDW+003: Number of Non-Downloaded Events After This Block
This is the number of events remaining in the log that have not yet been
downloaded, not counting the events in the current Fetch.