ProSoft Technology MVI56-AFC - Manual (Page 114)

Events MVI56-AFC ♦ ControlLogix Platform
User Manual Liquid and Gas Flow Computer
Page 114 of 316 ProSoft Technology, Inc.
February 25, 2011
9.1 The Event Log
An "event" is any occurrence that may affect the manner in which, or whether,
measurement is performed. Events include, for example:

Any change to a sealable parameter.

Power-up (product may have been lost during the power-down period).

A change in PLC operating mode (programming changes may alter
measurement).

A download of the event log (for audit trail purposes).
The Event Log occupies a block of 16000 Input registers in the Modbus table
starting at address 40000 and proceeding through address 55999. It consists of a
5-register "header" at address 40000 followed by 1999 8-register "event" records
starting at address 40008. As they are Input registers (read with Modbus function
code 4), no part of the Event Log can be written from outside the module, but it is
maintained exclusively by the AFC firmware.
As events occur they are recorded in the Log, which acts as a circular file. Each
new event record overwrites the oldest one, hence the log stores up to 1999 of
the most recent events. As each record is written the values in the header are
updated to reflect the new status of the log.
Auditors may require the Log to be "downloaded" from time to time; events are
read from the module and stored in a more permanent database, and the events
so copied and archived are marked in the module as "downloaded".
If all record positions contain events that have not yet been downloaded, the log
is full. In this case, the handling of a new event depends on the value of the
"Event log unlocked" site option:

If the option is set, then the log-full condition is ignored and the new event
overwrites the oldest one. Since the overwritten event was never
downloaded, it is permanently lost.

If the option is clear, then the Event Log is "locked", and the new event is
rejected if possible and otherwise ignored. Controllable events, that is,
changes to sealable parameters, are not allowed to occur; such datum points
remain unchanged retaining their current values and a Modbus command
that attempts such a change receives an "illegal data" exception response.
Uncontrollable events, such as PLC mode change, are simply not recorded.
The Log must be downloaded in order to unlock it for further events.