Rockwell Automation FactoryTalk Alarms and Events Quick Start - Manual (Page 205)

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Appendix F
Alarm time stamping
FactoryTalk Alarms and Events provides significantly improved time-stamp accuracy
over traditional alarm monitoring systems, you can obtain better time-stamp
resolution in some cases by using time stamps directly retrieved from peripheral
devices like I/O cards or other time-stamping hardware.
When an analog (ALMA) or digital (ALMD) alarm instruction executes when an
alarm event occurs, the current Universal Coordinated Time (UTC) value of the
controller’s wall clock is captured and delivered to the FactoryTalk Alarms and Events
system with the alarm.
This appendix shows how to replace the controller’s wall clock time in an alarm event
with a time stamp from an external source.
You can take events from an I/O card like a 1756-IB16ISOE and use the
50 microsecond accuracy from the card’s time stamp when delivering an alarm to the
rest of the alarm monitoring system.
Some of the things you must resolve when you replace the normal time stamp
provided with the alarm instructions are:
„ Alarm time stamps must be in UTC with no time zone or daylight savings time
offset included (these offsets are automatically added to the time stamp when it is
displayed).
„ The alarm time stamp is usually represented as a long integer (LINT) data type,
occupying one 64-bit word. Most I/O cards stamp time using two double integers
(DINT), occupying two 32-bit words.
Overview of Logix clocks
In the Logix environment, time is represented in three different ways — Local Time,
Coordinated System Time, and Universal Coordinated Time.
Coordinated System Time (CST)
Coordinated System Time is provided by a free-running backplane clock that starts
counting the first time the system is powered up. CST is used for time stamping,
event tracking between processors and I/O cards, as well as for motion planning
among multiple motion modules. CST has no relevance to real-world time, but can be
converted to Coordinated Universal Time (UTC) using an offset. CST time stamps
must be converted to UTC before passing them to the alarm monitoring system.