Rockwell Automation 1756-OF8H ControlLogix HART Analog I/O Modules - Manual (Page 23)

Rockwell Automation Publication 1756-UM533C-EN-P - February 2011 23
ControlLogix HART Analog I/O Modules Chapter 1
Timestamping Controllers within the ControlLogix chassis maintain a system clock. This clock
is also known as the coordinated system time (CST). You can configure your
analog I/O modules to access this clock and timestamp input data or output echo
data when the module multicasts to the system.
This feature provides accurate calculations between events to help you identify
the sequence of events in either fault conditions or in the course of normal I/O
operations. The system clock can be used between multiple modules in the
same chassis.
Each module maintains a rolling timestamp that is unrelated to the coordinated
system time. The rolling timestamp is a continuously running 15-bit timer that
counts in milliseconds.
For input modules, whenever a module scans its channels, it also records the value
of the rolling timestamp at that time. Your program can then use the last two
rolling timestamp values and calculate the interval between receipt of data or the
time when new data was received.
For output modules, the rolling timestamp value is updated only when new values
are applied to the Digital to Analog Converter (DAC).
IMPORTANT Changing electronic keying selections online may cause the I/O
communication connection to the module to be disrupted and may
result in a loss of data.