Rockwell Automation 1756-XXXX ControlLogix Analog I/O Modules - Manual (Page 44)

44 Publication 1756-UM009C-EN-P - December 2010
Chapter 3 ControlLogix Analog I/O Module Features
Access to System Clock for Timestamp Functions
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. You decide how
to timestamp data when you choose a Communication Format on the New
Module dialog box. For more information, see
page 205 .
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.
In systems using an EtherNet/IP network and 1588 Grand Master time, the
value of this timestamp still is the CST time. You must convert this CST value
to Grand Master time in the controller.
Rolling Timestamp
Each module maintains a rolling timestamp that is unrelated to the CST. 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. The user program can then use the
last two rolling timestamp values and calculate the interval between receipt of
data or the time when new data has been received.
For output modules, the rolling timestamp value is only updated when new
values are applied to the Digital to Analog Converter (DAC).
Producer/Consumer Model
By using the Producer/Consumer model, ControlLogix I/O modules can
produce data without having been polled by a controller first. The modules
produce the data and any owner or listen-only controller device can decide to
consume it.
For example, an input module produces data and any number of processors
can consume the data at the same time. This eliminates the need for one
processor to send the data to another processor.