Access to system clock for timestamping functions, Rolling timestamp, Producer/consumer model – Rockwell Automation 1756-IF4FXOF2F ControlLogix High-speed Analog I/O Module User Manual
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Rockwell Automation Publication 1756-UM005B-EN-P - January 2013
Chapter 3
Module Features
Access to System Clock for Timestamping Functions
Certain modules, such as controllers, in the ControlLogix chassis maintain a
system clock. The clock is a free-running, 64-bit number that increments every
microsecond. It is used to places a timestamp on the sampling of input data
within the local chassis.
You can configure your high-speed analog I/O modules to access this clock and
timestamp input data when the module multicasts to the system. You decide how
to timestamp data when you choose a communication format. For more
information about choosing a communication format, see
.
This feature allows for 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. This clock is also used to synchronize inputs across multiple modules
in the same chassis. For more information about synchronizing module inputs,
see
Rolling Timestamp
Each high-speed analog I/O module maintains a rolling timestamp that is
unrelated to the Coordinated System Time (CST). The rolling timestamp is an
on-board, continuously running 15-bit timer that counts in milliseconds.
For module inputs, when the module scans its input 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.
Because the high-speed analog I/O module offers sub-millisecond sample times
and the rolling timestamp counts in milliseconds, it is possible that a new sample
can be taken without altering the rolling timestamp. If accurate time deltas are
required in such sub-millisecond cases, the CST timestamp’s lower 32 bits offer
the necessary precision.
Producer/Consumer Model
The producer/consumer model is an intelligent data exchange between modules
and other system devices in which each module produces data without having
first been polled. The modules produce the data and any owner or listen-only
controller device can decide to consume it.
For example, module inputs produce 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. For a more detailed explanation of this
process, see
.