Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 21)

Rockwell Automation Publication 1756-UM540A-EN-P - May 2014 21
ControlLogix Isolated Analog I/O Module Features Chapter 2
Synchronized Sampling lets you configure a test stand, for example, and take
many measurements simultaneously across many modules, if needed, while still
precisely coordinating the sampling. With these modules, the synchronized
sampling should coordinate within approximately ± 20 μs.
Rolling Timestamp of Data
The rolling timestamp is a continuously running 16-bit rolling timestamp that
counts in milliseconds from 0…32,767 ms; where 1 ms = 1 count.
Rolling Timestamp with the 1756-IF8I and 1756-IRT8I Modules
The 1756-IF8I and 1756-IRT8I modules scan their inputs at the RPI, update the
input data, and update the rolling timestamp value. Other ControlLogix analog
input modules scan their inputs at the RTS, not the RPI.
In either case, though, the controller program uses the last two rolling timestamp
values to calculate the interval between the receipt of data or the time at which
new data is received.
The rolling value is commonly used with instructions such as the PID and PIDE
instructions. Every time a rolling timestamp changes, a PID or PIDE instruction
is executed. When you configure a PID instruction for use with a 1756-IF8I and
1756-IRT8I module, set the loop update time equal to the module’s RPI value.
Rolling Timestamp with the 1756-OF8I Module
For the 1756-OF8I module, the rolling timestamp value is updated only when
new values are applied to the Digital to Analog Converter (DAC).