Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules - Manual (Page 82)

82 Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
Chapter 4 1756-IRT8I Combined Temperature-sensing Isolated Analog Module
For example, if you have 12 input devices connected to two 1756-IRT8I modules
and one 1756-IF8I module in the same chassis, or different chassis synchronized
to the same CIP Sync TimeMaster, use Synchronized Sampling to take a snapshot
of the input data available at each input at a single moment in time.
The following conditions must exist to use this feature:
• A 1588 CIP Sync Time Master is configured for the chassis.
• All modules in the set use the same RPI value or values that are multiples
of each other.
• Synchronized Sampling with Other Synchronized Modules is enabled for
all input channels in the set.
For these input modules, configuring one channel for Synchronized
Sampling synchronizes all eight channels.
While setting the RPI to the same value on all 1756-IRT8I modules guarantees
that each module samples at the same rate, it does not guarantee that they sample
at the same time.
When enabled, Synchronized Sampling provides each module a synchronized
starting point for its respective input scans. Because the RPI values are the same,
the inputs on the modules are sampled at the same rate and the same time.
To see where to enable Synchronized Sampling, see page 127 .
Cold Junction Compensation
When using the 1756-IRT8I module with a thermocouple input type, the
channel must account for the thermoelectric effect of a junction of the
thermocouple field wires and the screw terminals of an RTB or IFM.
The junction at which temperature is measured is the hot junction. The junction
where the thermocouple wire interfaces with copper is the cold junction. The
transition from thermocouple wire to copper typically happens either on the
module screw terminal itself or at an IFM.