52 Rockwell Automation Publication 1756-UM540E-EN-P - December 2017
Chapter 3 1756-IF8I Isolated Analog Input Module
Synchronized Sampling
This feature lets you synchronize input sampling across inputs on multiple
modules, forcing those inputs to sample simultaneously within approximately 20
µS of each other. The modules do not need to be in the same chassis, if the system
clocks are synchronized via CIP Sync.
For example, if you have 12 input devices that are connected to one 1756-IF8I
module and two 1756-IRT8I modules in the same chassis, or different chassis
that are synchronized to the same CIP Sync Time Master, use Synchronized
Sampling to take a snapshot of the input data available at each input at a moment
in time.
For the example, you can't have 12 devices connected a single 1756-IF8I module -
either lower the count to <= 8, or make it 2 1756-IF8I modules.
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-IF8I modules makes sure
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 131
.
Synchronized Sampling is not limited to input samples across inputs of the
same module type.
You can use Synchronized Sampling across inputs on 1756-IF8I modules and
1756-IRT8I modules in the same system.
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