Relationship between module resolution and scaling, Module resolution – Rockwell Automation 1756-OF8I ControlLogix Eight-channel Isolated Analog I/O Modules User Manual
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Rockwell Automation Publication 1756-UM540A-EN-P - May 2014
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ControlLogix Isolated Analog I/O Module Features
Chapter 2
Relationship between
Module Resolution
and Scaling
The following concepts must be explained in conjunction with each other:
•
•
Module Resolution
Resolution is the smallest degree of change that the module is capable
of detecting. Module resolution represents a fixed number of counts across the
module’s theoretical operating range.
•
1756-IF8I and 1756-IRT8I modules support 24-bit resolution.
The 24 bits represent 16,777,216 counts.
•
1756-OF8I module supports 16-bit resolution.
The 16 bits represent 65,536 counts.
Resolution on Input Modules
The theoretical operating range is the full range across which the module can
operate. For example, a 1756-IF8I module in Current mode has a theoretical
operating range = -25.1…25.1 mA. The 24-bit resolution and 16,777,216 counts
are available across 50.2 mA which yields our calculated 2.99 nA/count
resolution.
However, when the 1756-IF8I module operates in Current mode, it is configured
for an input range = 0…20 mA. This range limits the input to a 0…21 mA actual
range capability.
The number of counts on a module is fixed. Module actual range capabilities,
however, narrow operating ranges from the theoretical and result in supporting
fewer counts. Using the example above, the 0…21 mA actual range capability
represents 5,815,117 counts, that is, slightly more than 22.5 bits.
Divide the actual range capability by the number of counts in that range to
determine the value of each count. The input range you choose during module
configuration determines the value of each count. It does not determine the
number of counts in that range. Therefore, module resolution across the usable
input operating range is not always 24 bits.
Resolution on Output Module
The module resolution for the 1756-OF8I module is always 16 bits, regardless of
operating mode and operating range.