Rockwell Automation Publication 1756-PM004D-EN-P - September 2012 15
Communicate with I/O Modules        Chapter 1
By doing this, you can avoid the case of a physical module rejecting the keying 
request because it is an earlier revision than the one configured in the software.
In the following scenario, Compatible Module prevents I/O communication:
The module configuration is for a 1756-IB16D module with module revision 3.3. 
The physical module is a 1756-IB16D module with module revision 3.2. In this case, 
communication is prevented because the minor revision of the module is lower 
than expected and may not be compatible with 3.3.
Module Configuration
Vendor = Allen-Bradley
Product Type = Digital Input 
Module
Catalog Number = 1756-IB16D
Major Revision = 3
Minor Revision = 3
Physical Module
Vendor = Allen-Bradley
Product Type = Digital Input Module
Catalog Number = 1756-IB16D
Major Revision = 3
Minor Revision = 2
Communication is prevented