The MVI56E-MCM and MVI56E-MCMR modules share nearly the same Modbus functionality, but the MVI56E-MCMR module pages user data in a block of 40 words at a time as opposed to 200 words for the MCM. This is due to the available bandwidth of ControlNet, and the smaller I/O sizes also allows for a smaller RPI setting, making the MCMR much easier to schedule in RSNetworx for ControlNet. Note that this applies to ControlNet only, for use in the local chassis or in a remote chassis via Ethernet, the MVI56E-MCM is the best choice.
There is one very important difference between the functionality of the 2 modules. The MCM solution provides an option called pass-thru. This option will allow a master system to issue Modbus Read and Write Commands to the exact same Modbus address in the module. This is sometimes required when the module is being used to replace a device that the master is already configured and cannot be changed. Although the MCMR module operating as a Modbus slave can support both Modbus Read and Modbus Write commands, you must read from one address range, and write to another address range.
If you are using the module in a remote rack over Control Net, then it is always recommended to use the MVI56E-MCMR module, unless the module must be configured as a Modbus slave and passthrough mode is required. If the module is to be used as a Modbus master, then it is always recommended to use the MCMR module in a remote I/O rack.
All of the above applies as well to the original MVI56-MCM and MVI56-MCMR modules.