Here's the info - specifically for adding I/O to the local chassis, but applies equally well for a remote chassis. The only fly in the ointment is if the remote chassis is on ControlNet (which is a scheduled network), it won't work until scheduled, but the I/O can be added as an un-scheduled connection - (regarded as a temporary solution).
Firstly (but not necessary to be first), understand that ControlLogix (1756) is just a collection of modules that share a communication "bus" (the backplane), and was designed to allow modules to be plugged in and out with the power ON. (Technical term is RIUP - Removal and Insertion Under Power). You can plug in your new modules without disruption to the running system, and you can plug them in any time you like, whether the I/O configuration exists or not.
Second, (but could be first), understand that in order to get I/O modules "working" within a processor's project, they have to be added to the I/O configuration tree. This can be done on-line, while the processor is running - for all revisions of firmware. You will get a warning saying that adding modules online can cause disruption to the I/O while connection to the new module is established, in truth I would not worry about this unless you have some super-critical or super-fast I/O (eg. motion control modules, or modules triggering an Event Task).
When you have correctly done both of the above, in either order, you will have working I/O.
Neither plugging modules in or adding them to the I/O tree while online will fault the processor. If the I/O tree does not match the physical I/O, you will get the I/O light flashing on the front of the processor.
