Since RSLogix5000 has supported Ethernet I/O, its been possible to setup two processors to control, or “own,” the same remote rack of Ethernet I/O. However, the processor which attached to the I/O first would be the one to control the I/O, and the second processor would receive an error stating that the I/O was already owned. If or when the first processor was disconnected or powered down, the I/O would be released and the second could then connect to the I/O and control it.
But what if you wanted two CompactLogix (or ControlLogix) processors to control the same remote Ethernet I/O at the same time? Well, lets think about that for a moment… having two processors reading the same inputs doesn't seem to be a problem, but having two processors trying to control the very same outputs at the same time could lead to unpredictable results.
With that in mind, Rockwell created the “listen only” mode for most (if not all) input modules, and for a limited number of output modules. This feature allows you to setup one of your processors as the “owner” of the input or output module, while the other controller can be set to “listen only” so it too will automatically receive I/O status updates.
So, how do you know which I/O modules support listen only? In my testing of 1734 Point I/O in version 20 of RSLogix, I found that when adding the I/O to the I/O tree the software will correctly present you with the available options for the module in question. In my tests, every 1734 Input module supported listen only, while only the subset of outputs modules listed below did:
The main downside I found to the “listen only” setting is that it only works when the processor which owns the I/O is also connected. If the owner is powered off or disconnected, the “listen only” processor loses it's connection to the I/O as well. Not good if you wanted to use the second controller to alarm on certain conditions if the first controller is taken out of service, but for those applications you should consider a redundant processor configuration.
I hope you’ve found the above information on multiple controllers controlling the same I/O helpful. As always, if you have any questions or would like to comment on the above article please click on the “post a comment or question” link below.