Adding I/O to ControlLogix Online

Installing I/O to a running controller reduces downtime and expands functionality.

Manufacturers need to update their equipment to increase efficiency, adapt to changing product or process needs, and keep up with regulations. However, many continuous process control applications must run for years without scheduled downtime because shutting down a line leads to significant product loss and lengthy start-up procedures.

To help applications deal with high availability requirements, the newest release of RSLogix 5000 software (v15) from Rockwell Automation allows users to add ControlLogix I/O (Bulletin 1756) to a running Allen-Bradley ControlLogix controller. The new I/O can be added to the same chassis as the ControlLogix Central Processing Unit (CPU), or in a remote chassis connected via ControlNet or EtherNet/IP.

Prior to v15, to expand the system, I/O spares had to be pre-installed into a ControlLogix system or production had to be stopped. If the new I/O were in a remote chassis connected via ControlNet, users would then need to reschedule the network. In addition, if multiple controllers were using the same ControlNet network as the I/O, users also would need to place these other controllers into program mode to complete the network scheduling changes. This meant that trying to expand a single process could result in having to shut down a whole line or even the whole plant.


Meanwhile, all I/O modules connected via ControlNet previously only communicated using the scheduled ControlNet bandwidth. This meant that if users wanted to add an additional module, they ad to place the system into idle or program mode to incorporate the new module's information into the network schedule, Now, with the new software version, I/O modules can communicate using unscheduled ControlNet bandwidth, allowing new modules to be added to an operating network.

Once the ControlNet network is scheduled, two methods are available for adding an I/O module to a running system. In the first method, a nondiagnostic 1756 digital I/O module can be added to an open slot within a pre-existing rack-optimised chassis connection with no impact on the scheduled ControlNet bandwidth. Because of scheduled bandwidth limitations, adding modules to a new chassis connection can only be performed when the system is off-line.

In the second method for adding an I/O module to a running system, a digital I/O module and more advanced modules, such as analog I/O or diagnostic discrete I/O, can be configured via an unscheduled direct connection over ControlNet.

While adding I/O to a running controller and ControlNet network provides more versatility, it also puts an additional burden on the system. Therefore, it's important to keep the following two points in mind:

1. The unscheduled bandwidth is shared by message traffic and human-machine interface (HMI) communications, so you should dedicate the network to I/O to ensure reliable operation. This means that HMI and RSLogix 5000 workstations should be attached via a separate supervisory ControlNet or EtherNet/IP network. Also, you should isolate message traffic to a separate network to avoid impacting the I/O communications. The key is to limit the use of unscheduled bandwidth and to help ensure that I/O connections are reliable.
   
2. If you have a system that utilises ControlNet and you anticipate adding digital modules that could leverage rack-optimised communications, you should add the chassis to your project up front so the rack connection is accounted for in the network schedule. If the chassis is not physically present, you should disable the ControlLogix ControlNet Bridge module (1756-CNB) from the module profile in RSLogix 5000. Then, when ready, you can simply add the chassis, engage the 1756-CNB module, and the controller will start using the pre-assigned scheduled bandwidth.
   
   In addition to supporting the ability to add I/O into existing rack-optimised scheduled connections and the ability to add new unscheduled I/O ControlNet connections, Logix also now supports the ability to add I/O to a remote chassis connected via EtherNet/IP. Because EtherNet/IP doesn't require the network to be scheduled, some of the communications restrictions are relaxed to the extent that anything you could do off-line to add direct or rack connections can also be accomplished online with a running system. However, it's still a good idea to lay out your system so that I/O for individual controllers is isolated on a single network. See the "EtherNet/IP Performance Applications Manual" publication ENET-AP001- for additional design criteria for EtherNet/IP.