ControlLogix

Layout

Memory and tasks

Data lives in tags: controller-scoped tags visible everywhere, program-scoped tags local to a program, and aliases that name a physical I/O point. User-defined data types group the data for a pump or a valve into a structure, and add-on instructions wrap the logic for that structure into a reusable instruction with its own parameters. Logic is organized in tasks, each holding programs that hold routines: a continuous task runs whenever nothing else is, periodic tasks run at a set rate, and event tasks run on a trigger. The scan of a periodic task is measured and has a watchdog, and I/O updates asynchronously to the tasks at the requested packet interval for each module, which is why a value can change in the middle of a scan unless it is buffered.

Networks

• EtherNet/IP is the current network for everything: remote I/O, drives, instruments, messaging between controllers, SCADA, and engineering. Ring topology is available with modules that support it.
• Produced and consumed tags exchange data between controllers on the same network at a set interval without message instructions.
• Messaging instructions read and write tags in other controllers and, through the right modules, older controllers on older networks.
• ControlNet and DeviceNet are found on plants from the 2000s; they work, they are aging, and their scanners and cables are the migration items.
• Data Highway Plus and the older remote I/O network connect a ControlLogix to PLC-5 and SLC systems that have not yet been replaced.

Redundancy

A redundant system is two chassis, each with a controller, a redundancy module, and communication modules, connected by fiber between the redundancy modules and both attached to the I/O networks. The primary runs the process and crossloads its data to the secondary each scan; on a failure or a manual switch, the secondary takes over and the I/O connections re-establish within the switchover time. Process I/O cannot be in the redundant chassis; every point is on remote I/O. The application is written with the redundancy rules in mind, particularly around scan time and which instructions are permitted, and the switchover is tested at commissioning and on a schedule.

On a site visit

1. 1

   Controller status

   Run, program, or fault; the OK indicator; the major and minor fault records in the software.

2. 2

   Module status

   Each module in the chassis reports its own state; a module the project expects but does not find is a fault the software names by slot.

3. 3

   I/O connections

   The I/O tree shows every remote module and its connection state; a module with a yellow triangle is not connected.

4. 4

   Network

   The Ethernet module web pages for connections in use, packet rate, and errors; the ring status where a ring is used.

5. 5

   Redundancy

   Primary and secondary status, synchronization state, and any disqualification reason on the redundancy module display.

6. 6

   Backup

   Upload with the matching software version; store the project with the site documentation; record the firmware.