A Complete Introduction to the Allen Bradley ControlLogix PLC System

An in-depth guide to the hardware that makes up the Allen Bradley ControlLogix PLC system including the chassis, power supply, IO modules, and communication modules.

The ControlLogix PLC system is a high-end PLC system designed for large, demanding automation applications that require specialty functions like redundancy, large numbers of I/O points, or many axes of motion control.

In this post, we will take a deep dive in the Rockwell Automation/Allen Bradley ControlLogix series of PLC. In the next sections, we will learn about all of the different hardware components that make up this PLC system including

  • Chassis
  • Power Supply
  • Controllers
  • I/O Modules
  • Communication Modules

ControlLogix PLC systems are configured, programmed, and monitored using Studio 5000 Logix Designer software from Rockwell Automation. If you are interested in learning how to program ControlLogix systems, I highly recommend checking out my course PLC Programming with Studio 5000 Logix Designer. In this course, I use my 10 years of industrial automation experience to show you exactly what you need to know to start programming ControlLogix and CompactLogix PLCs. It is the course that I wish I had when I was learning PLC programming many years ago.

Introduction to ControlLogix

The Logix 5000 ControlLogix system is made up of ControlLogix controllers, ControlLogix chassis, ControlLogix power supplies, and, optional I/O and communication modules.

The ControlLogix system is a multi-controller system. This means that it is possible to have more than one controller in the chassis. Controllers, I/O modules, and communication modules can be placed in any order and location within the chassis.

ControlLogix systems can range in size and complexity. A simple ControlLogix system may consist of a standalone controller and I/O modules in a single chassis. A more complex system may have multiple controllers, multiple chassis, and network connections to other devices.

Let’s look at the components that make up the ControlLogix system in more detail.

ControlLogix Chassis

The ControlLogix chassis is a physical housing for ControlLogix modules. Inside the chassis, there is a backplane that creates an electrical connection between the modules mounted in the chassis.

1756 ControlLogix Chassis

In the example above, you see a chassis with ten slots, however, ControlLogix chassis are available in various sizes to suit the requirements of different projects. The ControlLogix chassis is available in 4, 7, 10, 13, and 17 slot configurations.

Each slot in a chassis has a number. Slot numbering always goes from left to right starting at 0. So, a ten slot chassis has slots 0–9.

ControlLogix modules are mounted in the slots of the ControlLogix Chassis to build a system that meets an application's requirements.

ControlLogix Power Supply

1756 ControlLogix Power Supply

The ControlLogix power supply is located outside of the chassis. It provides electrical power that energizes all of the modules mounted in the chassis. The ControlLogix power supply does not provide power to field devices mounted outside of the chassis- these devices need a separate power supply.

The ControlLogix power supply is available with 120V AC, 230V AC, and 24V DC input.

ControlLogix Controllers

1756 ControlLogix Controllers

ControlLogix controllers are available with different combinations of memory and processor speed to suit the requirements of different applications. All ControlLogix controllers are capable of addressing large numbers of I/O points and networked devices.

The latest series of ControlLogix controllers are the 5571 L7 and 5581 L8 series of controllers. Let's look at the features of these controllers in detail.

The L7 and L8 series of controllers have many features in common, including;

A Scrolling Display

Both the L7x and L8x ControlLogix controllers have a scrolling display that shows

  • Project name
  • Fault information
  • Firmware revision
  • Other data

This scrolling display allows someone to easily understand the status of a controller at a glance.

Secure Digital (SD) Memory Card

Both the 5571 L7x and 5581 L8x ControlLogix controllers use a Secure Digital (SD) card to provide non-volatile memory for the controller. This non-volatile memory is used to store the controller’s project.

USB Port

Both series of controllers have a USB port. This USB port is used to create a temporary point-to-point connection between the controller and a computer for transferring project files or monitoring program execution and system status.

Energy Storage Module (ESM)

Both controllers have an Energy Storage Module, also known as an ESM.

The ESM allows the controller to retrieve the project from non-volatile memory after power loss or a power cycle. The ESM also keeps the controller’s wall clock running for up to 5 days without power.

In the 5571 L7x controllers, the ESM can be removed from the controller using the Release button located on the front of the controller.

5581 L8x Controller Features

The L8x is a newer controller that builds on the features of the 5571 L7x controller. The enhancements of the 5581 L8x controller includes;

Better Performance

The 5581 L8x controller can deliver up to 20x faster scan times and up to 45% more storage than an equivalent 5571 L7x controller.

Gigabit Ethernet Port

5581 L8x controllers feature a 1 Gigabit Ethernet port for lightning fast communication over an Ethernet network.

Increased Security

The 5581 L8x controller expands on the security features offered by the 5571 L7x controllers with;

  • Controller-based change detection and logging
  • Digitally signed controller firmware
  • Role-Based Access Control (also known as RBAC)

Role-based Access Control is a method of regulating access to a computer or network resources based on the roles of individual users within an enterprise. With RBAC, only specific users would be able to perform specific tasks like viewing, creating, or modifying files.

ControlLogix I/O Modules

1756 ControlLogix I/O Modules

In a ControlLogix System, I/O modules can be local I/O modules or distributed I/O modules.

A local I/O module is an I/O module that is mounted in the same chassis as the controller that contains the I/O module’s configuration information. In the image above, you can see examples of local  I/O modules in a ControlLogix system.

A distributed I/O module is an I/O module that is mounted in a different chassis than the controller that contains its configuration information. Distributed I/O modules help to minimize the amount of wiring needed to connect field devices to a controller.

In most ControlLogix systems, both local and distributed I/O come from the 1756 family of I/O modules. Let's look at the features of these I/O modules in more detail.

1756 I/O Modules

The ControlLogix system uses 1756 series input and output modules for local and distributed I/O. These I/O modules are mounted in a ControlLogix chassis. Both input and output modules have a very similar design.

Removable Terminal Blocks (RTBs)

The 1756 I/O modules are equipped with a Removable Terminal Block, also known as an RTB for easy wiring.

The Removable Terminal Block can is a terminal block which can be disconnected from the I/O module. Field devices are wired up to the Removable Terminal Block and once connected, the Removable Terminable Block can be reconnected to the I/O module. Connector pins on the I/O module create an electrical connection between the RTB and I/O module, passing both data and power through dedicated pins.

When the RTB is connected to an I/O module the locking tab, located on the front of the 1756 I/O module, is used to secure the RTB to the module. This locking tab helps to ensure that the electrical connection between the RTB and the module is secure.

ControlBus Backplane Connector

At the rear of 1756 I/O modules is a ControlBus connector. This connector connects the I/O modules to a ControlBus backplane, the communication bus found in a ControlLogix chassis, and allows the modules to communicate with a controller over the backplane.

Status Indicators

Each 1756 I/O module has status indicators on the front of the module. These status indicators provide an easy way to understand the health of the I/O module and its communication status. The status indicators are very useful for troubleshooting issues in a ControlLogix system.

Slots for Mechanical Keying

Finally, to prevent the wrong RTB from being connected to an I/O module, each module is equipped with slots for mechanical keying. This mechanical keying system prevents someone from connecting the RTB from an input module to an output module, for example.

1756 I/O Module Status Indicators

Let's talk a bit more about the status indicators included with 1756 I/O modules.

1756 Digital I/O Module Status Indicators

1756 ControlLogix Digital I/O Modules

A digital input reads an electrical input from a discrete device like a pushbutton and converts it to a signal that the controller understands. For example, a digital input may be connected to a pushbutton and tells the controller that the button is pressed or released.

In contrast, a digital output module actuates a discrete device like a light. The controller sends a signal to the digital output module which processes the information and turns the light on or off.

A 1756 digital I/O module includes the following status indicators;

  • I/O Status Indicators, which provides a yellow LED that indicates if each input or output point on the module is active
  • Fault Status Indicator, which provides an indication of a field side fault
  • Module Status Indicator, which indicates the module's health and connection to a controller.
  • Fuse Status Indicator, which indicates the health of the module's fuse

The Module Status Indicator and Fuse Status Indicator are not available on every 1756 I/O module.

1756 Analog I/O Modules Status Indicators

1756 ControlLogix Analog I/O Modules

An analog input reads data from an analog device like a temperature sensor. Analog devices typically communicate more complex data than digital input modules like temperature, flow, and rates.

In contrast, an analog output sends analog data to a device like a control valve or a motor controller.

A 1756 analog I/O module includes the following status indicators;

  • CAL indicator shows when an analog I/O module is in calibration mode
  • OK indicator shows that the module is healthy and communicating with a controller

ControlLogix Communication Modules

The ControlLogix system has communication modules available that enable ControlLogix systems to communicate on many common industrial networks including;

  • EtherNet/IP
  • ControlNet
  • DeviceNet
  • FOUNDATION Fieldbus
  • Data Highway Plus
  • Remote I/O
  • HART

Other communication modules, manufactured by third parties, allow ControlLogix systems to communicate with other manufacturers’ PLC systems.

In the rest of this section, we’ll look at the features of one of the most commonly used ControlLogix communication modules — the 1756-EN2T EtherNet/IP communication module. Most of the information about this communication module is applicable to other ControlLogix Communication Modules too.

1756-EN2T EtherNet/IP Communication Module

1756 ControlLogix Communication Modules

The 1756-EN2T EtherNet/IP communication module allows ControlLogix modules to communicate on an EtherNet/IP network.

The most important features of the communication module are the status indicators. The 1756-EN2T module has three status indicators. Let’s look in detail at each status indicator and what it means.

LINK Status Indicator

The LINK status indicator shows if the module is transmitting data on the EtherNet/IP network.

A flashing green LED indicates that data is being transmitted.

NET Status Indicator

The NET status indicator shows the state of the EtherNet/IP port on the communication module.

A solid green LED indicates that the module has an IP address and at least one active connection with another device.

OK Status Indicator

The OK status indicator shows the overall status of the module.

A solid green LED indicates that the module is operating normally.

1756-EN2TR EtherNet/IP Communication Module

Another common ControlLogix communication module is the 1756-EN2TR, which has two Ethernet ports.

The status indicators on this module are similar to the 1756-EN2T. The main difference is that the 1756-EN2TR has two LINK status indicators and no NET status indicator.

Wrap Up

In this article, we learned about the ControlLogix system and the components that make up the system. Specifically, we learned about;

  • The 1756 ControlLogix chassis and power supply
  • The 5571 L7x and 5581 L8x ControlLogix controllers
  • The 1756 ControlLogix I/O modules
  • The 1756 ControlLogix communication modules

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