Hardware Comparison: ControlLogix Processor Selection Guide

Before we dive into the specifics of this processor selection guide, it is important to bring up a few points about picking a controller. While the controller is the main piece to a control system, the best controller will fail if the remainder of the control system isn’t paired properly with it. The biggest, baddest controllers will cover for a lot of mistakes throughout the rest of your system, but they’ll expose just as many weaknesses as they make up for. This blog post will help you select the highest end family of Allen-Bradley’s controllers, the ControlLogix line. But, just as the article says, please sit down and converse with an expert. Plan the system, design it according to the plan, execute according to the design. With the knowledge you’ll gain from this article and the collaboration with those people who research these things on a daily basis, you’ll have more than a successful implementation, you’ll have a control system that will bestow great benefits upon you for decades to come.

You’re looking at either a new ControlLogix system, or you’re thinking about upgrading your current system. With a few simple questions, you can gain enough knowledge and understanding to have an intelligent conversation with a specialist on the subject. Yes, the knowledge you gain is not the final answer, as the selection of a controller (and the cost associated) is probably the most important part of control system design. Working with someone who knows and understands the controllers and can match one to your needs is an important aspect of that selection process. You need to have some knowledge though, to have that intelligent discussion. The questions you need to answer when it comes to selecting a controller start here.

The ControlLogix controllers are part of the 1756 family of large PAC’s for Rockwell Automation. They’re split into four versions, the 5570 and 5580 ControlLogix and GuardLogix controllers. Each has different levels of capability, although those levels are similar across all four versions. As the following questions are answered, it will start to create a part number for the desired controller, and will give insight into what the part number means.

One question that does not need answered for the ControlLogix is “Do you need motion capability?” ControlLogix controllers will do all motion functions, although you need to ensure it supports the number of motion axes you need. A good rule of thumb is adding 50% to your current needs and round up when it comes to desired axes.

Some pre-emptive questions first. Do you need redundant controllers? If yes, focus on the 5570 controllers until the 5580 controllers become redundancy capable. Do you want on-machine (not in a cabinet) installation of your controller? If yes, again focus on the 5570 controllers with their ArmoLogix lineup. Now, on to the big questions.

First question, are you upgrading an existing controller, or is this a new control system? The ControlLogix controllers come in two families, the 5570 and 5580 controllers. The 5580 are the newest version and have some differences from the 5570, mostly in performance enhancements but there are also different requirements for the controllers. If you’re upgrading an existing 5570 controller, the easiest way is to stay within the 5570 family. If you’re wanting the enhancements from the 5580, or you’re moving to a “fresh” ControlLogix system, the 5580 is the preferred version. In the 1756 family of controllers, 5570 versions have a 1756-L7* part number while 5580 versions have a 1756-L8* part number. One important consideration when deciding this is your software version. If you’re not on Rockwell’s software subscription service to get free version upgrades, the 5580 controllers require a minimum software version (I believe 28 at a minimum, and 30 for full capabilities.) If your software is older, the 5580 won’t work for you.

Second question, do you need or want safety? You’re already looking at a ControlLogix controller, so you’re interested in a large control system. Do you want to integrate safety into the control system? If so, look at the GuardLogix line of controllers, which are safety rated ControlLogix controllers. The 5580 GuardLogix controller by itself is rated to PLd/SIL2, and with a safety partner is capable of the highest levels of PLe/SIL3. The 5570 Guardlogix controller requires the safety partner to be safety rated, so unless you’re looking for the highest levels of safety (PLe/SIL3), this might not be the optimal choice. For either controller, the part number will be 1756-Lx*S, with the safety partners being 1756-LxSP. A secondary consideration is for the control system to be safety rated you also must use safety rated IO modules. If you’re planning for the future the minimal cost increase for the GuardLogix line can be worth it (the GuardLogix will work just like a ControlLogix when it doesn’t have safety functions.) Again, the 5570 (L7) controllers require the safety partner.

Third question, how much IO and how many nodes are you going to need? First, let’s talk about IO and nodes. IO are inputs and outputs, sensors and actuators, the senses and muscles of the control system. Nodes are devices on the network, which can consist of drives, IO modules, computers, servos, valves, cameras, and just about anything else you can think of that can communicate via Ethernet/IP (or whatever communication protocol you use.) As you’re building your control system, you’ll be adding these devices and increasing the node count. Once you’ve got a basic idea of which controller you want, the node count will be the next requirement to match up. For each version of the ControlLogix, 5570 and 5580, there are several levels of capability which are based on node capacity and memory size. With your initial part number of 1756-L7x or 1756-L8x, the (x) will give you the “size” or capacity of the controller. As you can see by the chart below, as the (x) number goes up, the capacity of the controller goes up.

Controller Part Number 5570 Controller 5580 Controller

Memory Size Max Nodes Memory Size Max Nodes
1756-Ly1 2MB 500 3MB 60
1756-Ly2 4MB 500 5MB 80
1756-Ly3 8MB 500 10MB 100
1756-Ly4 16MB 500 20MB 150
1756-Ly5 32MB 500 40MB 300

You’ll notice two things by the chart, the comparable 5580 controller has more memory, and the 5570 controllers all have the same (higher) maximum number of nodes. The 5570 controller is limited by two things, the communications modules used and the memory available. One benefit to the 5580 controller is the embedded 1GBe Ethernet port, which allows the 5580 controllers to access the control network without requiring a communications module. Again, the important aspect is how many nodes you have on the network (each stick of IO counts as a node – an analog card isn’t a node, but the ethernet adapter to connect it to the network is.) As a rule of thumb, at a minimum add 50% to your expected node count and then round up to the next size.

With these three questions answered, you’ll have the general part number of the controller you’re looking for. Again, this is just to get you started on the process of talking to an expert on Rockwell controllers. The reasons for having this discussion are many, but the overarching reason comes down to the expert can help with not only selecting a particular controller, but also ensuring all of the parts of the control system are coordinated to give you the greatest chance of a having a reliable and successful control system that is used for decades into the future.

1756 ControlLogix Controller Descriptions

Cat. No. Description User Memory
1756-L81E ControlLogix controller, 1 built-in USB port(1), single port EtherNet/IP 3 MB
1756-L82E
5 MB
1756-L83E
10 MB
1756-L84E
20 MB
1756-L85E
40 MB

GuardLogix safety controllers
1756-L81ES
3 MB standard

1.5 MB safety
1756-L82ES
5 MB standard

2.5 MB safety
1756-L83ES
10 MB standard

5 MB safety
1756-L84ES
20 MB standard

6 MB safety
1756-L8SP GuardLogix safety partner (in SIL 3 applications, one safety partner is required for each GuardLogix 5580 controller) Not applicable
1756-L71 ControlLogix controller, 1 built-in USB port(1) 2 MB
1756-L72
4 MB
1756-L73
8 MB
1756-L74
16 MB
1756-L75
32 MB
1756-L73XT ControlLogix-XT controller, extreme environment 8 MB
1756-L71S GuardLogix safety controllers 2M standard

1 MB safety
1756-L72S
4 MB standard


2 MB safety
1756-L73S
8 MB standard


4 MB safety
1756-L7SP GuardLogix safety partner (one is required for each GuardLogix L7 controller) Not applicable
1756-L72EROM Armor ControlLogix controllers, EtherNet/IP dual-port 4 MB
1756-L73EROM
8 MB
1756-L72EROMS Armor GuardLogix controllers, EtherNet/IP dual-port 4 MB standard

2 MB safety
1756-L73EROMS
8 MB standard


4 MB safety

1756- Controller Features

Feature 1756-L71, 1756-L72, 1756-L73, 1756-L74, 1756-L75 1756-L81E, 1756-L82E, 1756-L83E, 1756-L84E, 1756-L85E
Controller tasks • 32 tasks • 32 tasks
• 1000 programs/task(2) • 1000 programs/task
• Event tasks: all event triggers • Event tasks: all event triggers
Built-in communication ports • 1 port USB client • 1 port USB client

• Single-port EtherNet/IP
Communication options • EtherNet/IP • EtherNet/IP
• ControlNet • ControlNet
• DeviceNet • DeviceNet
• Data Highway Plus • Data Highway Plus
• Remote I/O • Remote I/O
• SynchLink • SynchLink
• 3rd-party process and device networks • Third-party process and device networks
Controller connections 500 connections Not applicable
Node capacity Not applicable Logix Designer application, version 30 or later:

• 1756-L81E: 100 EtherNet/IP nodes(3)

• 1756-L82E: 175 EtherNet/IP nodes(3)

• 1756-L83E: 250 EtherNet/IP nodes(3)

• 1756-L84E: 250 EtherNet/IP nodes(3)

• 1756-L85E: 300 EtherNet/IP nodes(3)
Network connections, per network module(1) • 256 EtherNet/IP; 128 TCP (1756-EN2x, 1756-ENxT(R)) Not applicable(4)
• 128 EtherNet/IP; 64 TCP ( 1756-ENBT)
• 128 ControlNet (1756-CN2/B)
• 100 ControlNet (1756-CN2/A)
• 40 ControlNet ( 1756-CNB)
Controller redundancy Full support Future
Integrated motion • EtherNet/IP connection • EtherNet/IP connection
• SERCOS interface • With Logix Designer, version 31 or later:
• Analog options (encoder, LDT, SSI) – SERCOS interface

– Analog options (encoder, LDT, SSI)
Programming languages • Relay ladder • Relay ladder
• Structured text • Structured text
• Function block • Function block
• Sequential function chart (SFC) • Sequential function chart (SFC)

(1) For ControlLogix 5580 controllers, total number of devices cannot exceed the total number of devices that the controller supports. The number of connections per network module shown is the maximum designed capacity of the modules. The device data size and requested data rate determine the actual device capacity.
(2) Studio 5000, version 23 and earlier, is limited to 100 Programs/Task.
(3) This is the maximum number of EtherNet/IP nodes the controller supports. Use Integrated Architecture® Builder to lay out and validate system design and additional node options. For further information on nodes on an EtherNet/IP network, see the ControlLogix 5580 and GuardLogix 5580 Controllers User Manual, publication 1756-UM543.
(4) This value is determined by the node capacity for the ControlLogix 5580 controllers. See the node capacity feature for more information.
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