Finding the Right ControlLogix Processor

Understanding ControlLogix Processor Generations

Every ControlLogix processor belongs to a generation, and the generation tells you most of what you need to know about a given controller: its performance, how it communicates, and where it sits in the product lifecycle. There are three you’ll encounter.

The 5560 series, the 1756-L6x controllers (such as the L61, L62, and L63), is the original ControlLogix generation. These are the processors you’ll find in systems that have been running for a decade or more. Rockwell has moved them to the end of their lifecycle, so they’re no longer in standard production. Importantly, though, end-of-life does not mean unobtainable. The L6x line remains widely available as new and new-old-stock inventory through the secondary market, which is often the fastest way to keep an existing system running exactly as it is.

The 5570 series, the 1756-L7x controllers (L71, L72, L73, and up), was the mid-generation step. It brought more memory, faster performance, a removable energy-storage module in place of the older battery, and a small status display on the front of the unit. These are current, available, and a common upgrade target for systems moving off the L6x.

The 5580 series, the 1756-L8x controllers (L81E through L85E and beyond), is the current top generation. The defining change here is communications: the 5580 controllers have an EtherNet/IP port built directly into the processor, along with significantly more memory and processing capacity for large or high-speed applications. If you’re building new or planning a long-term upgrade, this is where Rockwell is pointing you.

Why does this matter when you’re just trying to replace a part? Because your options depend on which generation you’re working with. If you have a healthy L7x or L8x, replacement is straightforward. If you have a discontinued L6x, you get to choose: source the same legacy processor and change nothing, or step up a generation and gain memory and connectivity, at the cost of some migration work. The next sections give you the data to make that call.

Official Documentation: Controller Technical Data

For deep architectural overviews and comprehensive structural specifications across processor generations, refer directly to the factory literature: 1756 ControlLogix and GuardLogix Controllers Technical Data (PDF).

ControlLogix Processor Selection & Replacement Table

The table below maps the ControlLogix processor families, with the user memory that drives most sizing decisions, the onboard communications, and the lifecycle status that tells you whether you’re buying current product or sourcing from secondary inventory. The L7x and L8x memory tiers line up closely, which makes finding an upgrade target straightforward: match your current memory, or step up.

A key ordering note runs through the communications column: the 5580 (L8x) controllers have an EtherNet/IP port built into the processor, while the older L6x and L7x controllers rely on a separate communication module in the chassis. If you upgrade from an L6x to an L8x, you may no longer need that standalone Ethernet module.

Standard ControlLogix processors

GuardLogix safety processors

If your application is safety-rated, the GuardLogix variants carry an “S” in the catalog number and add a dedicated safety memory partition. Worth including because these are searched separately and often confused with the standard line.

Reading the suffixes

The same processor often comes in several variants. Knowing the suffix saves you ordering the wrong one:

Important Note for beginners – A ControlLogix processor doesn’t work alone. It installs into a slot of a 1756 chassis, draws from a chassis power supply, and coordinates the input, output, and communication modules that share the rack. Replacing these components is normally a cheaper route. The processor you choose can affect the rest of that assembly, most notably communications: a current 5580 controller has EtherNet/IP built in, while an L6x or L7x relies on a separate communication module such as a 1756-EN2T or 1756-EN2TR. For help selecting system racks and power units, see our Chassis & Power Supply Guide, and for field wiring components, refer to our I/O Module Guide. The rest of this article stays focused on choosing and replacing the processor itself.

How to Read a 1756 ControlLogix Processor Catalog Number

Allen-Bradley processor numbers look cryptic, but once you know the structure you can read most of what a controller is straight from the catalog number. Here’s how a 1756-L processor breaks down. Start with the basics that never change. 1756 is the ControlLogix bulletin, the chassis-based platform the whole family lives in. The L that follows marks the part as a Logix controller (the processor), as opposed to an I/O or communication module.

The first digit after the L tells you the generation: a 6 is the 5560 series (1756-L6x, legacy), a 7 is the 5570 series (1756-L7x), and an 8 is the current 5580 series (1756-L8x). This single digit is the fastest way to know how old a controller is and where it sits in the lifecycle.

The second digit indicates the relative memory and performance tier within that generation. Higher numbers mean more memory: across the current 5580 line, for example, the L81E carries 3 MB and the L85E carries 40 MB, scaling up as the second digit climbs. The tiers track loosely with the older generations too, which is what makes the upgrade paths in the table above line up the way they do.

After the digits come the letters and suffixes that flag specific variants. On the 5580 line, an E signals the onboard EtherNet/IP port. An S marks a GuardLogix safety controller. A K means conformal coated for harsh environments, and XT means extended temperature. So a 1756-L83ES reads as a ControlLogix 5580, large-memory tier, with onboard Ethernet and integrated safety, while a 1756-L73XT is a 5570, 8 MB, built for temperature extremes.

Allen-Bradley’s numbering is consistent enough to orient you quickly, but it isn’t a perfect formula, and memory sizes and exact options vary by generation. Use the catalog number to narrow down what you’re looking at, or browse our dedicated 1756 ControlLogix processors catalog to confirm availability before you order.

Replacing a Discontinued Processor: Your Options

If you’re holding a 1756-L6x controller that’s failed or is nearing end of life, you have a real choice to make, and the right answer depends on your timeline and how much of the surrounding system you want to touch.

Option one: source the same processor and change nothing. This is the fastest path back to production. The L6x line is no longer in standard manufacture, but it remains widely available as new and new-old-stock inventory through the secondary market. Dropping in the identical controller means your program, your chassis, your I/O, and your communications all stay exactly as they are, with no software work and no requalification. When a line is down and you need it running today, this is almost always the right move.

Option two: upgrade to a current generation. If you have the runway, usually a planned shutdown, stepping up to a 5570 (L7x) or current 5580 (L8x) controller buys you years of supported availability, more memory, and, on the 5580, onboard EtherNet/IP. The upgrade paths in the table above are designed around matching memory: an L63 maps naturally to an L73 or L83E, and so on.

An upgrade is rarely just a hardware swap, though, and it’s worth knowing the work involved before you commit:

• Firmware and software. Moving to a newer controller generally means a newer firmware revision and a compatible version of Studio 5000 (the 5580 line in particular requires recent Studio 5000 versions). Your program must be converted to the new controller and revision.
• Communications. Upgrading from an L6x to an L8x can let you retire a separate Ethernet module, since the 5580 has a port onboard, but that also means reworking how the controller connects to the network.
• The rest of the rack. Most existing 1756 I/O modules carry forward, but it’s worth confirming module and chassis compatibility as part of the plan rather than discovering an exception mid-install.

For many plants the smartest sequence combines both options: source the exact L6x now to get running immediately, then schedule the upgrade to an L7x or L8x for the next planned downtime. That way a breakdown never forces an unplanned migration, and the upgrade happens on your terms.

Source Your ControlLogix Processor From TCI Supply

Whether you need to keep an older system running or move up to a current controller, the starting point is the same: knowing what’s genuinely available and getting the exact right part.

Sourcing discontinued and hard-to-find Allen-Bradley controls is our core business. The 1756-L6x processors that Rockwell no longer makes in standard production are still obtainable as new and new-old-stock inventory, and we locate, verify, and supply them so you can keep an existing system exactly as it is. If you’re upgrading instead, we’ll help you match your legacy controller to the right current-generation 5570 or 5580 processor and confirm what else in the rack is affected.

Either way, you get one place to ask instead of a dozen, with parts checked for authenticity before they ship.

Send us your processor catalog number and we’ll confirm availability, identify the correct replacement or upgrade, and get you a quote. Whether it’s a legacy 1756-L63 to keep a line running or a current 1756-L83E for a planned upgrade, contact TCI Supply or call us at 1-800-755-2883.

TCI Supply is not an authorized distributor of Allen-Bradley.

Companion ControlLogix Sourcing Guides

Building or modifying a 1756 configuration requires coordination between your processor capabilities, network cards, and field terminations. Review our companion hardware guides to map your complete hardware profile:

• Chassis & Power Supplies: Visit our 1756 ControlLogix Chassis & Power Supply Selection Guide to select system racks and power sources.
• Chassis Networking Bridges: Visit our 1756 ControlLogix Communication Module Guide to select EtherNet/IP, ControlNet, and legacy routing adapters.
• Field Wiring & Signal Cards: Visit our 1756 ControlLogix I/O Module Guide to select digital, analog, and isolated terminal cards.

Frequently Asked Questions