Finding the Right 1756 Communication Module

Companion ControlLogix Sourcing Guides

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

• Processor Sizing: Read our 1756 ControlLogix Processor Replacement Guide to evaluate memory capacities and CPU generations.
• Chassis & Power Sizing: Read our 1756 ControlLogix Chassis & Power Supply Selection Guide to plan backplane configurations and power requirements.
• Field Wiring & Signal Cards: Visit our 1756 ControlLogix I/O Module Guide to select digital, analog, and isolated terminal cards.

How 1756 Communication Modules Fit in a ControlLogix System

A communication module in a 1756 chassis does one job: it gives the controller a connection to a specific network. It installs in any available slot on the chassis, draws power from the chassis backplane, and exchanges data between the processor and whatever’s on the far end of the network, remote I/O, another PLC, a drive, an HMI, or the rest of the plant.

Most 1756 systems use one or more communication modules because the processor itself doesn’t include onboard network ports. The older 5560 (1756-L6x) and 5570 (1756-L7x) controllers have a USB port for local programming and nothing else, so every network connection runs through a dedicated module in the chassis. The current 5580 (1756-L8x) controllers are the exception: they have an EtherNet/IP port built into the processor, which can eliminate the need for a separate Ethernet module in simpler systems. For more on processor generations, see our ControlLogix processor selection and replacement guide.

That distinction is worth keeping in mind as you read the rest of this article, because it changes how you spec a system. A new build with a 5580 controller and a single Ethernet device network may need no separate comms module at all. A new build with a 5580 connecting to multiple networks, or any system built around a 5560 or 5570, will need one or more 1756 communication modules in the chassis. And replacing a failed module in an existing system is its own decision: source the same part and change nothing, or step up to a newer-generation module and gain capabilities like Device Level Ring or gigabit speed.

The sections that follow lay out the options by network, starting with EtherNet/IP.

Official Documentation: Controller Technical Data

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

EtherNet/IP Communication Modules (1756-EN Series)

EtherNet/IP is the standard network for current 1756 ControlLogix systems and the deepest of the four module families covered in this guide. The 1756-EN series has gone through four clear generations, each adding capability over the last. Knowing which generation you have, or which one fits your application, is most of the buying decision.

The four generations

• The 1756-ENBT is the original Ethernet bridge module for ControlLogix. Single port, 10/100 Mbps, 128 CIP connections. It’s no longer in standard production, but remains widely installed and obtainable through the secondary market for plants keeping existing systems exactly as they are.
• The 1756-EN2T is the direct successor: same single 10/100 port, but with significantly more processing capacity and connection handling than the ENBT it replaced. It’s active and the workhorse of the line.
• The 1756-EN2TR added a second Ethernet port and Device Level Ring (DLR) support, enabling ring topology with built-in switch redundancy. It’s the common choice for new builds where network resilience matters.
• The 1756-EN4TR is the current generation. It’s the gigabit step-up: two-port DLR like the EN4TR, but at full gigabit speed, with enhanced security features and support for redundant adapter pairs. For new builds where Rockwell is pointing buyers, this is it.

Specialty and variant modules

Beyond the four mainline modules, several variants exist for specific needs:

• 1756-EN2F: fiber optic Ethernet, for runs that need fiber rather than copper.
• 1756-EN2TP: Parallel Redundancy Protocol (PRP) variant, for applications requiring PRP network redundancy.
• 1756-EN3TR: a 1-port + DLR variant in the EN2-generation family.
• 1756-EN2TSC: secure-communication variant.
• 1756-EWEB: Ethernet/IP web server module, provides web-based access to controller data; end-of-life and a common sourcing request for plants relying on its specific capability.

And the same suffix system you’ve seen on processors applies here:

• K = conformal coated (harsh/corrosive environments). Example: 1756-EN2TK.
• XT = extended temperature (-25 to +70°C). Example: 1756-EN2TXT.

Table: 1756 EtherNet/IP communication modules

Choosing between generations

For a new build, the path is straightforward: a 5580 controller has Ethernet onboard and may not need a separate module at all; otherwise, the 1756-EN4TR is where Rockwell is pointing new installations. For ring topology at 10/100 speed, the EN2TR remains a fine and very common choice. For a single-port connection on an existing system, the EN2T is the standard module.

For a replacement on an existing system, the question is whether to source the same part or step up. A failed ENBT can be replaced with another ENBT (sourced through secondary market) to change nothing, or stepped up to an EN2T as a near-drop-in successor with more connection capacity. The right call depends on whether your program, network topology, and surrounding modules tolerate the swap, which is the kind of detail that’s worth a quick conversation before you order.

ControlNet Modules (1756-CN Series)

ControlNet remains in service across many process plants, pharmaceutical facilities, and other applications where its deterministic, scheduled-network behavior matters. It’s a smaller story than EtherNet/IP, but a critical one for plants that still depend on it, especially given a major lifecycle event that just happened.

The discontinuation that matters: 1756-CNB and 1756-CNBR. As of December 31, 2024, Rockwell Automation officially discontinued the 1756-CNB and 1756-CNBR ControlNet modules. Both are no longer available for sale through standard channels. Rockwell’s official engineered replacements are:

• 1756-CNB → 1756-CN2 (single-channel)
• 1756-CNBR → 1756-CN2R (redundant media)

If you’re maintaining a plant with installed CNB or CNBR modules, you now have a clear decision: source remaining CNB/CNBR inventory through the secondary market to keep your existing system running unchanged, or step up to the engineered replacement (CN2 or CN2R). Both paths are valid. The same-part route avoids any program or configuration work; the upgrade route gives you a current, supported module with significantly more processing capacity and connection handling.

The four ControlNet modules

And the standard suffix variants apply:

When ControlNet is still the right answer

It’s worth saying plainly: Rockwell is no longer adding new generations to the ControlNet family the way they are with EtherNet/IP. For brand-new builds, EtherNet/IP is the default. But ControlNet is far from dead in installed plants, and there are real reasons it remains in service:

• Deterministic behavior: ControlNet was designed to deliver scheduled, time-critical data with guaranteed timing, which still matters in batch process control and certain coordinated-machinery applications.
• Large installed base: many process plants have ControlNet networks running thousands of nodes that would be costly and risky to migrate. Replacing modules in place is often the right business decision.
• Bridge applications: ControlNet bridges between ControlLogix and older PLC-5 or SLC-500 equipment that won’t migrate to Ethernet.

If you’re maintaining one of these systems, the CN2 and CN2R remain actively supported, and CNB/CNBR are obtainable through secondary inventory while it lasts. For a comprehensive overview of hardware compatibility across the entire platform, you can view our dedicated 1756 communication modules catalog.

Legacy and Specialty Network Modules

Beyond EtherNet/IP and ControlNet, the 1756 platform supports several other networks. These don’t get their own generational stories the way Ethernet does, but each still has real demand in plants running older equipment or specialized field networks.

DH+ and Remote I/O: the 1756-DHRIO

Data Highway Plus (DH+) and Remote I/O were the dominant networks of the PLC-5 and SLC-500 era, and they’re still alive in thousands of plants that haven’t migrated. The 1756-DHRIO is the bridge module that lets a modern ControlLogix system talk to that legacy equipment, whether it’s a PLC-5 running on DH+, a remote I/O rack on RIO, or both (the DHRIO supports both networks on its two channels).

There’s no current replacement for the DHRIO; Rockwell has not introduced a successor product. For new builds, the path is replacing the legacy equipment itself, but for plants keeping their PLC-5s and SLCs in service, the DHRIO remains the standard answer. It’s increasingly hard to find through standard channels and a common sourcing request through secondary inventory.

A conformal-coated 1756-DHRIOK and extended-temperature 1756-DHRIOXT are also part of the family for harsh environments.

DeviceNet: the 1756-DNB

DeviceNet is the network of choice for field-device networks: sensors, valves, motor starters, and other industrial I/O that doesn’t need Ethernet-level bandwidth. The 1756-DNB is the ControlLogix scanner module for DeviceNet, and it remains active and lifecycle-stable. There’s no successor and none needed; DeviceNet is a mature, settled network, and the DNB does its job.

A conformal-coated 1756-DNBK variant is available for harsh environments.

Serial, DH-485, and specialty modules

For legacy serial devices, custom protocols, and specialty connections, two paths exist. Rockwell’s own catalog includes a few specialty modules: 1756-DH485 (for DH-485 networks, the smaller-PLC cousin of DH+), and 1756-SYNCH (for SynchLink fiber-optic time synchronization in coordinated motion applications). These are niche but real.

For broader serial and protocol-bridging needs, the standard approach is a third-party module in the 1756 chassis. The most common is the ProSoft MVI56-series (technically a 1756-compatible module family from ProSoft Technology, an Encompass partner), which handles dozens of protocols, including Modbus TCP, Modbus RTU, DNP3, and legacy serial protocols. These modules drop into a ControlLogix chassis like native 1756 modules and are the standard answer for non-native protocol connectivity.

The 1756 Communication Module Landscape at a Glance

Now that you’ve seen each network in depth, here’s the full 1756 communication module landscape side by side. Use this as a quick reference, a comparison tool when you’re weighing networks for a new build, or a cross-shop view when you’re not sure which family your part number belongs to.

A few patterns stand out across this table. EtherNet/IP is where Rockwell is investing, with a clear four-generation lineup and an active gigabit current module. ControlNet is at a clean inflection point, with both legacy modules discontinued and clean engineered replacements available, which means every ControlNet plant has a real decision to make on its own timeline. DH+ and Remote I/O have no future-direction story, only a present-day “keep it running” story, which is exactly what the DHRIO does and why it remains in demand. DeviceNet quietly persists as the right answer for field-device networks and isn’t going anywhere. Serial and specialty protocols mostly live with third parties in the 1756 chassis, which is the actual market answer rather than a workaround.

If you’re not sure which row matches your part number, the next section breaks down how to read a 1756 communication module catalog number.

Official Documentation: Communication Modules Technical Data

For explicit hardware ratings, exact package sizes, uncompressed capability matrices, and backplane current draw indices, download the factory specifications manual: 1756 ControlLogix Communication Modules Specifications Technical Data (PDF).

Final Architecture and BOM Validation

Before finalizing your parts order or initiating a scheduled plant shutdown, ensure that your communication architecture seamlessly matches your computing environment and termination hardware. Review our sister deployment resources:

• Allen-Bradley 1756 ControlLogix Processor Replacement Guide — Scale processing memory and match network bridges to processor backplanes.
• Allen-Bradley 1756 ControlLogix Chassis & Power Supply Selection Guide — Size racks and specify power delivery to prevent backplane drops.
• Allen-Bradley 1756 ControlLogix I/O Module Guide — Coordinate discrete and analog signal loops across remote network networks.

Pulling It Together

A 1756 communication module isn’t a complicated buy once you understand the landscape. Most decisions come down to three questions: which network are you on, what generation of module do you currently have, and is the right move to source the same part or step up to its current successor.

For new builds, the path is usually clear: EtherNet/IP is the default, the 1756-EN4TR is where Rockwell is pointing buyers, and a current 5580 controller may even let you skip a separate module entirely. For replacements in existing systems, the calculus is different. Sometimes a same-part replacement is the right call, especially when the existing system is running well and any change introduces risk, downtime, or program work you’d rather not take on. Other times, an upgrade to the engineered replacement is the smarter move, especially when you’d be sourcing an increasingly scarce legacy module anyway and a planned shutdown is on the calendar.

The pattern repeats across every network covered in this guide. Whether you’re replacing a 1756-ENBT, a 1756-CNB, a 1756-DHRIO, or any of the other modules above, the same two paths are open to you: keep your system as it is, or upgrade on a planned timeline. Both are valid. What matters is making the choice deliberately rather than under pressure during a breakdown.

Whichever path fits, the starting point is the same: knowing exactly what you have and what’s actually available. Send us your 1756 module catalog number and we’ll confirm what’s in stock, identify the right replacement or upgrade, and get you a quote. Whether it’s a discontinued 1756-CNB to keep a ControlNet plant running, a current 1756-EN4TR for a new build, or a legacy 1756-DHRIO bridging your PLC-5 equipment, contact TCI Supply or call us at 1-800-755-2883.

How to Read a 1756 Communication Module Catalog Number

Once you’ve seen the tables, the catalog-number pattern starts to make sense. Here’s the quick decoder for the 1756 communication module line.

Every module starts with 1756, the ControlLogix chassis bulletin. The letters that follow tell you the network: EN for EtherNet/IP, CN for ControlNet, DHRIO for DH+ and Remote I/O, DNB for DeviceNet, DH485 for DH-485, SYNCH for SynchLink.

For EtherNet/IP, the number after EN signals the generation: 2 for the EN2 series (1756-EN2T, EN2TR, EN2F), 3 for the EN3 family, 4 for the current gigabit EN4TR. After that, the letters flag the variant: T for a copper port, R for dual-port with Device Level Ring, F for fiber, P for Parallel Redundancy Protocol, SC for secure communications.

For ControlNet, the CN2 series is the current generation (CN2 single-channel, CN2R redundant media), succeeding the legacy CNB/CNBR.

Frequently Asked Questions