Replacing an Allen-Bradley 1756-L73 in a running ControlLogix system creates an important decision: should the replacement controller run the same firmware as the original, or should the maintenance event become an opportunity to move the application to a newer revision?

For a plant currently running firmware v19.013, there is no universal answer. Matching the existing revision usually creates the lowest-risk controller replacement, while moving to v21.011 or v31.011 can provide newer engineering features and compatibility with later Studio 5000 environments. The correct choice depends less on the controller hardware itself and more on the surrounding project, software tools, communication modules, HMI environment and plant change-control requirements.

The first fact to establish is that firmware does not change the physical memory capacity of the controller. The Allen-Bradley 1756-L73 is a ControlLogix 5570 controller with 8 MB of user memory and 0.98 MB of I/O memory. Those hardware capacities remain the same whether the controller is operating at revision 19, 21 or 31.

[Image Placeholder 1 — Allen-Bradley 1756-L73 ControlLogix controller installed in a 1756 chassis]

The Real Question Is Replacement or Migration

A controller replacement and a firmware migration are related tasks, but they are not the same engineering activity. If a running plant has a 1756-L73 at v19.013 and the controller fails, installing another L73 at the same major revision allows the maintenance team to preserve the existing software environment and project revision with the fewest intentional changes.

Moving the replacement to v21 or v31 changes that scope. The project must be converted to the target Logix revision, the correct programming software must be available, and communication with the HMI, network modules, drives and other devices should be validated before the system returns to production.

This distinction matters because a firmware upgrade can succeed electrically while the overall migration still fails operationally. A controller that boots normally at the new revision is only one part of the test; the project, I/O, messaging, HMI tags, motion configuration and network communications must also behave as expected.

First Correct the Software-to-Firmware Relationship

Firmware major revisions in the Logix platform are closely associated with the corresponding RSLogix 5000 or Studio 5000 Logix Designer project revision. This is one area where incorrect version tables can cause unnecessary confusion.

Version 21 is particularly significant because Rockwell renamed the RSLogix 5000 programming environment as the Logix Designer application within Studio 5000. It therefore represents both a controller firmware change and an important transition in the engineering software environment.

Version 31 is a considerably later generation, but it should not be described as the current 1756-L73 firmware. Rockwell has continued releasing later ControlLogix 5570 firmware revisions. For an existing plant, however, v31.011 may still be a deliberate target because the site's validated engineering environment is often more important than simply selecting the newest available firmware.

What Rockwell Actually Documents for Revision 19

Revision 19 is mature and widely encountered in long-running ControlLogix installations, but “old and stable” should not be interpreted as “free from documented anomalies.” Rockwell's revision 19 release notes contain both corrected and known anomalies affecting 1756-L7x controllers, including the 1756-L73.

For example, revision 19 documentation identifies conditions involving motion instructions, online edits and HMI activity that can produce abnormal controller behavior. Rockwell documented a condition in which applications with a large number of HMI tags being scanned could experience a nonrecoverable major fault during online editing. Other revision 19 anomalies are specific to motion configurations and therefore may have no relevance to a conventional process or machine-control application.

This is an important distinction when evaluating firmware risk. A documented anomaly should not automatically be treated as a reason to migrate every controller. Engineers should first determine whether the affected instruction, motion configuration, communication pattern or operating condition exists in the actual application.

Rockwell's official ControlLogix Revision 19 Release Notes should therefore be reviewed against the project before deciding that v19.013 must either be retained or abandoned.

Why v21.011 Is More Than a Simple Bug-Fix Revision

Revision 21 introduced several changes that are useful beyond individual anomaly corrections. Rockwell added improvements to HMI connectivity that allowed approved communication drivers to use symbolic data reads, helping improve HMI read performance and reducing certain data-access errors.

Another important feature was the ability to store project documentation in the controller. With Logix Designer v21, comments, tag descriptions and other project documentation can be stored with the controller project and recovered during an upload. For maintenance teams inheriting systems where the original workstation project may eventually disappear, this can be a meaningful lifecycle advantage.

It is worth describing this feature accurately. Revision 21 did not suddenly give the L73 more application memory, nor did it transform the controller into a different hardware platform. The benefit is that more of the engineering documentation associated with the project can remain available with the controller rather than existing only in a workstation ACD file.

The official ControlLogix Revision 21 Release Notes provide the detailed list of enhancements, corrected anomalies and known restrictions.

[Image Placeholder 2 — Studio 5000 Logix Designer controller properties or firmware revision comparison]

Do Not Treat an HMI Upgrade as Automatically Mandatory

A common mistake in firmware migration guides is to publish a fixed statement such as “firmware v20 or later requires FactoryTalk View v11” or “RSLinx Classic must be v3.91.” That is too broad for a 1756-L73 migration because HMI compatibility depends on the actual communication path, driver, FactoryTalk generation and features used by the project.

Moving from revision 19 into a later Logix generation should trigger an HMI compatibility review, but not an automatic assumption that every HMI package must be replaced. FactoryTalk View, FactoryTalk Linx or RSLinx, OPC servers and third-party drivers should be checked against the selected controller and Studio 5000 revision using Rockwell's Product Compatibility and Download Center and the vendor's own compatibility information.

This is particularly important with older SCADA systems. A production HMI may have operated unchanged for many years, and changing several software layers during an emergency controller replacement creates far more risk than simply matching the failed controller's firmware.

Legacy DeviceNet and ControlNet Need Their Own Review

Many 1756-L73 systems from the revision 19 era still contain DeviceNet or ControlNet. Their presence does not automatically prevent a move to a later controller firmware, but it does increase the number of components that must be checked before migration.

A system using a 1756-DNB DeviceNet Scanner, for example, may have scanner firmware, RSNetWorx configuration, EDS dependencies and connected field devices that have remained untouched for years. The controller migration plan should preserve those known-good relationships unless there is a specific reason to change them.

The same principle applies to ControlNet. Network scheduling and configuration should not be treated as incidental details simply because the ControlLogix controller itself supports later firmware. Rockwell's ControlNet documentation remains useful when evaluating older ControlLogix architectures.

If the plant already uses EtherNet/IP, communication through a 1756-EN2T EtherNet/IP Bridge should also be reviewed as part of the complete chassis architecture. Controller firmware should never be evaluated as though the CPU were the only intelligent device in the system.

v19.013: The Lowest-Change Replacement Strategy

If the existing production system is stable, the project is available in RSLogix 5000 v19, and there is no migration requirement, matching v19.013 is usually the most conservative replacement strategy. The objective in this scenario is not modernization; it is restoring the machine or process with the smallest possible change footprint.

This strategy is particularly attractive when the system includes old HMIs, DeviceNet, ControlNet, third-party communication drivers or OEM equipment that has not been validated against later software revisions. It also avoids converting the project during an emergency maintenance event.

The disadvantage is obvious: the plant remains dependent on an old engineering environment. Workstations, operating-system compatibility, software licensing and long-term support become increasingly important as the installation ages. A successful same-version replacement solves today's controller failure without necessarily solving tomorrow's engineering-support problem.

v21.011: A Moderate Step Into Studio 5000

Revision 21 can make sense when a plant wants to move beyond the RSLogix 5000 v19 environment without making a much larger modernization jump. It introduces the Studio 5000 generation, HMI communication improvements and controller-stored project documentation while remaining much closer historically to the original revision 19 system.

That does not make v21 automatically safer than v31. It simply means the migration distance is smaller. The converted project still needs to be reviewed, downloaded and functionally tested, and all important communications should be validated before production resumes.

For an older ControlLogix system where documentation recovery is a concern, the ability to retain more engineering documentation with the controller can be a strong argument for v21 or later.

v31.011: Better Viewed as a Planned Modernization

Revision 31 is more appropriate when the plant already maintains a later Studio 5000 environment or has decided to standardize older controllers around a newer engineering baseline. The 1756-L73 supports firmware v31.011, so the controller hardware itself is not the limiting factor.

The challenge is migration scope. Jumping from v19 to v31 crosses many generations of Logix software and firmware changes. A project that converts without errors still requires engineering review because controller behavior, module profiles, communication software and integrated devices may have changed across those releases.

For this reason, v31 should normally be approached as a controlled modernization project rather than an incidental step performed while replacing a failed processor. It becomes much easier to justify when the site already has v31 engineering workstations, later FactoryTalk software and a tested hardware configuration.

The 1756-L73 Memory Does Not Increase With Firmware

One technical misconception deserves special emphasis because it appears frequently in unofficial comparison tables. The 1756-L73 does not have 2 MB at revision 19 and 3 MB at later firmware.

Firmware conversion can change how much memory a specific project requires because firmware generations may allocate memory differently for instructions, tags and system features. That is different from changing the controller's physical memory capacity. Rockwell recommends using the project memory estimation tools when moving applications between revisions.

For full hardware specifications, see Rockwell's ControlLogix 5570 and 5560 Controllers User Manual. Additional Bulletin 1756 reference material is available in the Rockwell Automation 1756 documentation.

A Safer Firmware Change Workflow

Firmware migration on a production controller should be handled as a controlled engineering change, not simply as a flash operation. Before changing anything, preserve the original project at its current revision, document the controller and communication-module firmware, record important network configuration, and confirm that the engineering workstation can open the original application.

Create a separate converted copy for the target revision rather than treating the converted file as the only remaining project. Logix projects can be migrated forward, but returning a controller to an older firmware revision does not magically convert a newer project back into the old software format. If rollback is required, the original project at the original revision is extremely valuable.

Rockwell firmware can be installed using the supported ControlFLASH or ControlFLASH Plus workflow, depending on the software environment and firmware package. USB or an appropriate network path may be used where supported. During the firmware operation, power and communication should remain stable, and the controller should not be treated as available for production control.

After flashing, verify the controller revision before downloading the converted project. The complete application should then be tested, including local and remote I/O, produced and consumed tags, MSG instructions, HMI/SCADA data, alarms, drives, motion functions and any DeviceNet or ControlNet communications relevant to the plant.

[Image Placeholder 3 — Firmware upgrade workflow: backup, compatibility check, flash, download and functional validation]

Do Not Assume Downgrade Equals Instant Rollback

The 1756-L73 can be flashed to supported lower firmware revisions when the hardware and firmware package permit it, but operational rollback requires more than changing the firmware number. A project converted to a later Logix revision should not be assumed to open directly in an earlier RSLogix or Studio 5000 version.

This is why the original v19.013 ACD file should be preserved before any migration. If the v31 project causes an unexpected integration problem and the plant decides to return the processor to v19.013, the clean rollback path is the original firmware plus the original revision-19 project, followed by verification of the original communication architecture.

Which Revision Makes Sense for Your 1756-L73?

The Practical Decision

For a replacement 1756-L73 in a stable revision-19 system, matching firmware v19.013 remains the lowest-change option. There is little value in turning an emergency processor replacement into a multi-generation controls migration unless the existing firmware or engineering environment is already creating a real operational problem.

Revision 21.011 is a reasonable intermediate target when the plant deliberately wants to enter the Studio 5000 generation and benefit from features such as controller-stored project documentation. Revision 31.011 can also run on the L73, but a jump from v19 to v31 should be treated as a planned migration with compatibility testing rather than a routine firmware update.

The most important rule is therefore not “always stay old” or “always upgrade.” Preserve the known-good application first, identify the dependencies around the controller, and change only as much of the system as the maintenance objective requires.

For engineers maintaining mixed-generation systems, the broader Allen-Bradley ControlLogix platform includes controllers, communication modules and I/O spanning several firmware generations. Evaluating the entire chassis and network architecture is more useful than selecting a controller revision in isolation.

FAQ: Can a 1756-L73 Be Downgraded After a Firmware Upgrade?

Yes, supported firmware revisions can generally be flashed onto a compatible 1756-L73. However, firmware rollback and project rollback are separate issues. Preserve the original project at its original Logix revision before upgrading, because a project converted to a later Studio 5000 revision should not be assumed to convert backward automatically.

FAQ: What Software Is Used With 1756-L73 Firmware v31.011?

Firmware revision 31.011 belongs to the Studio 5000 Logix Designer v31 generation. It is incorrect to state that v31.011 requires Studio 5000 v32 merely because v32 is newer. The engineering software and controller project revision must be selected according to Rockwell's compatibility information for the target firmware.

FAQ: Does Firmware v21 Increase 1756-L73 Memory?

No. The 1756-L73 has 8 MB of user memory as a hardware specification. Later firmware can change the memory required by an application, but it does not convert the controller into a larger-memory model.

FAQ: What Did Revision 21 Add for Project Documentation?

Studio 5000 Logix Designer v21 introduced the option to store project documentation such as comments, tag descriptions and other descriptors in the controller. This improves project recovery when the original workstation ACD file is unavailable.

FAQ: Can a v31 Controller Still Use DeviceNet?

A ControlLogix 5570 architecture can include DeviceNet communication through compatible chassis modules such as the 1756-DNB. However, controller firmware alone does not prove that an entire installed DeviceNet system is ready for migration. Scanner firmware, engineering software, network configuration and connected devices should be checked before upgrading a production system.

FAQ: Should I Upgrade a Stable v19.013 System Just Because the Firmware Is Old?

Not necessarily. Age alone is not a sufficient engineering reason to modify a stable control system. The stronger reasons are documented anomalies relevant to the application, loss of engineering-software support, workstation migration, cybersecurity or lifecycle requirements, and a planned controls modernization strategy.