1756-L73 Processor Replacement Guide

Overview

The Allen-Bradley 1756-L73 is an industry-standard programmable automation controller (PAC) belonging to the ControlLogix 5570 family. Offering 8 megabytes of user memory and high-performance processing capabilities, the 1756-L73 has served as the backbone for complex, high-speed industrial applications, including motion control, process sequencing, and safety integration.

As industrial automation systems evolve, facilities face decisions regarding the repair, direct replacement, or modernization of their aging ControlLogix 5570 hardware. Whether you need a standard "like-for-like" replacement to keep your current configuration intact, or you are preparing to migrate to the next-generation ControlLogix 5580 platform, this guide provides the detailed technical specifications, migration options, and physical replacement procedures necessary to ensure a smooth transition with minimal downtime.

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Legacy Product Information

The 1756-L73 controller features a dual-core CPU designed to split control execution and background communications, improving performance over older ControlLogix 5560 (L6x) series.

Key Technical Specifications

• User Memory: 8 MB (configured for both application code and data storage)
• I/O Memory: 0.98 MB
• Onboard Communication Port: USB 2.0 (Type B) for programming, configuration, and firmware flash updates only (no industrial protocol support)
• Energy Storage Module (ESM): Captive capacitor-based storage (1756-ESMCAP or 1756-ESMNSE). Eliminates the need for a lithium battery.
• Current Draw: 800 mA at 5.1V DC; 5 mA at 1.2V DC
• Power Dissipation: 2.5 Watts
• Thermal Dissipation: 8.5 BTU/hr
• Supported Tasks: 32 tasks (1,000 programs per task using Event, Periodic, or Continuous execution)
• Firmware Compatibility: RSLogix 5000 / Studio 5000 Logix Designer, Version 20.011 through Version 35 (and subsequent releases matching the ControlLogix 5570 family)

Lifecycle Status

The 1756-L73 is currently designated as Active Mature by Rockwell Automation. This status indicates that while the module is still manufactured and fully supported, it is no longer recommended for brand-new system designs. New system installations should prioritize the ControlLogix 5580 series, whereas active operations can continue utilizing direct replacements or migrate existing panels over scheduled maintenance shutdowns.

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Recommended Replacements

When planning a replacement strategy for the 1756-L73, industrial engineers have three primary paths. The choice depends on licensing constraints, physical space, budget, and performance needs.

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Compatibility Considerations

Before replacing or upgrading your 1756-L73 controller, evaluate the following hardware and software dependencies:

Chassis Compatibility

The 1756-L73 is compatible with all standard ControlLogix ControlLogix 1756 chassis, including Series A, B, and C (such as the 1756-A4, 1756-A7, 1756-A10, 1756-A13, and 1756-A17).

• Note: If migrating to the 1756-L83E, Series B or C chassis are highly recommended to support the higher backplane speeds of the 5580 platform.

Energy Storage Modules (ESMs)

The 1756-L73 utilizes capacitor-based storage to write the volatile RAM contents to non-volatile flash memory upon power-down.

• 1756-ESMCAP: Standard capacitor module (removable).
• 1756-ESMNSE: No-electrolyte capacitor module for locations with strict shipping safety constraints or classified hazardous environments. Ensure that your replacement module matches the required ESM for your facility's hazard classification.

Electronic Keying Configurations

When performing a direct swap with another 1756-L73, confirm the Electronic Keying parameter in Studio 5000:

• Compatible Module: Allows the replacement controller to accept the code if the major/minor revisions match within parameters.
• Exact Match: Requires the replacement hardware to run the precise firmware version configured in the project file. If your program is configured for "Exact Match," you must flash the new controller to the identical firmware revision before downloading the program.

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Upgrade Benefits

Upgrading from the legacy 5570 architecture (1756-L73) to the 5580 architecture (1756-L83E) offers significant system-level enhancements:

• Processing Speed: The 1756-L83E utilizes a high-speed multi-core processing engine, executing boolean operations and motion profiles up to 5 to 20 times faster than the 1756-L73.
• Embedded Gigabit Ethernet: The L83E includes a built-in 1 Gbps Ethernet port, removing the requirement for dedicated communication modules (like the 1756-EN2T) for primary control networks, freeing up crucial chassis slots.
• Expanded Memory: The 1756-L83E provides 10 MB of user memory, which represents a 25% increase over the L73, allowing room for future expansion or diagnostics.
• Enhanced Security: Legacy controllers lack advanced hardware-based security. The 5580 series features digitally signed controller firmware, role-based access control, and encrypted communication paths to mitigate cyber threats.

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Common Migration Challenges

While migrating or swapping processors is generally straightforward, keep these common roadblocks in mind:

• Firmware Conversions: If upgrading to a 1756-L83E, your minimum software version moves to Studio 5000 Logix Designer v28 or higher. If your plant standard is locked to v20, a direct 1756-L73 replacement is the only viable path without an engineering overhaul.
• Consumed/Produced Connection Paths: When migrating to an L83E and leveraging the onboard ethernet port, you must update the communication paths for all Consumed/Produced tags and Message (MSG) instructions pointing to or from external controllers.
• Hardware Slot Allocation: Setting up standard I/O connections through an onboard port rather than a dedicated bridge card shifts your network architecture. Make sure your IP addresses, subnets, and gateway configurations are thoroughly documented prior to hardware removal.

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Step-by-Step Replacement Procedure

Follow these detailed steps to safely swap a 1756-L73 processor:

Phase 1: Preparation & Backup

1. Connect a laptop to the active 1756-L73 via the front USB port or network connection.
2. Open Studio 5000 Logix Designer and go Online.
3. Save a complete, runtime backup of the program (Upload and Save the .ACD file). Note the current firmware revision (e.g., v24.012 or v30.011).
4. Verify that the OK LED on the processor is solid green and no faults are present.

Phase 2: Isolation & Disassembly

1. Put the connected process elements into a safe state. Turn the key switch on the front of the 1756-L73 to PROG (Program) mode.
2. De-energize the ControlLogix chassis power supply (e.g., toggle the switch on the 1756-PA72 or 1756-PB72 module). Verifying that all LEDs on the chassis are completely dark.
3. Wear a grounded Electrostatic Discharge (ESD) wrist strap.
4. If a key is present in the mode switch, remove it to prevent damage during extraction.
5. Push the plastic tabs on the top and bottom of the 1756-L73 module inward and pull the module out of the chassis slot.
6. Remove the Energy Storage Module (ESM) gate from the side of the legacy processor and slide out the ESM.

Phase 3: Hardware Verification & Installation

1. Inspect the new/refurbished 1756-L73 unit. Ensure the backplane connectors are free of dust or bent pins.
2. Slide the saved Energy Storage Module (ESM) into the slot on the side of the new processor until it clicks, then close the protective card gate.
3. Set the key switch of the replacement module to PROG mode.
4. Align the replacement module with the guide rails of the designated chassis slot (usually slot 0).
5. Slide the module firmly into the chassis until the top and bottom locking tabs click into place.
6. Turn the chassis power supply switch back to the ON position. Validate that the module performs its power-up self-test sequence.

Phase 4: Firmware Flashing & Program Download

1. Connect your computer to the USB port on the front panel of the new controller.
2. Open ControlFlash or ControlFlash Plus utility software.
3. Select the target 1756-L73 controller via the USB path.
4. Select the target firmware version analyzed during Phase 1. Start the flash sequence and wait for the transfer to complete. Do not interrupt power during the firmware upgrade.
5. Once the flash succeeds, open the backup .ACD file in Studio 5000.
6. Go to Communications > Who Active, find the USB-connected controller, and click Download.
7. Once the download completes, turn the key switch on the controller face to RUN (or REM) and verify that the OK LED turns solid green and the I/O LED is solid green (indicating active, fault-free field communications).

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Frequently Asked Questions

Can I hot-swap a 1756-L73 controller?

No. While ControlLogix utilizes RIUP (Removal and Insertion Under Power) technology for many of its digital and analog I/O modules, removing or inserting a system-critical controller under power can cause immediate backplane noise, communication dropouts, system faults, or hardware damage. Always isolate the chassis power supply before replacing a controller module.

How do I clear an intermittent "Non-Volatile Memory" error on my 1756-L73?

First, check the health of your Energy Storage Module (ESM). If the ESMCAP capacitor has decayed, it will fail to provide enough hold-up power during isolation to write volatile memory to the onboard non-volatile flash. Replace the ESM card with a new 1756-ESMCAP component.

Is the 1756-L73 compatible with older ControlLogix 1756-L61 programs?

Yes, but you must perform a controller type conversion in Studio 5000. Change the controller configuration from 1756-L61 to 1756-L73 and match the project database elements. Be sure to check task execution rates and motion profiles, as the faster CPU clock of the L73 can alter execution timing in unscheduled loops.

What happens if I replace a 1756-L73 with a 1756-L73S guard controller?

The "S" suffix designates a GuardLogix Safety Controller. You cannot download a standard 1756-L73 program directly into a L73S safety processor without revising the controller type in the programming software. Safety-rated systems also require an accompanying safety partner module (1756-L7SP) placed in the adjacent right-side slot.

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Related Products & Families

To complete your system infrastructure or support your 1756-L73 migration project, verify safety stock levels on these related industrial parts:

• Chassis Options: 1756-A4, 1756-A7, 1756-A10, 1756-A13 (Series B & C)
• Power Supplies: 1756-PA72 (85-265V AC input), 1756-PB72 (18-32V DC input)
• EtherNet/IP Modules: 1756-EN2T (Standard EtherNet/IP bridge), 1756-EN2TR (Dual-port ring support), 1756-EN3TR
• Replacement ESMs: 1756-ESMCAP, 1756-ESMNSE (non-hazardous cap)

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Need Help?

Whether you are seeking an in-kind 1756-L73 controller to maintain system configuration or are preparing to execute a planned plant-wide upgrade to the 5580 platform, Palm Parts Solution can assist. We supply a comprehensive inventory of new, refurbished, and certified surplus Allen-Bradley ControlLogix components—each backed by an industry-leading warranty.

Contact our technical support and sales team today to source replacement components or consult on your next migration project.