ControlLogix Processor 1756L55 1756 L55 1756L55

Allen-Bradley 1756-L55 | ControlLogix Logix5555 Processor Module — No Onboard User Memory, Requires 1756-M Series Memory Board, Multi-Network Support

Part Number: 1756-L55

Product Designation: Logix5555 (ControlLogix Processor)

Manufacturer: Allen-Bradley / Rockwell Automation (USA)

Product Line: ControlLogix — 1756 Series

Function: Central execution processor for the ControlLogix programmable automation controller chassis; executes ladder logic, function block diagram, structured text, and sequential function chart programs

Onboard User Memory: None — requires a separate 1756-M series memory board 

Weight: Approx. 0.35 kg (12.5 oz) 

Humidity: 5 to 95% (non-condensing)

Vibration: 2g at 10–500 Hz

Operating Shock: 30g

Non-Operating Shock: 50g

Isolation Voltage (continuous): 30V

Isolation Test: 707V DC for 60 seconds

Battery: 1756-BA1 or 1756-BATM (for SRAM retention during power loss) 

Programming Software: RSLogix 5000

Status: Discontinued — active legacy/surplus market

Applications: Discrete manufacturing, process control, motion control, batch processing, safety-integrated systems (with appropriate safety modules)

Overview

The Allen-Bradley 1756-L55 is the Logix5555 processor — the execution engine at the centre of a ControlLogix 1756 Series chassis, responsible for running the Logix5000 program that controls the machine or process.

What makes the 1756-L55 architecturally distinctive from later ControlLogix processors is its memory design: the processor module itself carries no user program memory. To function, it must be paired with a 1756-M series memory board that plugs into the processor module's expansion connector.

The combined assembly — processor base plus memory board — forms the complete controller, and the memory board's capacity determines how large a program the system can hold.

This separation of processor and memory was deliberate at the time of the Logix5555 platform's design. It allowed users to upgrade memory capacity independently of the processor — adding a larger memory board to the same 1756-L55 when the application program grew beyond the original memory allocation, without replacing the processor itself. The 1756-M family offered memory capacities from 750KB through 7.5MB, in both volatile (SRAM-backed by the 1756-BA1 battery) and non-volatile (Flash memory with automatic backup) configurations.

The 1756-L55's position in the ControlLogix processor family sits at the high end of the original Logix5000 generation — the "5555" designation indicated the largest, most capable processor in the original Logix5000 series.

Where lower Logix controllers (Logix5550, Logix5553) were sized for smaller applications, the Logix5555 was the platform for complex, high-channel-count control systems requiring multi-network integration, coordinated motion control across multiple axes, and programs of significant size and complexity.

Key Specifications

Memory Architecture — The 1756-M Board Requirement

The most operationally critical thing to understand about the 1756-L55 is that it cannot run without a memory board.

At power-up without a 1756-M board installed, the processor will enter a memory fault condition and will not execute any user program.

The controller's RUN LED will indicate the fault, and RSLogix 5000 will report the absence of installed memory.

The 1756-M memory boards divide into two categories:

Volatile (battery-backed SRAM) boards: 1756-M12 (750KB), 1756-M13 (1.5MB), 1756-M14 (3.5MB), 1756-M16 (7.5MB).

These boards hold their contents as long as the 1756-BA1 or 1756-BATM battery maintains SRAM power. When the battery fails or the chassis loses power while the battery is depleted, the program is lost and must be redownloaded from RSLogix 5000.

The BATT LED on the 1756-L55 indicates low battery status — when it turns red, replace the battery with chassis power applied to avoid data loss.

Non-volatile (Flash memory) boards: 1756-M22 (750KB), 1756-M23 (1.5MB), 1756-M24 (3.5MB).

These boards combine SRAM working memory with integrated Flash backup — the program is automatically saved to Flash, and at power-up, the controller restores the program from Flash to SRAM automatically. 

Non-volatile memory boards eliminate the battery-dependency concern for program retention, though a battery is still recommended to maintain clock data and certain system parameters.

Multi-Tasking and Multi-Network Architecture

The ControlLogix platform's defining engineering advantage was its multi-tasking execution model combined with its network-agnostic communication architecture. The 1756-L55 inherits both:

Multitasking: The Logix5000 operating environment supports multiple concurrent tasks — continuous task (running perpetually in background), periodic tasks (triggered at fixed time intervals), and event tasks (triggered by I/O changes or program events).

Task priorities are configurable, allowing time-critical functions (motion control, safety monitoring) to preempt less time-critical sequences (data logging, HMI communication) at defined priority levels.

This deterministic task execution lets engineers guarantee specific update rates for critical control loops regardless of the overall program complexity.

Network architecture: The ControlLogix chassis accepts communication modules for any Rockwell Automation network — EtherNet/IP (1756-EN series), ControlNet (1756-CN series), DeviceNet (1756-DNB), DH+ (1756-DHRIO), Remote I/O (1756-DHRIO), and SynchLink (1756-SYNCH) — in any combination of slots.

A single ControlLogix chassis can simultaneously participate in multiple networks, bridging data between them through the 1756-L55's messaging architecture.

Remote I/O racks, drives, operator terminals, and other controllers on different networks all appear to the program as data tags, abstracting the network topology from the control logic.

Dual CPU Architecture — Logix CPU and Backplane CPU

The 1756-L55 contains two processing cores: the Logix CPU that executes the application program (user tasks, logic routines, motion instructions) and a dedicated backplane CPU that manages all communication with the chassis backplane and installed communication modules.

This separation was central to the ControlLogix's performance claim: the application program's execution is not slowed by I/O communication tasks, because the backplane CPU handles all data traffic independently.

The Logix CPU passes data to and from the backplane CPU's shared memory region; the backplane CPU handles the actual backplane bus protocol and buffers incoming and outgoing I/O data.

This dual-CPU design made the ControlLogix platform's I/O scan time essentially independent of the application program's complexity — adding more logic rungs to the program does not slow down I/O updates, because the two CPUs run in parallel.

FAQ

Q1: The 1756-L55 is listed with no user memory. Can it be purchased without a memory board and have one installed later?

Yes. The 1756-L55 base processor and the 1756-M memory board are ordered separately and assembled by installing the memory board into the expansion connector on the processor module.

The assembly is straightforward: the board plugs directly into the 1756-L55's memory connector and is secured mechanically.

The processor can then be inserted into any ControlLogix 1756 chassis slot. 

The memory board must be installed before attempting to download or run any program — operating without a memory board produces a fault condition that prevents program execution.

Q2: What battery is required, and what happens if the battery fails?

The 1756-L55 uses the 1756-BA1 lithium battery (or 1756-BATM battery module for some configurations). The battery maintains SRAM memory when the chassis loses power.

The BATT LED on the processor front face illuminates red when battery voltage drops below the threshold — this is the warning to replace the battery while the chassis is powered, which avoids the brief power interruption to SRAM during battery swap. 

If the battery is allowed to fully discharge and the chassis then loses power, all program data in the SRAM memory board is lost. Non-volatile memory boards (1756-M22/M23/M24) automatically restore their contents from Flash backup at power-up, eliminating this risk.

Q3: Can the 1756-L55 be used in a redundant ControlLogix system?

Yes. The ControlLogix platform supports redundant controller configurations where two 1756-L55 controllers (with identical memory boards and communication modules) operate in primary/secondary pairs.

The redundancy is coordinated through a 1756-RM redundancy module installed in each chassis, and the synchronisation channel uses ControlNet or a dedicated SynchLink connection between the two redundant chassis.

When the primary controller faults, the secondary automatically assumes control within the redundancy switchover time.

The ControlNet communication modules (1756-CN2) are required for redundant system communication; EtherNet/IP does not support the synchronisation requirements of the redundant system in the 1756-L55 generation.

Q4: The 1756-L55 is discontinued. Which current Allen-Bradley controller replaces it?

Rockwell Automation's current ControlLogix generation — the 1756-L8x series (1756-L82, 1756-L83, 1756-L84, 1756-L85) — is the recommended replacement for the 1756-L55. The 1756-L8x processors have integrated non-volatile memory (no separate memory board required), faster processors, built-in USB and SD card interfaces, and run the same Logix5000 programming environment (now Studio 5000 Logix Designer).

Programs written for the 1756-L55 can be imported into Studio 5000 with appropriate version updates.

The 1756 chassis and most 1756 I/O and communication modules are retained in the migration, so the mechanical infrastructure of an existing 1756 installation typically transfers to the new processor.

Q5: What programming languages does the 1756-L55 / Logix5000 support?

The Logix5000 controller runtime supports all four IEC 61131-3 programming languages: Ladder Diagram (LD), Function Block Diagram (FBD), Structured Text (ST), and Sequential Function Chart (SFC).

Each language can be used in separate routines within the same program, and routines in different languages can call each other — allowing engineers to use ladder for discrete logic where they are most comfortable, structured text for mathematical algorithms and string handling, and function block diagram for PID loops and continuous control.

The languages can coexist within the same RSLogix 5000 project, with the compiler generating a unified executable program for the controller.