1756-L55M14 Allen-Bradley ControlLogix 5555 Processor (1.5 MB)

Product Overview and Industrial Computing Heritage

The 1756-L55M14 (1756L55M14) is a robust, high-performance central processing unit within the ControlLogix 5555 family. It was engineered to manage complex control tasks in the era of early Integrated Architecture, providing a bridge between legacy automation and modern Ethernet-based systems. Featuring 1.5 MB of user memory, this controller is ideal for medium-to-large scale applications that require extensive data tracking, complex floating-point math, and coordinated motion. The 1756-L55M14 utilizes a modular memory design (M14), allowing for reliable program execution in industries ranging from automotive assembly to high-volume pharmaceutical packaging.

Hardware Architecture and Memory Dynamics

The 1756-L55M14 is a chassis-based processor that occupies one slot in a 1756 ControlLogix rack. Its architecture is designed for deterministic logic execution and high-speed backplane communication.

• User Memory (1.5 MB): The "M14" suffix designates the specific non-volatile memory module installed. This capacity allows for substantial program logic and tag storage.

• Non-Volatile Storage: Compatible with 1756-M1, -M2, and -M3 expansion memory modules. For permanent backup, it supports the 1756-BA1 or 1756-BA2 lithium batteries to retain the volatile RAM contents during power-down.

• Multitasking Operating System: Supports multiple tasks with different priorities, including continuous, periodic, and event-based tasks, ensuring critical control loops are never delayed by background communication.

• Serial Interface: Includes a built-in RS-232 (DF1/DH-485) port, enabling direct connection for local programming, HMI interfacing, or modem communication.

Technical Specifications

Integrated Technical FAQ

What is the difference between the 1756-L55 and the newer 1756-L6x processors?

The 1756-L55 series utilizes a physically separate memory module (like the M14) that is plugged into the processor board, whereas the 1756-L6x (5560) series has integrated onboard memory. The L55 also relies on a battery for RAM retention, while newer models use energy storage modules (ESMs) and internal flash.

How do I clear a "Solid Red OK" LED on this processor?

A solid red OK light typically indicates a non-recoverable hardware fault. However, you can attempt to cycle power and perform a factory reset by removing the battery and shorting the battery terminals on the processor (with power off) to clear the memory. If the red light persists after a firmware re-flash, the unit may require hardware repair.

Is the 1756-L55M14 compatible with the latest version of Studio 5000?

The 1756-L55 series is supported up to RSLogix 5000 version 16. It is not compatible with version 17 or higher (including Studio 5000). For modern software compatibility, a migration to the 1756-L7x or L8x series is required.

Engineering & Installation Protocols

• Battery Maintenance: The 1756-BA1 battery should be replaced annually. If the "BATT" LED illuminates, you have approximately two weeks to replace the battery before the program in the volatile RAM is lost during a power cycle.

• Firmware Management: Ensure the firmware revision (e.g., v15 or v16) matches the version of RSLogix 5000 used for the project. Mismatched firmware will prevent the processor from entering "Run" mode.

• Electrostatic Discharge (ESD): When handling the 1756-L55M14, particularly when installing or removing the M14 memory module, always use an ESD wrist strap. The internal CMOS circuits are highly sensitive to static and can be permanently damaged by invisible discharges.

Engineering Advantages

The 1756-L55M14 remains a critical component for legacy system maintenance. Its modular memory approach allows for easy upgrades; an "M12" processor can be upgraded to an "M14" simply by swapping the memory card. By supporting the full suite of Logix5000 instructions—including Sequential Function Charts (SFC) and Structured Text—it provides sophisticated control capabilities that were far ahead of its time. For facilities looking to maintain existing ControlLogix installations without the risk of a full-scale control system overhaul, the 1756-L55M14 provides the perfect balance of memory capacity and proven industrial reliability.