Product Overview

The Allen‑Bradley 1756‑L1/A is a pioneering processor module from the ControlLogix family of programmable automation controllers (PACs) by Rockwell Automation. As one of the earliest models in the renowned Logix 5550 series, the 1756‑L1/A established the foundation for the modern Logix control architecture that has become an industry standard-1-10. This single-slot controller is designed for installation in a 1756 I/O chassis, delivering deterministic, high-performance control for a wide range of industrial automation applications.

The 1756‑L1/A provides 64 KB of user memory for logic programs, tag data, and I/O configuration, making it well-suited for small to medium-scale discrete control tasks-1-6-10. Its 64KB capacity, while modest by contemporary standards, was sufficient to drive many manufacturing processes and represented a significant step forward from earlier PLC platforms. The processor features an integrated RS-232 serial communication port for programming and data exchange, supporting the DF1 protocol-1-3. Additionally, the 1756‑L1/A is equipped with a removable memory option and battery-backed RAM (requiring a 1756‑BA1 or 1756‑BA2 lithium battery) to retain programs and data during power interruptions-1-3. Front-panel status LEDs—including OK, RUN, I/O, FORCE, and BAT—provide immediate visual feedback on the processor’s operational state-1-10.

 

Technical Specifications

Parameter Name	Value
Product Model	1756-L1/A
Manufacturer	Rockwell Automation / Allen‑Bradley
Product Type	ControlLogix Processor Module (Logix 5550)
User Memory	64 KB (for logic, tags, and I/O configuration)-1-3-6
I/O Memory	4 KB to 64 KB (local I/O capacity)-1-5
Maximum Local I/O	Up to 256 digital or 128 analog points (system-dependent)-6
Total I/O Capacity	128,000 Discrete, 4,000 Analog (theoretical maximum)-3
Execution Time	Approx. 0.4 ms / K instruction (mixed logic)-1
Communication Port	1 x RS-232 Serial (DF1 / DH-485 protocol)-1-3
Backplane Current (5V)	120 mA (typical) / 800 mA (peak)-3-10
Backplane Current (24V)	2 mA-3
Power Dissipation	3.3 W to 4.1 W (depending on load)-1-3
Slot Width	1 (ControlLogix chassis)-3
Operating Temperature	0°C to 60°C-1-3
Storage Temperature	-40°C to 85°C-3
Compatible Memory Modules	1756‑M1, 1756‑M2, 1756‑M3-3-4
Compatible Battery	1756‑BA1, 1756‑BA2-1-3
Programming Software	RSLogix 5000 (Version 10 or earlier)-1-6-10

 

Main Features and Advantages

The 1756‑L1/A is valued for its foundational role in the ControlLogix platform and its ability to execute the robust RSLogix 5000 programming environment. This processor supports a rich instruction set that includes traditional ladder logic, structured text, and function block diagrams, enabling engineers to implement complex control strategies. The 1756‑L1/A supports a powerful multitasking operating system that allows configuration of up to 32 continuous or periodic tasks, each with its own priority level for optimized code execution. Each task can be subdivided into up to 32 programs with local data, providing excellent modularity and code reuse.

A significant advantage of the 1756‑L1/A lies in its compatibility with legacy systems and program migration. It supports the transition of logic from older Allen‑Bradley PLC-5 and SLC 500 platforms, allowing users to modernize their control architecture while preserving established control strategies-6. This backward compatibility makes the 1756‑L1/A a valuable asset for facilities with decades-old automation infrastructure.

The 1756‑L1/A also offers flexible expansion through its support for optional memory modules. Users can enhance the processor’s storage capacity for program and data retention using 1756‑M1, 1756‑M2, or 1756‑M3 memory expansion cards, with the 1756‑M2 providing an additional 1 MB of memory-3-4. Additionally, the processor’s RS-232 port facilitates direct connection to a programming terminal, enabling convenient firmware updates, program upload/download, and online diagnostics. Multiple 1756‑L1/A controllers can even be installed within the same ControlLogix chassis, communicating across the backplane while operating independently, increasing system redundancy and flexibility-3.