1756-M3 | AB Allen-Bradley | ControlLogix Logix5550 Expansion Memory Module

Product Summary

The Allen-Bradley 1756-M3 is a 2 MB Expansion Memory Module by Rockwell Automation, designed for use with the legacy Logix 5550 (1756-L1) ControlLogix processor. It provides additional non-volatile memory to expand the controller's user program capacity from the base 160 KB to 2 MB total, enabling larger and more complex industrial automation applications on early ControlLogix platforms.

Core Specifications

• Model Number: 1756-M3
• Type: ControlLogix Expansion Memory Module
• Memory Capacity: 2 MB (2 megabytes)
• Compatible Controllers: 1756-L1 Logix 5550 (memory range 750 KB to 32 MB)
• Backplane Current: 1.20–1.25 A at 5.1 VDC; 0.014 A (14 mA) at 24 VDC
• Power Dissipation: <6.2 W typical
• Weight: 0.22 kg (7.7 oz)
• Manufacturer: Allen-Bradley by Rockwell Automation

Electrical & Functional Characteristics

• Memory Type: Non-volatile expansion memory for user program and data storage
• Installation: Plugs into dedicated memory expansion slot on 1756-L1 processor
• Base Memory: 1756-L1 base controller includes 160 KB user memory; 1756-M3 expands to 2 MB total
• Data Retention: Battery-backed RAM with lithium battery (1756-BA1 or 1756-BA2)
• Status Indicators: Controller status LEDs indicate memory health and battery status

System Integration & Compatibility

• Primary Platform: ControlLogix 1756 chassis (first generation)
• Compatible Processor: 1756-L1 Logix 5550 Controller
• Related Memory Modules:
  • 1756-M1 — 512 KB expansion memory
  • 1756-M2 — 1 MB expansion memory
  • 1756-M3 — 2 MB expansion memory (this part)
  • 1756-M1K — 512 KB conformal coated
  • 1756-M2K — 1 MB conformal coated
  • 1756-M3K — 2 MB conformal coated
• Related Processors: 1756-L1M1 (512 KB base), 1756-L1M2 (1 MB base), 1756-L1M3 (2 MB base with integrated memory)

Environmental & Operational Limits

• Operating Temperature: 0°C to 60°C (32°F to 140°F)
• Storage Temperature: -40°C to +85°C (-40°F to 185°F)
• Humidity: 5% to 95% RH, non-condensing
• Vibration: 2G, 10 to 500 Hz
• Shock: 30G operating
• ESD Immunity: 6 kV indirect contact discharge; 8 kV air discharge (IEC 61000-4-2)
• Emissions Class: Group 1, Class A
• IEC Temperature Code: T4A / T4

Industrial Applications

Deployed in legacy ControlLogix systems from the late 1990s to early 2000s, primarily in manufacturing, material handling, and process control applications where expanded program memory was required for complex ladder logic, sequential function charts, and structured text programs on the original Logix 5550 platform.

Frequently Asked Questions

What is the 1756-M3 used for?

It is a 2 MB memory expansion module for the 1756-L1 Logix 5550 ControlLogix processor, expanding user program capacity from 160 KB base memory to 2 MB total.

Which controllers are compatible with the 1756-M3?

The 1756-M3 is designed specifically for the 1756-L1 Logix 5550 first-generation ControlLogix controller. It is not compatible with later 1756-L6x, L7x, or L8x series controllers which have integrated memory.

What is the difference between 1756-M3 and 1756-M3K?

The 1756-M3 is the standard version. The 1756-M3K features conformal coating on the circuit boards for enhanced protection against moisture, dust, and corrosive gases in harsh industrial environments.

Is the 1756-M3 still supported by Rockwell Automation?

No — the 1756-L1 Logix 5550 platform and its memory modules are legacy/obsolete products. Rockwell Automation no longer manufactures or supports these modules. Replacement units must be sourced from authorized surplus dealers or refurbishment services.

What are the related part numbers?

• 1756-M1 — 512 KB expansion memory
• 1756-M2 — 1 MB expansion memory
• 1756-M3 — 2 MB expansion memory (this part)
• 1756-M1K — 512 KB conformal coated
• 1756-M2K — 1 MB conformal coated
• 1756-M3K — 2 MB conformal coated
• 1756-L1 — Logix 5550 base processor (160 KB)
• 1756-L1M3 — Logix 5550 with integrated 2 MB memory