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Allen-Bradley 1756-L55M14: The ControlLogix5555 Processor with 3.5 MB Memory Explained
Allen-Bradley 1756-L55M14: The ControlLogix5555 Processor with 3.5 MB Memory Explained
The Allen-Bradley 1756-L55M14 is a Logix5555 processor module from Rockwell Automation’s classic ControlLogix platform. It pairs the 1756-L55 controller base with a 1756-M14 memory board to deliver 3.5 MB of user memory, making it one of the most widely used CPUs in legacy mid-size automation systems.
What Is the 1756-L55M14?
The 1756-L55M14 is a ControlLogix 5555 processor:
- 1756-L55 = CPU base (Logix5555 architecture)
- M14 = 3.5 MB battery-backed RAM memory board installed on the CPU
- No on-board non-volatile flash by default — program retention relies on the controller battery (1756-BA1 / 1756-BATM)
It mounts in a standard 1756 ControlLogix chassis (A4/A7/A10/A13/A17 racks) and executes logic under RSLogix 5000 (up to roughly v16 firmware; not supported by Studio 5000 v17+).
Key Specifications
|
Parameter
|
1756-L55M14
|
|---|---|
|
Platform
|
ControlLogix (Logix5555)
|
|
User memory
|
3.5 MB (battery-backed RAM)
|
|
I/O memory
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~208 KB
|
|
Max I/O points
|
128,000 (4,000 AI / 2,000 AO)
|
|
Scan time
|
~0.08 ms typical bit logic
|
|
Comms
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RS-232 (CH0); Ethernet/ControlNet/DeviceNet via add-on modules
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|
Backplane current
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1.23 A @ 5V, 0.014 A @ 24V
|
|
Battery
|
1756-BA1 / 1756-BATM
|
|
Temp range
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0–60 °C operating
|
|
Status
|
Discontinued by Rockwell
|
Why the 1756-L55M14 Still Matters
Proven Mid-Tier Control Power
For years, the L55M14 was the sweet spot for line control, batch systems, and machine supervision — enough memory for complex routines without jumping to the larger L6x/L7x tiers.
Battery-Backed Program Model
The 3.5 MB RAM is fast and editable, but depends on a healthy lithium battery. With a good battery and clean power, the controller retains the project through shutdowns.
Drop-In to Existing 1756 Racks
It uses the same chassis, backplane, power supply, and I/O modules as other ControlLogix generations — ideal as a like-for-like spare while a plant delays a full migration.
Network Flexibility via Modules
No native Ethernet port, but with 1756-ENBT/EN2T, CNB, or DNB cards it talks EtherNet/IP, ControlNet, and DeviceNet — enough for most legacy integrations.
Typical Application Scenarios
Manufacturing Lines
Packaging, conveying, assembly, and batch sequencing where RSLogix 5000 v13–v16 projects are already validated.
Power & Utility Aux Systems
Switchgear interlocks, motor control centers, turbine aux controls, and substation SCADA front-ends built on ControlLogix 5555.
Oil & Gas and Compressor Stations
Pipeline compressor control, ESD aux logic, and vendor-package controllers that were shipped with L55-class CPUs.
Legacy System Life-Extension
Many 90s/2000s ControlLogix panels are still running. A tested L55M14 keeps the system alive while the migration business case is evaluated.
Migration Path (When You’re Ready)
If you’re outgrowing 3.5 MB or fighting spare availability:
- Short-term: 1756-L62 (4 MB) or 1756-L63 (8 MB) in same chassis (firmware/project update required)
- Modern drop-in: 1756-L71/L73/L74 (Logix5570, built-in Ethernet, larger memory)
- Current gen: 1756-L8x (Logix5580) with Studio 5000, CIP Security, redundant RM2
Note: L55 → L7x/L8x is not a pure download. You’ll need a controlled project migration, tag reconciliation, and firmware alignment.
Conclusion
The Allen-Bradley 1756-L55M14 is a field-proven ControlLogix5555 CPU with 3.5 MB of battery-backed memory. It remains a practical spare for legacy 1756 racks and a low-risk way to keep an aging automation system running — as long as you source tested units and respect its battery/firmware limits.
For plants not ready to migrate, stocking a verified L55M14 is cheaper insurance than an unplanned shutdown.
FAQ
Is the 1756-L55M14 still manufactured?
No. Rockwell discontinued the L55 family. Available units are refurbished, surplus, or repaired stock.
Does it have built-in Ethernet?
No. Use a 1756-ENBT/EN2T module in the chassis for EtherNet/IP.
What software programs it?
RSLogix 5000 up to ~v16. It is not supported on Studio 5000 v17+.
What battery does it use?
1756-BA1 or 1756-BATM lithium battery; replace before it flags LOW BAT.
Can I just swap the M14 board for more memory?
No. Memory tier is tied to the CPU configuration; field-upgrading usually means replacing the whole controller.
