Allen Bradley1756-M12/A&1756-L55ControlLogix5555 Processor with 750 KB Memory Expansion Board (Equivalent to 1756-L55M12)

Product Description

What this part is

An Allen-Bradley 1756-L55 ControlLogix5555 processor with a 1756-M12/A memory expansion board fitted to it. The two together are what Rockwell catalogues as a 1756-L55M12: the L55 is the controller platform and the M12 is the memory daughter board that gives it its usable program capacity. It occupies one slot in a 1756 ControlLogix chassis and runs the Logix application — ladder, function block, structured text and sequential function chart — for the whole control system.

Decoding the catalogue numbers

1756 — the ControlLogix platform.L55 — the Logix5555 controller. The L5x numbering identifies the processor generation; the L55 is the mid-life ControlLogix controller that sat between the L1/L5x early units and the later L6x (ControlLogix5560) and L7x families.M12 — the memory expansion board, 750 KB. The rest of the family reads M13, M14, M16, M22, M23 and M24 in ascending capacity, and all of them fit an L55 base.1756-L55M12 — the combined catalogue number for exactly this pairing, which is the number to search if you are matching an existing installation.SER A — hardware series A on the processor.

Published specifications

Controller: ControlLogix5555 (Logix5555)User memory: 750 KB (supplied by the fitted 1756-M12 board)I/O memory: 208 KBMemory type on the M12 expansion board: volatile — it is battery-backed working memory, not flash. This is the specification most often got wrong on these modules, and it has a direct practical consequence (see below).Slots occupied: 1 in a 1756 chassisProgramming: RSLogix 5000 / Studio 5000 Logix Designer of a firmware-compatible revisionManufacturer status: discontinued; superseded within the ControlLogix line by the 1756-L6x and 1756-L7x controller families

Battery and memory retention: read this before you buy:
The memory on this controller is volatile and depends on a backup battery. The 1756-L55 uses a 1756-BA1 lithium battery, and if that battery is flat or absent when chassis power is removed, the program in the controller is lost — you get a corrupt-memory or non-recoverable fault on the next power-up and you must download the project again. No battery is included with this unit, and any battery in a used or shelf-stored controller of this age should be treated as expired regardless of appearance.

Two practical consequences. First, fit a fresh 1756-BA1 before you commit the controller to a machine. Second — and this is the one that catches people — make sure you have the project file. A used controller arrives with no program, and if the original RSLogix 5000 ACD file for your machine has been lost, buying the hardware does not get you back into production. Nothing on this listing includes or implies a program.

Firmware and software compatibility

Logix controller firmware and the RSLogix 5000 / Studio 5000 major revision must match. A controller flashed to firmware v16 will not accept a v20 project and vice versa; you flash the controller to the revision your project was authored in using ControlFLASH. Confirm you have both the correct ControlFLASH firmware kit and a licensed copy of the matching RSLogix 5000 version before ordering — on a legacy L55 that is often the harder half of the job.

Quantity and configuration

One processor with the 1756-M12/A memory board fitted, tested, as photographed.

What is not included

1756-BA1 backup battery, 1756 chassis, chassis power supply, communication modules (1756-ENBT, 1756-CNB, 1756-DHRIO and similar), I/O modules, slot filler modules, serial programming cable (1756-CP3) or Ethernet cable, CompactFlash card, RSLogix 5000 / Studio 5000 software or licence, ControlFLASH firmware kits, project file or program, and documentation.

Installation and setup notes

Remove chassis power before inserting or removing the controller. Fit a fresh 1756-BA1 and confirm the BAT indicator is not lit before downloading. Set the keyswitch to PROG for the download, then to RUN or REM as your site standard requires. Verify the controller firmware revision against the project’s major revision before you attempt a download rather than after. On a system with existing communication modules, check their firmware revisions too — a mismatched comms module in the same chassis will fault even when the controller is correct.

Interchange guidance

Interchangeable with a 1756-L55M12, and with an L55 base carrying any other M12 board. Related but different parts, not equivalents: 1756-L55M13, M14, M16, M22, M23 and M24 (larger memory — they will run a smaller project, but a project built for a larger memory board may not fit this one), 1756-L55M12 versus 1756-L61/L62/L63 (ControlLogix5560, a different controller generation with different firmware and a different memory architecture), and 1756-L1/L5x early controllers. The M-series memory boards are specific to the L55-class base — they do not fit an L6x or L7x controller, which have integral memory.