Allen-Bradley ControlLogix 1756 Modules – 1756-L71/B

Technical Dossier

Product Details And Specifications

Allen-Bradley ControlLogix 1756 Series Technical Notes

The Allen-Bradley ControlLogix 1756 series is Rockwell Automation's flagship programmable automation controller (PAC) platform, deployed across the full spectrum of heavy industrial operations — including petrochemical refineries, nuclear power generation facilities, offshore oil & gas platforms, pulp and paper mills, and large-scale water treatment infrastructure. Since its commercial introduction in the mid-1990s, the 1756 chassis-based architecture has become the de facto standard for high-availability process control and discrete manufacturing automation in facilities requiring deterministic scan times, redundant controller configurations, and multi-discipline integration (motion, safety, process, and discrete) within a single backplane.

The Evolution of ControlLogix 1756 Architecture

The ControlLogix platform has undergone four distinct architectural generations since its launch:

Generation 1 (1997–2003): Initial release with Series A controllers (1756-L1, 1756-L20, 1756-L40). RSLogix 5000 v1–v8 firmware. Limited to 250 I/O modules per system. No integrated motion or safety. Backplane communication at 100 Mbps. These units are now fully obsolete and unsupported by Rockwell Automation.

Generation 2 (2003–2010): Introduction of Series B and C controllers with expanded memory (1756-L55, 1756-L60, 1756-L61, 1756-L62, 1756-L63). RSLogix 5000 v13–v17. Added integrated motion control via SERCOS and analog servo. ControlNet redundancy modules (1756-CNB) became standard in critical process applications. Many of these models are now in the mature/end-of-life phase.

Generation 3 (2010–2018): The L7x controller family — including the 1756-L71/B, 1756-L72, 1756-L73, 1756-L74, 1756-L75 — represented a major architectural leap. 1.5 GHz dual-core processors, USB programming port, SD card slot for firmware/program backup, and support for up to 128,000 I/O points. Studio 5000 Logix Designer replaced RSLogix 5000 as the primary programming environment. EtherNet/IP became the primary network backbone.

Generation 4 (2018–Present): The L8x family (1756-L81E, 1756-L82E, 1756-L83E, 1756-L84E, 1756-L85E) introduced embedded dual-port EtherNet/IP, eliminating the need for a separate 1756-EN2T communication module in standard topologies. These are the current production models and the recommended migration target for all L6x and L7x installations.

Compatibility note: L7x controllers running firmware v20+ are compatible with all 1756 Series B and later I/O modules. L6x controllers cannot execute programs compiled for L7x firmware without re-compilation. Chassis and power supplies are backward-compatible across all generations.

ControlLogix 1756 Full Catalog & Functionalities (SKU List)

Controllers (CPU Modules)

1756-L71/B: ControlLogix 5571 controller, 2MB user memory, Series B, USB/SD port, supports up to 128K I/O.
1756-L72: ControlLogix 5572 controller, 4MB user memory, dual-port EtherNet/IP ready, L7x generation.
1756-L73: ControlLogix 5573 controller, 8MB user memory, high-capacity process & motion applications.
1756-L74: ControlLogix 5574 controller, 16MB user memory, large-scale redundant system deployments.
1756-L75: ControlLogix 5575 controller, 32MB user memory, maximum capacity L7x platform controller.
1756-L81E: ControlLogix 5580 controller, 3MB memory, embedded dual-port EtherNet/IP, current generation.
1756-L83E: ControlLogix 5580 controller, 20MB memory, embedded EtherNet/IP, replaces 1756-L73 in new designs.

Digital Input (DI) Modules

1756-IB16: 16-point 24VDC sinking digital input module, standard density, 1756 backplane compatible.
1756-IB32: 32-point 24VDC sinking digital input, high-density, reduced chassis slot consumption.
1756-IA16: 16-point 120VAC digital input module, isolated, for legacy AC field device integration.

Digital Output (DO) Modules

1756-OB16E: 16-point 24VDC sourcing digital output, electronically fused, diagnostic feedback per point.
1756-OB32: 32-point 24VDC sourcing digital output, high-density, standard industrial applications.
1756-OA16: 16-point 120/240VAC digital output module, triac-based, for AC actuator control.

Analog Input/Output (AI/AO) Modules

1756-IF16: 16-channel analog input, 4–20mA / 0–10V, 16-bit resolution, process instrumentation standard.
1756-OF8: 8-channel analog output, 4–20mA / 0–10V, 16-bit resolution, valve and positioner control.

Communication & Network Adapter Modules

1756-EN2T: Single-port EtherNet/IP communication module, 100 Mbps, standard network bridge for L6x/L7x.
1756-EN2TR: Dual-port EtherNet/IP with Device Level Ring (DLR) support, fault-tolerant network topology.
1756-CNB: ControlNet bridge module, coax-based, used in legacy redundant process control architectures.
1756-DHRIO: Data Highway Plus / Remote I/O bridge, legacy PLC-5 and SLC migration gateway.

Power Supply Modules

1756-PA72: 120/240VAC input power supply, 13A backplane output, standard chassis power for 1756 systems.
1756-PB72: 18–32VDC input power supply, 13A backplane output, for DC-powered panel installations.
1756-PSCA2: Power supply chassis adapter for redundant power supply configurations.

Sourcing Hard-to-Find & Obsolete ControlLogix 1756 Parts

Q: How are RFQ terms confirmed?
A: Quantity, required condition, documentation needs, destination and sourcing route are confirmed during RFQ review before quotation.

For obsolete or allocation-constrained SKUs, DriveKNMS provides cross-reference analysis to identify compatible substitute modules where direct replacements are unavailable, including firmware compatibility assessment and backplane slot impact evaluation.

Quality Control for the ControlLogix 1756 Range

The 1756 platform's backplane operates at a proprietary high-speed serial bus protocol. Functional verification of 1756 modules requires live backplane testing — bench-level power-on testing alone is insufficient to confirm communication integrity, I/O channel accuracy, or firmware state.

DriveKNMS applies the following test protocol to all 1756 series units prior to shipment:

I/O modules: Each digital input channel tested with 24VDC / 120VAC stimulus signal as applicable. Each analog channel calibrated against NIST-traceable reference standard (±0.1% full-scale accuracy verification). Electronic fuse reset function tested on all OB16E and similar diagnostics-capable output modules.

Communication modules (1756-EN2T, 1756-CNB, 1756-DHRIO): Live network connection test with active EtherNet/IP, ControlNet, or DH+ traffic. Port-level LED diagnostic verification. Backplane communication integrity test at rated scan rate.

Q: How is condition confirmed before quotation?
A: Available condition, photos, test records and documentation are checked according to the requested model and sourcing channel before a formal RFQ response.