Allen-Bradley ControlLogix 1756 Modules - 1756-OF8K

Technical Dossier

Product Details And Specifications

Allen-Bradley ControlLogix 1756 Series Technical Notes

The Allen-Bradley ControlLogix 1756 platform is one of the most widely deployed programmable automation controller (PAC) architectures in global heavy industry. Installed across petrochemical refineries, nuclear power facilities, LNG terminals, steel mills, and continuous process plants, the 1756 chassis-based system has served as the backbone of safety-critical and high-availability automation since its commercial introduction in the late 1990s. Its modular, hot-swappable architecture and deterministic backplane communication make it the reference standard against which competing DCS and PLC platforms are measured. As of 2026, the 1756 series remains in active production for core CPU and I/O families, while a significant portion of the catalog—particularly legacy communication adapters and early-generation analog modules—has entered end-of-life status, creating sustained demand for verified surplus and refurbished inventory.

The Evolution of ControlLogix 1756 Architecture

The 1756 platform was engineered around a producer/consumer network model, enabling deterministic peer-to-peer data exchange across the ControlNet and EtherNet/IP backbones without polling overhead. First-generation controllers (1756-L1, 1756-L20) used SRAM-based memory with battery backup; these have been fully superseded by the L7x and L8x series featuring non-volatile energy-stored memory (ESM) and SD card program storage.

The transition from Series A to Series B and C I/O modules introduced improved isolation ratings, HART pass-through on analog modules, and enhanced diagnostics. The 1756-L8x generation (introduced circa 2015) added dual-port EtherNet/IP, integrated motion over EtherNet/IP (MoE), and CIP Security support—features absent in all prior generations. Facilities running mixed L6x/L7x/L8x environments must account for firmware compatibility constraints when performing chassis-level upgrades, as Studio 5000 Logix Designer version requirements differ across controller generations. The 1756-OF8K specifically belongs to the HART-capable analog output family, supporting 8-channel current/voltage output with HART communication on each channel—a capability introduced to bridge legacy field instrument protocols with modern DCS integration requirements.

1756 ControlLogix Full Catalog & Functionalities (SKU List)

The following SKUs represent the core functional categories of the 1756 platform. Each entry reflects a verified, commercially available or historically documented module within the ControlLogix ecosystem.

Controllers (CPU)

• 1756-L71: ControlLogix 2MB controller, Series B, EtherNet/IP single-port, Studio 5000 compatible
• 1756-L72: ControlLogix 4MB controller, dual-port EtherNet/IP, standard workhorse for mid-scale process applications
• 1756-L73: ControlLogix 8MB controller, expanded task capacity, suited for multi-axis and complex batch processes
• 1756-L74: ControlLogix 16MB controller, high-density logic and data logging applications
• 1756-L75: ControlLogix 32MB controller, maximum memory L7x generation, large-scale continuous process
• 1756-L81E: ControlLogix 3MB controller, L8x generation, dual-port EtherNet/IP, CIP Security enabled
• 1756-L83E: ControlLogix 10MB controller, L8x generation, integrated motion over EtherNet/IP
• 1756-L85E: ControlLogix 40MB controller, flagship L8x, maximum I/O and motion axis capacity

Analog Output Modules

• 1756-OF8K: 8-channel analog output (current/voltage), HART-enabled, Series C, isolated channel architecture
• 1756-OF8: 8-channel analog output (current/voltage), non-HART, standard process control applications
• 1756-OF4: 4-channel analog output, current/voltage selectable, compact footprint for smaller chassis
• 1756-OF6CI: 6-channel current output, individually isolated, high-accuracy process trim applications
• 1756-OF6VI: 6-channel voltage output, individually isolated, servo reference and positioner control

Analog Input Modules

• 1756-IF8K: 8-channel analog input, HART pass-through, differential input, 24-bit resolution
• 1756-IF16: 16-channel analog input, single-ended, high-density process measurement
• 1756-IF6I: 6-channel isolated analog input, individually isolated, precision measurement in noisy environments

Digital I/O Modules

• 1756-IB16: 16-point 24VDC digital input, standard sink/source, general-purpose discrete sensing
• 1756-OB16E: 16-point 24VDC digital output, electronically fused, diagnostics per point
• 1756-IA16: 16-point 120VAC digital input, field-side isolated, motor control center integration

Communication & Adapter Modules

• 1756-EN2T: EtherNet/IP communication module, single-port, 100Mbps, standard network bridge
• 1756-EN2TR: EtherNet/IP communication module, dual-port, Device Level Ring (DLR) support
• 1756-CN2: ControlNet communication module, coax media, legacy plant network integration
• 1756-DNB: DeviceNet scanner module, legacy field device integration, end-of-life status

Power Supply Modules

• 1756-PA72: 120/240VAC power supply, 13A backplane current, standard chassis power
• 1756-PB72: 18-30VDC power supply, 13A backplane current, battery-backed UPS compatible installations

Sourcing Hard-to-Find & Obsolete 1756 ControlLogix Parts

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

Quality Control for the 1756 ControlLogix Range

The 1756 backplane operates at a proprietary high-speed serial bus. Counterfeit or improperly refurbished modules that pass visual inspection can still introduce latent backplane faults, causing intermittent communication errors that are difficult to diagnose under production conditions. DriveKNMS applies a structured test protocol for all 1756 modules prior to shipment:

• Backplane communication verification: Each module is seated in a live 1756 chassis and confirmed to enumerate correctly via RSLinx or Studio 5000 I/O tree.
• Analog channel calibration check: For modules such as the 1756-OF8K and 1756-IF8K, each channel is exercised across its full output/input range and measured against NIST-traceable reference instruments.
• HART communication test: HART-capable modules are tested for correct HART frame transmission and response using a calibrated HART communicator.
• Firmware revision audit: Module firmware is read and documented. Customers are informed of the revision prior to shipment and advised on compatibility with their controller firmware.
• Cosmetic and connector inspection: Backplane connector pins, front-panel terminals, and status LED functionality are inspected and documented.

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.