Allen-Bradley 1756-OF8I: ControlLogix Analog Output Module – Specs, Alternatives & Availability

Technical Dossier

Product Details And Specifications

Allen-Bradley 1756 Series Technical Notes

The Allen-Bradley 1756 ControlLogix platform is one of the most widely deployed programmable automation controller (PAC) architectures in global heavy industry. Installed across petrochemical refineries, nuclear power generation facilities, offshore oil & gas platforms, pulp and paper mills, and large-scale water treatment plants, the 1756 backplane system has served as the backbone of process and discrete control since its commercial introduction in the late 1990s. The 1756-OF8I is a high-density, individually isolated analog output module within this platform, designed for applications where channel-to-channel isolation is a non-negotiable safety and signal integrity requirement.

The ControlLogix chassis accepts up to 17 modules per rack and supports redundant controller configurations via the 1756-RM2/A redundancy module. The 1756 series communicates natively over the ControlNet, DeviceNet, and EtherNet/IP networks, making it compatible with Rockwell Automation's Integrated Architecture framework. Its modular, hot-swappable design allows maintenance personnel to replace individual I/O modules without de-energizing the chassis — a critical operational requirement in continuous-process industries.

The Evolution of 1756 Architecture

The 1756 ControlLogix platform was introduced by Rockwell Automation in 1997 as a successor to the PLC-5 and SLC 500 families. The original architecture used a proprietary backplane bus operating at 200 Mbps. Over successive hardware revisions, Rockwell introduced the Series B and Series C module variants, which addressed firmware compatibility issues and expanded the supported I/O point density.

Early 1756 analog modules (circa 1998–2005) used 12-bit resolution DACs. The 1756-OF8I and its contemporaries moved to 16-bit resolution, significantly improving control precision for valve positioners and variable-speed drive reference signals. The introduction of the Logix5000 firmware v16 and later v20 brought producer/consumer communication model enhancements that reduced I/O scan latency across large 1756 installations.

Compatibility considerations are significant for maintenance engineers: modules manufactured before Series B may not support newer firmware features available in Studio 5000 Logix Designer v30+. When replacing legacy 1756 modules, engineers must verify the catalog number suffix (e.g., /A, /B) and cross-reference against the Rockwell Automation Product Compatibility and Download Center (PCDC) to confirm firmware alignment with the resident controller.

1756 Series Full Catalog & Functionalities (SKU List)

Analog Output Modules

• 1756-OF8I: 8-channel individually isolated analog output; ±10V / 0–20mA; 16-bit resolution
• 1756-OF8: 8-channel non-isolated analog output; voltage and current output; 16-bit resolution
• 1756-OF4: 4-channel analog output; voltage/current configurable per channel; 16-bit resolution
• 1756-OF6CI: 6-channel current-only isolated analog output; 0–20mA / 4–20mA; 16-bit resolution
• 1756-OF6VI: 6-channel voltage-only isolated analog output; ±10V; 16-bit resolution

Analog Input Modules

• 1756-IF16: 16-channel single-ended analog input; voltage and current; 16-bit resolution
• 1756-IF8: 8-channel differential analog input; ±10V / 0–20mA; 16-bit resolution
• 1756-IF8I: 8-channel individually isolated analog input; voltage and current; 16-bit resolution
• 1756-IF6I: 6-channel individually isolated analog input; thermocouple/RTD compatible variant available
• 1756-IT6I: 6-channel isolated thermocouple input; J, K, T, E, R, S, B, N type support
• 1756-IR6I: 6-channel isolated RTD/resistance input; Pt100, Pt1000, Ni120 compatible

Digital Output Modules

• 1756-OB16D: 16-point 24VDC diagnostic output; electronically fused; per-point fault detection
• 1756-OB32: 32-point 24VDC sourcing output; high-density discrete control
• 1756-OA16: 16-point 120/240VAC output; triac-based solid-state switching

Digital Input Modules

• 1756-IB16D: 16-point 24VDC diagnostic input; wire-break and short-circuit detection
• 1756-IB32: 32-point 24VDC sinking input; high-density discrete sensing

Controller & Communication Modules

• 1756-L73: ControlLogix 5573 controller; 8MB user memory; EtherNet/IP native port
• 1756-L83E: ControlLogix 5583 controller; 20MB user memory; dual EtherNet/IP ports
• 1756-EN2T: EtherNet/IP communication bridge; 100 Mbps; supports CIP routing
• 1756-CN2: ControlNet communication module; dual-port coax; redundant media support
• 1756-DNB: DeviceNet scanner module; supports up to 64 nodes per scanner
• 1756-RM2/A: Redundancy module; synchronizes primary and secondary controller chassis

Power Supply Modules

• 1756-PA75: 120/240VAC input power supply; 13A backplane current at 5VDC
• 1756-PB75: 18–32VDC input power supply; 13A backplane current at 5VDC

Sourcing Hard-to-Find & Obsolete 1756 Series Parts

Rockwell Automation has progressively moved earlier 1756 module variants to End-of-Life (EOL) and Discontinued status. Modules such as the 1756-OF8I/A (Series A) and early-revision communication adapters are no longer manufactured. However, the installed base of these modules in operating plants remains substantial, and replacement demand persists across the full lifecycle of the host facility — often 20–40 years for petrochemical and power generation assets.

Quality Control for the 1756 Series Range

The 1756 backplane operates at a proprietary high-speed bus protocol. Functional verification of 1756 modules requires chassis-level testing — bench-level power-up alone is insufficient to confirm backplane communication integrity. DriveKNMS applies the following test protocol to all 1756 modules prior to dispatch:

• Backplane communication test: Module is seated in a live 1756 chassis running a compatible controller firmware version. Module identity, revision, and electronic keying data are read and logged via RSLinx Classic or Studio 5000.
• Analog output calibration verification (for OF-series modules): Each output channel is loaded with a precision resistive load. Output voltage and current values are measured at 0%, 25%, 50%, 75%, and 100% of full scale using a calibrated Fluke 87V or equivalent instrument. Results are compared against Rockwell's published accuracy specifications (±0.1% of full scale for 1756-OF8I).
• Isolation integrity test: For isolated modules (1756-OF8I, 1756-IF8I, etc.), channel-to-channel and channel-to-backplane isolation resistance is verified at 500VDC using a calibrated insulation resistance tester.
• Thermal cycling: Modules are subjected to a 4-hour thermal soak at 55°C operating temperature to screen for latent component failures prior to shipment.
• Firmware revision logging: Module firmware revision is recorded and included in the shipment documentation to support the customer's change management process.