Allen-Bradley ControlLogix 1756-OF8IH 8-Point Isolated HART Analog Output Module

Module Overview

The 1756-OF8IH is a high-performance analog output module for the ControlLogix 1756 platform, specifically engineered for precision process control and advanced asset management. It features eight fully isolated differential channels, each equipped with its own dedicated HART (Highway Addressable Remote Transducer) modem. This architecture allows the controller to simultaneously command a 4–20 mA actuator and communicate digitally with "smart" field devices to retrieve diagnostics, secondary variables, and configuration data. By providing channel-to-channel isolation, the 1756-OF8IH ensures that electrical noise or ground loops in one field circuit do not affect the accuracy of the other seven channels.

Detailed Engineering Specifications

The 1756-OF8IH utilizes a 15-bit digital-to-analog converter (DAC) to provide high-resolution control over standard industrial current loops.

• Module Type: ControlLogix HART Analog Output

• Number of Channels: 8 Isolated Differential Outputs

• Integrated Modem: 1 HART Modem per channel

• Output Signal Ranges: 0 to 20 mA and 4 to 20 mA

• Resolution: 15-Bit across 24 mA

• HART Scan Time: Approximately 1 second per module (with HART enabled)

• Overvoltage Protection: ±24V DC

• Backplane Current (5.1V DC): 300 mA

• Backplane Current (24V DC): 200 mA

• Power Dissipation: 5.8 Watts

• Thermal Dissipation: 19.8 BTU/hr

• Isolation Voltage: 250V (Continuous), reinforced insulation type

• Operating Temperature: 0°C to 60°C (32°F to 140°F)

• Physical Weight: 1.00 lbs (0.45 kg)

Advanced HART Functionality

Unlike standard analog modules, the 1756-OF8IH acts as a bridge between the plant floor and asset management software (such as FactoryTalk AssetCentre). Each channel can pass HART "Pass-Through" messages, allowing maintenance personnel to remotely calibrate valve positioners or check the internal health of a transmitter without physical field access. The module supports HART versions 5, 6, and 7, and can be configured to trigger alarms in the Logix5000 controller based on the device status bits returned via the HART protocol.

Installation and Hardware Integration

The 1756-OF8IH is a chassis-based module compatible with any standard 1756 ControlLogix rack. It requires a Removable Terminal Block (RTB) for field wiring, with the following models being the most common:

• 1756-TBNH: 20-position NEMA screw terminal block.

• 1756-TBSH: 20-position spring-clamp terminal block for high-vibration applications.

  The module supports "Removal and Insertion Under Power" (RIUP), allowing for hardware replacement without disrupting the power to the rest of the chassis or the communication on the backplane.

Technical FAQ

Do I need a separate HART multiplexer to use this module?

No. The 1756-OF8IH has an integrated HART modem for every channel. The HART digital data is superimposed on the 4–20 mA signal and processed directly by the module hardware.

Can I use this module for 0–10V voltage signals?

No. The 1756-OF8IH is strictly a current-output module (0–20 mA / 4–20 mA). If your application requires voltage outputs, you should consider the 1756-OF8 or a similar voltage-capable module.

What is the benefit of channel-to-channel isolation?

In many industrial plants, different field devices may be powered by different sources or have different ground potentials. Isolation prevents current from flowing between these different grounds, which would otherwise cause signal inaccuracies or hardware damage.