Product Overview

The Allen-Bradley 1756-OF8IH is a ControlLogix HART output module. This device provides 8 isolated differential current outputs with one HART modem per channel. It supports HART protocol versions 5, 6, and 7 with read/write capability.

The module installs in any 1756 I/O chassis slot. Furthermore, it offers open-style enclosure (none). Therefore, it suits demanding industrial applications requiring smart device communication.

General Specifications

The 1756-OF8IH offers robust analog output capabilities with HART integration.

• Manufacturer: Rockwell Automation.

• Brand: Allen-Bradley.

• Series: ControlLogix.

• Part Number/Catalog No.: 1756-OF8IH.

• Module Type: ControlLogix HART I/O Module.

• Channels: 8 Isolated Differential Outputs.

• HART Modems: 1 per channel.

• Enclosure Type: None (Open-Style).

Output Specifications

The 1756-OF8IH provides flexible output ranges for field devices.

• Input Current Range: 0-20 mA, 4-20 mA.

• Resolution: 15 bits across 24 mA.

• Output Conversion Method: R-Ladder DAC.

• Drive Capability: 50-750 Ω (with short circuit survival).

• Load Reactance: 10 µH.

• Output Short Circuit Protection: Electronically limits output current to ≤21 mA.

• Open Circuit Detection: Yes.

• Overvoltage Protection: ±24 Volts DC.

Settling Time

The 1756-OF8IH has load-dependent settling times.

• HART Enabled: Less than 100 ms.

• HART Disabled: Less than 23 ms.

Therefore, disable HART for faster analog response when digital communication is not required.

Accuracy and Calibration

The 1756-OF8IH delivers high precision for critical applications.

• Calibration Accuracy: 0.15% at 4-20 mA.

• Calibration Interval: 12 months (standard).

• Module Error: 0.3% at 4-20 mA.

• OFFSET Drift: ±3 ppm (full scale).

• GAIN Drift: ±4 ppm.

• Data Format: IEEE 32-bit floating point.

HART Communication

The 1756-OF8IH supports comprehensive HART functionality.

• Supported HART Versions: 5, 6, and 7.

• Capability: Read/Write.

• HART Modems: One per channel (8 total).

• Module HART Scan Time: 1 second estimated.

Consequently, the module communicates with smart field devices for configuration, diagnostics, and process data.

Isolation and Voltage Ratings

The 1756-OF8IH provides multiple levels of electrical isolation.

Outputs-to-Backplane:

• Isolation Type: Reinforced insulation.

• Voltage Rating: 250V.

• Dielectric Test: 3535V DC for 60 seconds.

Output-to-Output and Output-to-Ground:

• Isolation Type: Basic insulation.

• Voltage Rating: 250V.

• Dielectric Test: 2120V DC for 60 seconds.

Backplane Current Draw

The 1756-OF8IH draws power from the ControlLogix backplane.

• Estimated Backplane Current (24V DC): 400 mA.

Environmental Specifications

The 1756-OF8IH withstands typical industrial conditions.

• Working Temperature Range: 0 to 60°C.

• Ambient Temperature: 60°C (140°F) maximum.

• Relative Humidity: 5 to 95% (non-condensing).

Temperature Codes

The 1756-OF8IH carries hazardous location temperature ratings.

• North America: T4A.

• ATEX and IECEx: T4.

Key Features

The 1756-OF8IH offers several advantages for analog output applications.

Eight Isolated Outputs:
Provides 8 independent, isolated analog output channels. Therefore, it eliminates ground loops and simplifies wiring.

Per-Channel HART Modems:
Each output includes a dedicated HART modem. Consequently, all 8 channels can communicate simultaneously with smart field devices.

Wide HART Version Support:
Supports HART 5, 6, and 7 with read/write capability. Hence, it works with most modern smart positioners and I/P converters.

Selectable Output Ranges:
Supports 0-20 mA or 4-20 mA output ranges. Thus, it adapts to different field device requirements.

High Resolution:
15-bit resolution across 24 mA provides fine control. For this reason, it suits precision analog applications.

Fast Settling Time:
Less than 23 ms with HART disabled. Therefore, it responds quickly to control loop changes.

Robust Protection:
Includes short circuit protection, open circuit detection, and overvoltage protection (±24V). Consequently, it withstands field wiring faults.

High Isolation Ratings:
Reinforced isolation (3535V DC test) between outputs and backplane. Hence, it meets safety standards for industrial control systems.

Low Drift:
±3 ppm offset drift and ±4 ppm gain drift ensure long-term stability. Thus, calibration intervals can be extended.

R-Ladder DAC Conversion:
Provides monotonic output with excellent linearity. For this reason, output accuracy is maintained across the full range.

Typical Applications

The 1756-OF8IH suits various analog output and HART applications.

• Controlling I/P converters on control valves.

• Driving HART-enabled positioners and actuators.

• Analog setpoint control for VFDs and drives.

• Retrofitting existing 4-20mA loops with HART capability.

• Process control applications requiring isolated analog outputs.

Installation Guidelines

Install the 1756-OF8IH in any slot of a ControlLogix chassis. First, verify the chassis power supply can provide the estimated 400 mA at 24V DC. Then, insert the module into an available slot.

Connect field devices to the 8 isolated differential output channels. Each channel has its own HART modem. For 4-20mA loops, calibration accuracy is 0.15%. The recommended calibration interval is 12 months.

For fastest analog response, disable HART communication. The settling time is then less than 23 ms. When HART is enabled, settling time increases to less than 100 ms. The module provides short circuit protection (output limited to ≤21 mA) and open circuit detection.

What is in the Box

Please inspect all items upon delivery.

• Allen-Bradley 1756-OF8IH analog output module with HART.

• Installation instruction manual.

• Anti-static protective bag.

Frequently Asked Questions (FAQ)

1. How many analog output channels does the 1756-OF8IH have?

The 1756-OF8IH provides 8 isolated differential analog output channels. Each channel includes a dedicated HART modem.

2. What HART protocol versions does this module support?

The 1756-OF8IH supports HART versions 5, 6, and 7. It offers both read and write capability for full device configuration and diagnostics.

3. What is the output current range of this module?

The 1756-OF8IH supports both 0-20 mA and 4-20 mA output ranges. Resolution is 15 bits across 24 mA.

4. What is the settling time with HART enabled?

With HART enabled, the 1756-OF8IH has a settling time of less than 100 ms. With HART disabled, settling time is less than 23 ms.

5. What isolation ratings does this module provide?

The 1756-OF8IH provides 250V reinforced insulation between outputs and backplane (tested at 3535V DC). It also provides 250V basic insulation between output-to-output and output-to-ground (tested at 2120V DC).