Allen-Bradley 1756-OF8 ControlLogix Analog Output Module

The Allen-Bradley 1756-OF8 is an 8-channel analog output module for the ControlLogix platform. It provides voltage and current outputs for driving variable speed drives, control valves, recorders, and other analog field devices. The module supports multiple output ranges, high-resolution signal generation, and direct integration into a ControlLogix chassis while occupying a single slot.

Technical Specifications

Analog Output Capability

The module provides eight independently configurable analog outputs that can be assigned for either voltage or current operation. Supported voltage outputs include -10 VDC to +10 VDC, while current outputs support both 0-20 mA and the widely used 4-20 mA control standard.

This flexibility allows a single module type to interface with a wide range of industrial devices, including actuators, drives, positioners, and process control equipment.

Resolution and Signal Accuracy

The 15-bit digital-to-analog conversion architecture provides fine output granularity for control applications that require smooth signal changes and precise setpoint control.

For current outputs, the effective resolution is approximately 650 nA per bit across the 21 mA operating span. For voltage outputs, the resolution is approximately 320 µV per bit across a 10.4 V range.

ControlLogix Integration

The 1756-OF8 installs directly into a standard ControlLogix chassis and occupies a single slot. Power is supplied through the ControlLogix backplane, eliminating the need for a dedicated module power source.

The module draws 150 mA from the 5.1 VDC backplane supply and 210 mA from the 24 VDC backplane supply, resulting in a total backplane power consumption of 5.8 W.

Channel Isolation Considerations

The standard 1756-OF8 does not provide individual channel-to-channel isolation. Applications requiring isolated analog outputs should consider models specifically designed with isolated channels, such as the 1756-OF8I.

When connecting field instruments powered from different grounding systems, isolation requirements should be reviewed during system design.

Installation and System Design

The module's compact design allows efficient use of ControlLogix chassis space while providing eight analog output channels from a single slot.

Typical applications include:

• Variable frequency drive speed reference control

• Control valve positioning

• Analog setpoint transmission

• Process control loop outputs

• Recording and monitoring systems

Frequently Asked Questions

Q: Does the 1756-OF8 provide isolated output channels?

A: No. The 1756-OF8 uses eight single-ended non-isolated output channels. Applications requiring channel isolation should use an isolated alternative such as the 1756-OF8I.

Q: What output signal ranges are supported?

A: The module supports voltage outputs from -10 VDC to +10 VDC and current outputs of 0-20 mA or 4-20 mA.

Q: How many chassis slots are required?

A: The module occupies one slot in a ControlLogix chassis.

Q: What types of field devices can be controlled by the module?

A: The analog outputs can be used with drives, valve positioners, actuators, process controllers, recorders, and other equipment that accepts standard voltage or current control signals.