Facing Mixed Voltage Loads in Your Control System?
Managing multiple output devices with different voltage requirements can complicate wiring and increase panel costs. The Allen-Bradley 1756-OX8I ControlLogix Isolated Relay Output Module simplifies this by providing eight individually isolated Form C relay contacts, each capable of switching AC loads from 10 to 265 Volts AC or DC loads from 5 to 150 Volts DC. This flexibility allows you to drive contactors, motor starters, pilot lamps, and annunciators from a single module without needing separate interface relays.
Product Overview
The 1756-OX8I is a discrete output module from Rockwell Automation's Allen-Bradley ControlLogix series. It features eight normally open (N.O.) and eight normally closed (N.C.) contact outputs per module, with each output point individually isolated. This design enables independent wiring of loads operating at different voltage levels, reducing external component count and simplifying system architecture.
Technical Specifications Breakdown
Key parameters for the 1756-OX8I are listed below for quick reference:
| Feature | Specification |
|---|---|
| Module Type | Isolated Relay Output Module |
| Outputs | 8-Point Individually Isolated |
| Contact Output | Form C contacts |
| Operating Voltage | 10-265 Volts AC, 5-150 Volts DC |
| Output Current | 2, 0.5, 0.25 Amps |
| Initial Contact Resistance | 100 mΩ @ 6V 1 A |
| Maximum Switching Frequency | 1 operation/3 s (0.3 Hz at rated load) |
| Mean Bounce Time | 1.2 ms |
| Maximum Output Delay Time | 13 milliseconds |
| Expected Contact Life | 300 kHz resistive; 100 kHz inductive |
| Backplane Current at 5 Volts | 100 mA |
| Power Dissipation (Max) | 3.1 Watts at 60 Celsius |
| Isolation Voltage | 250V (continuous), basic insulation type, outputs-to-backplane, and output-to-output |
| Removable Terminal Block(s) | 1756-TBCH, 1756-TBS6H |
Industry Applications
This module excels in applications where loads have mixed voltage requirements. For example, in a packaging line, you might need to switch 120V AC pilot lights and 24V DC solenoid valves. With the 1756-OX8I, each output can be wired to its respective voltage source, eliminating the need for additional interposing relays. In material handling systems, the isolated contacts can drive motor starters at 240V AC while simultaneously controlling lower-voltage status indicators. The individual isolation also simplifies troubleshooting by allowing you to isolate a single output without affecting others.
Installation & Setup Guidance
Mount the 1756-OX8I directly into any 1756 local or remote chassis. Use the compatible removable terminal blocks (1756-TBCH or 1756-TBS6H) for wiring. These RTBs are sold separately and allow modular termination, making installation and future replacement faster. For non-isolated operation, you can short the L1 connections across groups using a 1756-JMPR jumper or a third-party conductor. Always verify that the load voltage and current are within the module's ratings before energizing.
Maintenance & Reliability
Periodically inspect the module for signs of overheating or contact wear. The expected contact life is 300 kHz for resistive loads and 100 kHz for inductive loads. Because the module has no enclosure, ensure it is installed in a clean, dry environment. Regular visual checks of the terminal block connections can prevent intermittent faults.
Compatibility & Integration
The 1756-OX8I is designed for the ControlLogix platform and integrates seamlessly with 1756 backplanes. It supports both local and remote chassis configurations. The isolated outputs allow mixing AC and DC loads without commoning issues.
Selection Guide & Purchasing Advice
When selecting this module, consider your load types and voltage ranges. The 1756-OX8I is ideal for systems requiring mixed-voltage switching. SpareTechPLC stocks this module and can confirm current lifecycle status and availability before ordering.
Troubleshooting & Common Issues
If an output fails to energize, check the wiring and verify the load voltage is within the 10-265V AC or 5-150V DC range. Use a multimeter to measure contact resistance; the initial value should be 100 mΩ at 6V 1 A. If the module does not communicate with the backplane, reseat it and check the backplane power supply. Ensure the removable terminal block is fully locked.
