Why the Allen-Bradley 1756-OX8I Is the Smart Replacement Choice
When a relay output module fails, production stops. The Allen-Bradley 1756-OX8I offers a direct drop-in replacement for your existing ControlLogix discrete output module, restoring full control with minimal downtime. This 8-channel isolated relay output module handles both AC and DC loads, making it a versatile workhorse for diverse industrial applications.
What Is the 1756-OX8I?
The 1756-OX8I is a ControlLogix isolated relay output module from Allen-Bradley / Rockwell Automation. It provides eight individually isolated Form C contacts—each configurable as normally open (N.O.) or normally closed (N.C.). The module operates on a wide voltage range: 10-265V AC and 5-150V DC, so it can directly switch contactors, motor starters, pilot lamps, annunciator panels, and other field devices without additional interposing relays.
Key Technical Specifications That Matter
- Outputs: 8-point individually isolated, Form C contacts
- Operating Voltage: 10-265V AC, 5-150V DC
- Output Current: 2 A, 0.5 A, 0.25 A (per point, depending on load)
- Isolation: 250V continuous, outputs-to-backplane and output-to-output
- Backplane Current: 100 mA at 5V
- Power Dissipation: 3.1 W max at 60°C
- Switching Frequency: 0.3 Hz (1 operation per 3 seconds at rated load)
- Expected Contact Life: 300 kHz resistive, 100 kHz inductive
- Terminal Blocks: Requires 1756-TBCH or 1756-TBS6H (sold separately)
These specs mean you can mix AC and DC loads on the same module, reduce spare parts inventory, and simplify wiring. The 250V isolation protects your backplane from field transients, enhancing system reliability.
Comparison with Similar Output Modules
| Feature | 1756-OX8I | Typical 8-Ch DC Output | Typical 8-Ch AC Output |
|---|---|---|---|
| Output Type | Isolated Relay (Form C) | Solid-State DC | Solid-State AC |
| Voltage Range | 10-265V AC / 5-150V DC | 24V DC nominal | 120/240V AC nominal |
| Isolation | 250V continuous | Typically 250V | Typically 250V |
| Contact Life | 300 kHz resistive | Infinite (no moving parts) | Infinite |
| Switching Speed | 0.3 Hz | Up to 1 kHz | Up to 60 Hz |
| Best For | Mixed AC/DC loads, high inrush | High-speed DC loads | High-speed AC loads |
Choose the 1756-OX8I when you need true dry contacts, mixed voltage compatibility, or high isolation. For high-speed switching, solid-state modules may be better, but they lack the flexibility of relay outputs.
Where the 1756-OX8I Excels
1. Mixed-Voltage Control Panels – With a single module, you can control 24V DC pilot lights and 120V AC contactors, reducing panel space and wiring complexity.
2. Legacy System Upgrades – If you are replacing an older 1756-OX8I or migrating from another PLC platform, this module integrates seamlessly into any ControlLogix chassis (local or remote).
3. High-Inrush Applications – Relay contacts handle inductive loads like motor starters better than many solid-state outputs, thanks to their robust construction and 2 A rating.
Installation and Setup Tips
- Mounting: Install in any 1756 local or remote chassis. Ensure power is off before insertion.
- Terminal Blocks: Use 1756-TBCH (36-pin) or 1756-TBS6H (spring-clamp) RTBs. These are not included and must be ordered separately.
- Wiring: Each channel is isolated. For non-isolated operation, jumper the L1 terminals using 1756-JMPR or a third-party jumper.
- Load Compatibility: Verify that your load current does not exceed the module’s rating (2 A, 0.5 A, or 0.25 A depending on voltage and duty cycle).
Maintenance and Reliability
Relay outputs have moving parts, so periodic inspection is wise. Check for contact wear if switching inductive loads frequently. The expected contact life is 300 kHz resistive and 100 kHz inductive, which translates to years of service in typical applications. Keep the module clean and ensure adequate ventilation to dissipate the 3.1 W max power. For safety-related systems, always follow the manufacturer’s safety manual.
Compatibility and Integration
The 1756-OX8I works with all ControlLogix processors and communicates via the backplane. No special firmware is required. It is compatible with remote chassis over ControlNet, EtherNet/IP, or other supported networks. This makes it a flexible choice for both new builds and retrofit projects.
Selection Guide
Before purchasing, verify your chassis slot availability, required terminal block type, and load voltage/current. If you need a different channel count or solid-state output, consider other modules in the ControlLogix family. For genuine Allen-Bradley parts and expert support, trust SpareTechPLC. We stock the 1756-OX8I and can help you confirm compatibility.
Frequently Asked Questions
Q: What is the difference between the 1756-OX8I and a standard DC output module?
A: The 1756-OX8I uses isolated relay contacts, so it can switch both AC and DC loads. Standard DC modules only switch DC and are typically solid-state.
Q: Can I use the 1756-OX8I in a remote chassis?
A: Yes, it works in any 1756 local or remote chassis.
Q: What terminal blocks do I need?
A: You need either 1756-TBCH or 1756-TBS6H. They are sold separately.
Q: How many outputs does the 1756-OX8I have?
A: It has 8 individually isolated Form C relay outputs.
Q: What is the maximum switching frequency?
A: 0.3 Hz (1 operation per 3 seconds at rated load).
Upgrade with Confidence
The Allen-Bradley 1756-OX8I delivers the reliability and flexibility you need for critical control applications. With its wide voltage range, high isolation, and robust relay contacts, it minimizes downtime and simplifies spare parts management. Contact SpareTechPLC today to order your 1756-OX8I and keep your production running.
