The Evolution of Discrete Control Demands Better Isolation
For decades, plant engineers accepted a hard truth: a single field fault could take down an entire output card. Shared commons meant shared risk. As automated processes grew denser and more demanding, that risk became unacceptable. The Allen-Bradley 1756-OB16I ControlLogix isolated output module emerged from this shift, giving each of its sixteen channels its own electrical island. This design changed how engineers planned panel layouts and fault containment.
What Makes the 1756-OB16I Different
The 1756-OB16I is a single-slot, 16-point DC output module for the ControlLogix platform. Each point is individually isolated, so you can wire different voltage sources to separate channels without creating a common return path. The module operates in both sinking and sourcing configurations at a 12/24 VDC voltage category, with an operating range of 10-30 VDC. Sixteen LED indicators show channel status at a glance.
Technical Specifications That Matter
Three parameters define real-world performance for this module.
Isolation architecture. True channel-to-channel isolation means a shorted solenoid on point 3 will not drag down point 4. This reduces troubleshooting time and protects upstream devices. The module provides 50 volts continuous isolation voltage between outputs and backplane, and 125 volts continuous between output channels.
Current handling. Each point delivers up to 2 A at 30 °C, derating linearly to 1 A at 60 °C. The module total is 8 A at 30 °C, dropping to 4 A at 60 °C. Plan your load budget around the ambient temperature inside the enclosure, not the room temperature.
Switching speed. Output delay is 1 ms maximum from OFF to ON and 2 ms maximum from ON to OFF. This suits relay, pilot lamp, solenoid, and horn control where millisecond timing is adequate.
| Specification | Value |
|---|---|
| Outputs | 16 individually isolated |
| Voltage Category | 12/24 VDC sink/source |
| Operating Voltage Range | 10-30 VDC |
| Current per Point | 2 A @ 30 °C; 1 A @ 60 °C (linear derating) |
| Current per Module | 8 A @ 30 °C; 4 A @ 60 °C (linear derating) |
| Output Delay OFF to ON | 1 ms max |
| Output Delay ON to OFF | 2 ms max |
| Backplane Current at 5 V | 350 mA |
| Backplane Current at 24 V | 3 mA |
| Power Dissipation | 3.6 W at 60 °C |
| Isolation Voltage | 50 V continuous |
| Terminal Block | 1756-TBCH or 1756-TBS6H |
Where the 1756-OB16I Fits Best
Mixed-voltage panels. Because each point is isolated, you can drive 12 VDC pilot devices on some channels and 24 VDC solenoids on others from the same card. This eliminates the need for separate output modules in panels with mixed field voltages.
High-availability processes. Safe-state configuration lets you define what happens to each channel when the controller enters Program or Fault mode. Choose Hold Last State, On, or Off per point. This prevents unexpected actuator movement during a processor fault.
Retrofit and upgrade projects. If you are replacing a non-isolated output card, the 1756-OB16I drops into any 1756 local or remote chassis. The single-slot footprint and removable terminal block simplify field wiring changes.
Installation and Wiring Notes
The module installs directly into a 1756 chassis. Use the 1756-TBCH or 1756-TBS6H removable terminal block for signal termination. For inductive loads, install an external surge protection device to manage inrush current. The recommended fuse is a 5×20 mm 4 A quick-acting type. The module does not include internal fusing, so external protection is required. Follow all manufacturer wiring guidelines and local electrical codes.
Maintenance and Reliability
Inspect terminal connections periodically for tightness and corrosion. Verify that ambient temperature inside the enclosure stays within the 0 to 60 °C operating range. At higher temperatures, derate current per point and per module according to the linear derating curve. Keep the chassis ventilation path clear. The module has no moving parts, so wear is minimal. The primary failure mode in the field is external: shorted loads or loose wiring. Address those first during troubleshooting.
Compatibility and Integration
The 1756-OB16I works with ControlLogix controllers and communicates over the backplane. It supports multiple communication formats including output data, listen-only output data, rack optimization, and scheduled output data. This flexibility allows integration into both new and existing ControlLogix architectures without special firmware or configuration tools beyond standard programming software.
Selection Checklist Before You Buy
Confirm your field voltage falls within 10-30 VDC. Calculate total module current against your ambient temperature. Verify you have the correct terminal block: 1756-TBCH or 1756-TBS6H. Check that your chassis has an open slot. For availability and current lifecycle status, contact SpareTechPLC before ordering.
Frequently Asked Questions
Can I mix 12 V and 24 V loads on the same 1756-OB16I module?
Yes. Each of the sixteen outputs is individually isolated, so you can connect different voltage sources to different channels as long as each source is within the 10-30 VDC operating range.
What happens to my outputs when the controller faults?
You can configure a safe state per point. Choices are Hold Last State, On, or Off. This behavior activates when the controller switches to Program or Fault mode.
What terminal block do I need?
Use either the 1756-TBCH or 1756-TBS6H removable terminal block. Both are 36-pin housings.
Does this module have internal fusing?
No. The 1756-OB16I does not include internal fusing. Rockwell recommends an external 5×20 mm 4 A quick-acting fuse for protection.
What is the maximum current per point?
2 A at 30 °C, derating linearly to 1 A at 60 °C. Plan your load budget based on the actual ambient temperature inside the enclosure.
