Allen-Bradley 1756-OX8I ControlLogix Isolated Contact Module

Allen-Bradley 1756-OX8I ControlLogix Isolated Contact Module

The Allen-Bradley 1756-OX8I, also cataloged as the 1756-OX8I Isolated Contact Module, operates as a dedicated hardware component for switching discrete electrical circuits within ControlLogix chassis architectures. The hardware provides direct physical execution via eight individually isolated Form C contact pairs, containing eight Normally Open (N.O.) and eight Normally Closed (N.C.) contacts. Technicians interface the hardware to both AC and DC field loads, allowing the module to close circuits handling 10 to 240 VAC or 5 to 125 VDC without cross-channel electrical interference.

Hardware Specifications

Deterministic Network and I/O Density Scaling

The Allen-Bradley 1756-OX8I relies on high-velocity backplane bus communication to transfer channel state data between the local processor and the physical relay coils. When integrated into distributed configurations over EtherNet/IP deterministic networks, the module updates its contact coil states synchronously with the controller scan cycle. Maintaining consistent firmware flash compatibility across the I/O rack ensures that backplane communication velocity remains steady, preserving the 1.2 ms mean bounce time and enforcing predictable timing even as the overall I/O density scales across the plant floor topology.

Frequently Asked Questions

Q: Can the 1756-OX8I be converted into a non-isolated, common-line relay module?

A: Yes. The installation technician can convert the module to a non-isolated configuration by installing a 1756-JMPR jumper or a third-party conductor to short the L1 connections on the Removable Terminal Block (RTB).

Q: What is the operational benefit of using the 1756-TBCH or 1756-TBS6H RTB with this module?

A: The separately purchased RTB allows full disconnection and replacement of the hardware module without terminating or altering the existing field-side signal wiring, which minimizes maintenance downtime.

Q: What limits the maximum cycle speed of the contacts on this module?

A: The physical relay mechanism limits the switching frequency to 0.3 Hz (one operation every three seconds) at full rated load to prevent premature contact wear and ensure the module meets its engineered cycle life expectancy.

Field Installation Guidelines

• Chassis Engagement and Latches: Insert the 1756-OX8I directly into any available slot of the ControlLogix chassis. Press firmly until the top and bottom retention latches click into place, securing the physical module against backplane vibration.
• Shield Grounding Regulations: Run inductive load lines through separate conduit paths away from low-voltage communication paths. Ground the chassis frame to a low-impedance earth ground to dissipate inductive kickback voltage spikes.
• Inductive Load Suppression: Install external surge suppression devices across external inductive loads (such as motor starter coils or large contactors) to protect the internal mechanical contacts from excessive arcing and pitting.