Allen-Bradley 1756-OX8I ControlLogix Relay Output Module

Configured for high-voltage discrete load switching in ControlLogix chassis architectures, the Allen-Bradley 1756-OX8I (1756-OX8I Digital Isolated Relay Output Module) provides direct physical/electrical execution. The hardware establishes contact closure control for field actuators across 5-125 VDC and 125/240 VAC control circuits. Each channel offers complete channel-to-channel galvanic isolation, preventing common-mode voltage transfer between independent power sources. The module interfaces directly with the ControlLogix backplane system for power distribution, status synchronization, and deterministic command parsing without requiring interposing external relays for standard industrial control loops.

Hardware Specifications

ControlLogix System Architecture & Backplane Protocol Features

The module communicates over the local ControlLogix backplane bus utilizing high-velocity CIP (Common Industrial Protocol) framing. Backplane bus communication velocity guarantees low deterministic latency during field state updates. High-density I/O routing protocols ensure synchronous cyclic data transmission to the system controller, matching configured Requested Packet Interval (RPI) rates down to sub-millisecond thresholds. Firmware flash compatibility enables online updates through Studio 5000 Logix Designer without physically dismounting the chassis assembly.

Frequently Asked Questions

Q: Can the 1756-OX8I module be hot-swapped (RIUP) while backplane power is actively supplied?

A: Yes, the module supports Removal and Insertion Under Power (RIUP). However, control power wired to the Removable Terminal Block (RTB) must be de-energized or safely isolated prior to disconnecting the RTB header to eliminate arc flash exposure and contact degradation on external field terminals.

Q: Are adjacent channels on the 1756-OX8I capable of operating at different field voltage levels?

A: Yes, each output channel features complete individual galvanic isolation. Channel 0 can switch 24 VDC control signals while Channel 1 simultaneously switches 240 VAC loads, provided maximum continuous insulation voltage limits between channels are strictly observed.

Field Installation Guidelines

• Ensure backplane chassis grounding complies with IEEE 1100 standards; ground wire connection to the enclosure star ground point must maintain low impedance using copper conductors rated for site environment requirements.
• Inspect standard 36-pin Removable Terminal Block (RTB) retaining screws before insertion. Ensure mechanical locking latches engage fully with the chassis top and bottom alignment keyways.
• Maintain a minimum wiring separation distance of 50 mm (2 inches) between high-voltage AC load lines and low-voltage DC communication or analog signal wiring inside the wire ducting.
• Install appropriate external inductive load suppression (RC snubbers for AC loads, flyback diodes for DC loads) across switched field devices to prevent contact welding and electromagnetic interference back-propagation.