Allen-Bradley 1746-IO4 SLC 500 Digital Combination I/O Module

The 1746-IO4 is a 4-point digital combination I/O module manufactured by Allen-Bradley for the SLC 500 programmable controller platform. It integrates 2 digital inputs rated for 120 VAC and 2 electromechanical relay outputs featuring normally open (N.O.) contacts on a single compact chassis slot. Designed for applications requiring localized control with limited point counts, the 1746-IO4 minimizes backplane rack space requirements while providing isolation between control logic and field signals.

Input Specification and Signal Characteristics

The two AC input channels operate at a nominal voltage of 120 VAC, converting high-level plant signals into low-voltage logic levels across the SLC backplane. Each input circuit includes filtering and isolation to buffer internal processor logic against field voltage surges, line noise, and transient spikes common in industrial environments.

Relay Output Capabilities

The two relay output channels utilize electromechanical Form A (normally open) contacts capable of switching AC or DC load voltages. This dry-contact design provides voltage flexibility, allowing maintenance teams to interface directly with control relays, small solenoids, indicator lights, and motor starters operating across different control voltages without external interposing relays.

System Compatibility and Installation

The module installs directly into any available I/O slot of an SLC 500 chassis, including 1746-A4, 1746-A7, 1746-A10, or 1746-A13 racks. It communicates with SLC 5/01, 5/02, 5/03, 5/04, and 5/05 processors via the standard backplane bus. Field wiring connects via a removable terminal block (RTB), allowing quick module replacement without disconnecting individual field conductors.

Frequently Asked Questions

Q: How is the 1746-IO4 identified by the SLC 500 processor in programming software? A: The module is configured in RSLogix 500 / APS software as a 4-point combination module, assigning 2 input bits and 2 output bits to the CPU's I/O image table for the assigned chassis slot.

Q: Can the relay outputs on the 1746-IO4 handle DC control voltages? A: Yes, because the output channels are isolated electromechanical relay contacts (dry contacts), they can switch compatible DC voltage loads within the relay's rated current limits.

Q: Does replacing a 1746-IO4 require rewiring field devices? A: No, field wiring is landed on a removable terminal block. Disconnecting the terminal block assembly allows the module to be swapped without disturbing existing field wiring.