Allen-Bradley 1771-IM Discrete Input Module

The 1771-IM is a high-voltage discrete AC/DC input module engineered for the Allen-Bradley PLC-5 system architecture. It interfaces high-voltage field sensing devices—such as limit switches, pushbuttons, and selector switches—to the controller's logic level by converting incoming 220/240 V signals into digital status data. Featuring 8 discrete input channels, the module draws 75 mA of current directly from the 1771 I/O chassis backplane.

Signal Processing and System Interface

The module processes high-voltage AC or DC control signals, converting line-level voltages into internal logic signals sent across the backplane to the main PLC-5 processor. Each of the 8 channels is opto-isolated to shield sensitive backplane electronics from field electrical noise, high-voltage spikes, and ground loops. Status indicators on the module front panel provide real-time visual verification of individual channel states, simplifying field diagnostics. 

Field Wiring and Chassis Integration

Field wiring connects via a removable 1771-WA wiring arm, enabling module installation or replacement without disconnecting individual field leads. Occupying a single slot in any standard 1771 I/O chassis, the module features mechanical keying slots on its rear circuit card to prevent accidental insertion into slots configured for incorrect voltage levels.

Frequently Asked Questions

Q: Can the 1771-IM module accept direct DC inputs as well as AC inputs? A: Yes, the module is rated for 220/240 V AC/DC line-level field inputs, accepting high-voltage signals from both AC control circuits and direct-current field devices.

Q: Is it necessary to disconnect field wiring when swapping a damaged 1771-IM module? A: No, the module utilizes a swing-arm style 1771-WA field wiring connector that latches into the chassis, allowing the module to be unplugged and replaced without rewiring individual terminal connections.