1794-IM8 Allen Bradley Flex I/O 8 Channel Digital AC Input Module

Product Overview

The 1794-IM8 is a high-voltage digital input module from the Allen-Bradley Flex I/O Series by Rockwell Automation. Built for modular distributed control systems, this module provides 8 non-isolated AC input channels operating at a nominal 220V/240V AC line voltage. The unit mounts directly onto a 1794-TBN terminal base unit, combining the flexibility of remote I/O with a compact footprint (46mm x 94mm x 53mm). Keying is enforced via an 8-position mechanical keyswitch to prevent improper module insertion during maintenance. Front-panel field-side yellow LED indicators give technicians immediate visual feedback for each channel state.

Technical Specifications

Input Filter Time Configurations

Engineers can adjust software filter settings via the output image table (Bits 00-02) to compensate for contact bounce, line transients, or noisy field environments:

• Filter Time 0 (Bits 02:0, 01:0, 00:0 - Default): OFF to ON = 7.5 ms | ON to OFF = 26.5 ms
• Filter Time 1 (Bits 02:0, 01:0, 00:1): OFF to ON = 8.0 ms | ON to OFF = 27.0 ms
• Filter Time 2 (Bits 02:0, 01:1, 00:0): OFF to ON = 9.0 ms | ON to OFF = 28.0 ms
• Filter Time 3 (Bits 02:0, 01:1, 00:1): OFF to ON = 10.0 ms | ON to OFF = 29.0 ms
• Filter Time 4 (Bits 02:1, 01:0, 00:0): OFF to ON = 12.0 ms | ON to OFF = 31.0 ms
• Filter Time 5 (Bits 02:1, 01:0, 00:1): OFF to ON = 16.0 ms | ON to OFF = 35.0 ms
• Filter Time 6 (Bits 02:1, 01:1, 00:0): OFF to ON = 24.5 ms | ON to OFF = 44.0 ms
• Filter Time 7 (Bits 02:1, 01:1, 00:1): OFF to ON = 42.0 ms | ON to OFF = 60.5 ms

Target Machinery & System Compatibility

The 1794-IM8 module interfaces seamlessly with standard high-voltage AC discrete sensors, mechanical switches, and field devices. It fits within wider Allen-Bradley automation architectures using Flex I/O adapters (EtherNet/IP, ControlNet, or PROFIBUS DP). Target equipment includes:

• 220V AC line-powered limit switches and heavy-duty pushbuttons
• Motor starter auxiliary contact feedback circuits
• Process solenoid valve limit switch position monitoring
• High-voltage pressure and float switches in water/wastewater treatment
• Industrial conveyor control panels utilizing 230V AC control voltage loops

Diagnostic & Monitoring Capabilities

This module provides real-time signal conditioning and hardware verification for discrete high-voltage inputs:

• Field-Side LED Status: Eight individual yellow LEDs indicate valid ON-state field signals directly from the terminal base without depending on controller backplane communications.
• Selectable De-glitching: Eight configurable filter time settings allow software engineers to suppress contact bounce and electrical noise spikes without adding hardware RC filters.
• Backplane Verification: Built-in Flexbus monitoring ensures full backplane contact and power delivery prior to sending valid input data to the main PLC processor.

Packaging Details, Fast Delivery & Warranty

• Item Condition: New & Original in Factory Packaging
• Warranty Terms: 12 Months full replacement warranty
• Standard Dispatch: Ships within 24 to 48 hours after payment confirmation
• Express Freight Carriers: FedEx, UPS, and DHL worldwide express shipping

Global PLC Spare HUB's Insight

In modern industrial retrofits, engineering teams often face legacy control panels that use 220V/240V AC control logic rather than 24V DC. The 1794-IM8 allows direct interface to high-voltage AC field switches without requiring intermediate interposing relays. When installing, always verify that your terminal base is set to Keyswitch Position 8 before snapping the module into place. If you encounter erratic input switching, check the off-state leakage current of solid-state sensors connected to the inputs — any leakage exceeding 2.6 mA can keep the 22.3 kΩ input circuit above the 40V AC OFF threshold, causing false ON indications in your Logix controller.

Frequently Asked Questions (FAQ)

Q: What terminal base unit must be paired with the 1794-IM8 module?

A: This module mounts directly on the Cat. No. 1794-TBN terminal base unit, which provides screw-clamp terminals for high-voltage AC wiring connections.

Q: Why does an input point stay ON even when the field switch is open?

A: Solid-state field devices (like proximity switches) often have off-state leakage current. If leakage exceeds 2.6 mA into the 22.3 kΩ input impedance, the voltage stays above the 40V AC OFF threshold. Installing a resistor in parallel across the input terminals solves this issue.

Q: How do I change the input filter times on the 1794-IM8 in Studio 5000 or RSLogix 5000?

A: Input filter times are set by writing to configuration bits 00, 01, and 02 in the module output image table. Selecting values from 0 to 7 adjusts the OFF-to-ON filter time from 7.5 ms up to 42 ms.

Q: Can I mix 24V DC modules and 220V AC modules on the same Flex I/O rail assembly?

A: Yes. Flex I/O backplanes support mixed voltage modules on the same rail. However, you must route high-voltage 220V AC field wiring separately from low-voltage 24V DC field wiring according to Category 1 isolation standards.

Q: Selection Guide: When should I choose the 1794-IM8 over the 1794-IA8 module?

A: Choose the 1794-IM8 for 220V/240V AC nominal field circuits (operating between 159V and 264V AC). Choose the 1794-IA8 for lower 120V AC nominal field circuits (operating between 85V and 132V AC).

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