Description

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Allen‑Bradley 1791‑8AR – Remote I/O Block Module

The Allen‑Bradley 1791‑8AR is a legacy Remote I/O Block I/O module from Rockwell Automation’s 1791 Block I/O family. It was designed for use in distributed control and remote I/O applications where both field input sensing and output actuation were required in one compact package.

📌 Overview

Part Number: 1791‑8AR

Product Type: Block I/O Remote I/O Module (Discrete I/O)

Manufacturer: Allen‑Bradley

Status: Discontinued/Obsolete (Rockwell ended manufacture in 2015)

📊 Key Features

I/O Configuration:

8 AC inputs (120 VAC discrete sensing)

8 relay outputs (electromechanical relay contacts)

 

Remote I/O Support:

Integrated Remote I/O (RIO) interface for communication with compatible Allen‑Bradley controllers (e.g., PLC‑2, PLC‑3, PLC‑5, SLC500 with RIO scanner modules)

Input & Output Characteristics:

AC input on‑state voltage range ~79–132 VAC

Relay outputs suitable for switching AC or DC loads

Power Supply: External supply typically 85–132 VAC, ~150 mA

Mounting: Block I/O module form factor for panel or machine mounting

🛠️ How It Works

The 1791‑8AR combines input sensing and output actuation in one unit:

Inputs: Reads up to eight 120 VAC discrete signals from field devices such as limit switches, proximity switches, pushbuttons, and auxiliary contacts.

Outputs: Provides eight relay outputs, useful for switching loads like lamps, solenoids, coils, or other actuators within its voltage/current ratings.

Communication: Uses Allen‑Bradley’s Remote I/O protocol to transmit input status and receive output commands from a central controller over a RIO network.

Because it has an embedded RIO interface, integrated power components, and terminals, the 1791‑8AR acts as a complete Remote I/O node — reducing the need for separate I/O cards and simplifying decentralized I/O architecture.

🏭 Typical Applications

Although obsolete, the 1791‑8AR was widely deployed in industrial automation systems where remote or distributed I/O was needed, such as:

Factory automation panels

Machine control interfaces

Process industry remote I/O racks

Control systems using SLC/PLC families with RIO scanners

It offered a compact solution for combining sensing and actuation at the field level without bulky relay panels.

📌 Lifecycle & Alternatives

The 1791‑8AR is no longer manufactured and is classified as obsolete by Rockwell Automation (manufacturer support ended around 2015).

In modern architectures, Allen‑Bradley POINT I/O, Flex I/O, or CompactLogix I/O systems (with Ethernet/IP communication) are used instead for distributed I/O with improved performance and support.

For legacy systems using 1791 modules, third‑party surplus or refurbished units are sometimes used to maintain existing installations.