Allen-Bradley 1791-8AR Block I/O Remote Module

Allen-Bradley 1791-8AR Block I/O Module for Remote I/O Architecture

Product Introduction

The Allen-Bradley 1791-8AR belongs to the legacy 1791 Block I/O family. This module integrates a power supply, RIO interface, signal conditioning, and I/O terminals into a single housing. Therefore, it acts as a complete remote I/O node without needing extra components.

You will find eight 120VAC discrete input channels on this device. Their on-state voltage range is 79–132VAC. Additionally, the module provides eight relay outputs. These outputs can switch both AC and DC loads up to 125VDC at 1A (resistive).

This model works perfectly with legacy Allen-Bradley controllers. For example, you can connect it to PLC-2, PLC-3, PLC-5, or SLC 500 systems. The SLC 500 requires a 1747-SN RIO scanner module for proper operation.

Technical Specifications of the 1791-8AR

The 1791-8AR follows Allen‑Bradley’s Remote I/O (RIO) protocol. This protocol uses a daisy‑chain topology. It supports three baud rates: 57.6K, 115.2K, and 230.4K.

Cable length depends directly on the baud rate. For instance:

• 10,000 feet at 57.6K baud

• 5,000 feet at 115.2K baud

• 2,500 feet at 230.4K baud

You must install terminating resistors at both ends of the RIO line. Use either 82 ohms or 150 ohms for reliable communication.

Input Specifications

• Number of inputs: 8 (one group of 8)

• On-state voltage: 79–132VAC

• On-state current: 12.3mA max at 132VAC

• Off-state voltage: 35VAC max

• Off-state current: 2.3mA min

Output Specifications

• Number of outputs: 8 relay outputs

• Output voltage range: 125VDC at 1A resistive, 20–30VDC at 1A, or 125VAC at 0.2A

• Output type: Relay (dry contact)

Power and Environment

• External power: 85–132VAC at 150mA

• Maximum power dissipation: 12.9 watts

• Maximum thermal dissipation: 56.1 BTU/hr

• Mounting orientation: Vertical or horizontal

Mechanical and Environmental Parameters

The 1791-8AR weighs only 0.45 kg (1.00 lbs). Its compact size saves valuable panel space. You can mount it vertically or horizontally without any performance derating.

Moreover, this module operates reliably in standard industrial environments. It has no built-in cooling fan. Therefore, you should maintain proper spacing around the unit for adequate airflow.

Installation Guidelines for the 1791-8AR

First, disconnect all power before starting the installation. Then mount the 1791-8AR on a DIN rail or directly onto a panel. Use the mounting holes provided on the module housing.

Next, connect the external AC power supply (85–132VAC) to the designated terminals. After that, wire your input devices, such as limit switches or pushbuttons, to the eight input channels. Then wire your output loads, like solenoids or pilot lamps, to the relay output terminals.

Finally, connect the RIO communication cable to the module. Remember to set the correct baud rate and node address using the DIP switches. Terminate both ends of the RIO trunk line with either 82Ω or 150Ω resistors.

Packaging List

• 1 x Allen-Bradley 1791-8AR Block I/O module

• 1 x Terminal block (factory-installed, no separate part)

• 1 x Quick reference guide (printed)

• No calibration certificate included

• No warranty document included

Shipping Information

We ship all orders via FedEx, UPS, or DHL. Delivery times depend on your specific location. Typically, you will receive the module within 3–7 business days.

Frequently Asked Questions (FAQ)

1. Can I use the 1791-8AR with a modern ControlLogix PLC?
Yes, but you will need a 1756-RIO or 1756-DHRIO module to bridge the RIO network. The 1791-8AR itself speaks only the legacy RIO protocol.

2. What happens if I skip the terminating resistors?
Communication errors will appear randomly. Data corruption and node dropouts occur frequently. Therefore, always use 82Ω or 150Ω resistors at both cable ends.

3. Can the relay outputs switch 240VAC loads directly?
No. The maximum AC voltage rating is 125VAC at 0.2A resistive. Never exceed this rating. For higher voltages, use an external interposing relay.

4. How do I set the baud rate on this module?
You set it via DIP switches located inside the module. Refer to the user manual for exact switch positions. Typical settings are 57.6K, 115.2K, or 230.4K baud.

5. Is the 1791-8AR still supported by Rockwell Automation?
This product is a legacy item. Rockwell no longer offers direct factory support or repairs. However, many third‑party suppliers provide refurbished units and technical assistance.