Allen-Bradley 1791-24AR Block I/O 24 Inputs 8 Relays

1791-24AR Block I/O with Relay Outputs for Mixed Loads

The Allen-Bradley 1791-24AR solves a common problem in industrial controls. Many machines need AC inputs but also require switching low-voltage DC signals. This module gives you 24 AC input channels and 8 relay outputs that work with either AC or DC power. Therefore, you avoid using separate interface relays. The 1791-24AR belongs to the 1791 Block I/O series, known for distributed remote I/O architectures.

Detailed Technical Specifications of the 1791-24AR

This module operates with a nominal 120VAC input. Its on-state voltage range runs from 79VAC to 132VAC. The on-state current reaches 12.3mA at 132V max. For off-state conditions, the voltage stays below 35V max, and the current stays below 2.3mA min. The 1791-24AR organizes its 24 inputs into three isolated groups of 8 channels each.

The relay outputs are the key feature of the 1791-24AR. Each output is a normally open electromechanical relay. The output voltage depends entirely on your external load. For AC loads, the range is 20-132VAC at 1A resistive max. For DC loads, you get 20-30VDC at 1A resistive max. Additionally, at lower currents, the voltage can go higher. Specifically, at 0.2A resistive max, the output reaches 125VDC.

Mechanical Dimensions and Mounting

The 1791-24AR measures 3.85 x 4.35 x 6.95 inches. It weighs 2.50 lbs (1.13 kg). You can mount the unit vertically or horizontally inside a control cabinet. The housing is rugged enough for industrial environments. Moreover, the built-in Remote I/O (RIO) interface eliminates extra communication cards. Therefore, installation is straightforward.

Installation and Wiring Best Practices

First, mount the 1791-24AR close to your field devices. Second, supply external power between 85-132VAC at the dedicated terminals. Third, connect your PLC using Belden 9463 blue hose cable. Set the baud rate switch to match your RIO scanner (57.6K, 115.2K, or 230.4K). For daisy-chain networks, install a terminating resistor (82 or 150 ohm) on the last node. Without this resistor, you risk intermittent communication failures.

Furthermore, keep relay output wiring separate from input wiring. Relay contacts can generate electrical noise when switching inductive loads. Additionally, use external surge suppression for heavy inductive loads. This practice extends relay life significantly.

Compatibility with Controllers

The 1791-24AR works with multiple Allen-Bradley controllers. Use it with PLC-2, PLC-3, or PLC-5 families through their embedded RIO scanner. For SLC 500 systems, add the 1747-SN Remote I/O scanner module. The 1791-24AR then acts as an adapter on the RIO link. ControlLogix users need a 1756-RIO scanner module. Thus, the module fits into both legacy and modern systems.

Why Choose the 1791-24AR Over Standard AC Output Blocks?

Standard AC output blocks only switch AC voltage. The 1791-24AR switches both AC and DC. This feature reduces panel space because you no longer need interposing relays. Additionally, the relay outputs provide complete isolation between the logic side and the field side. Maintenance becomes simpler too. If a relay fails, you replace the entire module easily.

Packaging List

• 1791-24AR block I/O module (1 unit)

• Built-in RIO adapter and power supply

• Removable terminal blocks (factory included)

• Quick start installation guide

Note: No certificates or warranty documents are included.

Shipping Method

All orders ship via FedEx, UPS, or DHL. Tracking information is provided within 24 business hours.

Frequently Asked Questions (FAQ)

1. Can the 1791-24AR switch 24VDC sensors directly?
Yes. The relay outputs handle 20-30VDC at 1A resistive max. Therefore, you can switch small 24VDC loads without extra relays.

2. What is the lifespan of the relay outputs on the 1791-24AR?
Electrical life depends on the load. At 1A resistive, expect 100,000 operations. For inductive loads, the life drops to 30,000-50,000 cycles. Use external suppression to maximize life.

3. Do I need a separate power supply for the 1791-24AR?
No. The module has a built-in power supply. Simply connect 85-132VAC to the external power terminals. The internal circuitry then powers the logic and RIO interface.

4. How do I know which baud rate to select?
Match the baud rate of your RIO scanner. For short distances under 2,500 feet, use 230.4K. For longer runs up to 10,000 feet, use 57.6K. Set the switches on the 1791-24AR before powering up.

5. Can I mix the 1791-24AR with other 1791 blocks on the same RIO link?
Yes. The 1791 series works together on one Remote I/O network. You can combine AC input blocks, relay output blocks, and DC blocks on the same daisy-chain. Only the terminating resistor at the last node matters.