Allen-Bradley 1791-8BR Block I/O 8 Input 8 Relay Output

Allen-Bradley 1791-8BR Block I/O Module with 24VDC Inputs and Relay Outputs

Product Introduction

The Allen-Bradley 1791-8BR belongs to the 1791 Block I/O family. This combination module provides eight 24VDC input channels and eight normally open relay output channels. The relay outputs are dry contacts. Therefore, they are compatible with both AC and DC voltages.

You can use this module as a stand-alone distributed I/O station. Its built-in power supply and communication interface eliminate the need for a separate backplane. Consequently, you can install the 1791-8BR close to field devices. This placement reduces long cable runs significantly.

This module supports the Remote I/O (RIO) protocol. It connects directly to RIO scanners and controllers. Compatible systems include the PLC-2, PLC-3, PLC-5, and SLC 500 families. For instance, you can use it with a 1771-SN scanner module or a 1772-SD module.

Technical Specifications of the 1791-8BR

The 1791-8BR receives external DC power through specific terminals. Connect +24V to terminal 1, RET +24 to terminal 3, and GND to terminal 2. The external power range is 19.2–30VDC.

RIO communication connections use three terminals. Specifically, connect to BLU (terminal 6), CLR (terminal 8), and SHD (terminal 7). This module supports standard RIO baud rates for reliable long-distance communication.

Input Specifications

• Number of inputs: 8

• Nominal input voltage: 24VDC

• On-state voltage range: 10–30VDC

• On-state current: 11mA max at 30V

• Off-state voltage: 5VDC max

• Off-state current: 1.5mA min

Output Specifications

• Number of outputs: 8 normally open relay contacts

• Output voltage (AC): 20–132VAC

• Output voltage (DC): 20–30VDC at 1A resistive max, or 125VDC

• Output type: Dry contact (relay)

Power and Wiring

• External power: 19.2–30VDC

• Recommended wire: 14-gauge stranded with 3/64 inch max insulation

• No internal fusing (external protection required)

• Parallel output connections are not permitted

Important Output Characteristics

The 1791-8BR receives DC power as an input but delivers AC or DC power through its relay outputs. Output voltage ratings vary based on the load type. For resistive loads, the maximum is 20–30VDC at 1A or 125VDC.

Surge current is the most critical parameter you must consider. This module has no internal surge current limiting. Therefore, you need external fuses to protect both the module and your field devices.

Internal protection circuits guard the module's internal electronics only. They do not protect external wiring or loads. Always install appropriate external overcurrent protection for each output channel.

Mechanical and Environmental Parameters

The 1791-8BR weighs only 0.45 kg (1.00 lbs). Its rackless design allows direct mounting on a standard DIN rail. Alternatively, you can panel mount the module using its mounting holes.

You can mount this module vertically or horizontally. The mounting orientation does not affect the current rating. However, always maintain adequate spacing for heat dissipation.

Installation Guidelines for the 1791-8BR

First, disconnect all power before starting the installation. Then mount the 1791-8BR on a DIN rail or panel. Ensure the module is secure and properly aligned.

Next, connect the external DC power supply (19.2–30VDC) to terminals 1, 2, and 3. After that, wire your input devices to the eight input channels. Compatible devices include proximity switches, limit switches, pushbuttons, float switches, and auxiliary contacts.

Then wire your output loads to the relay output terminals. You can connect solenoid actuators, motor starters, pilot lamps, and other inductive or resistive loads. Remember to install external fuses for each output circuit.

Finally, connect the RIO communication cable to terminals 6 (BLU), 7 (SHD), and 8 (CLR). Set the node address and baud rate using the DIP switches. Terminate both ends of the RIO trunk line with the appropriate resistor.

Important Wiring Notes

Use 14-gauge stranded wire with a maximum insulation thickness of 3/64 inch. High-quality conductors with low resistance help maintain load current. Do not connect outputs in parallel. Parallel connections lower the overall circuit resistance and can damage the module.

Packaging List

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

• 1 x Terminal block (factory-installed)

• 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 typically takes 3–7 business days depending on your location.

Frequently Asked Questions (FAQ)

1. Can the 1791-8BR switch 240VAC loads?
No. The maximum AC voltage rating is 132VAC. Do not exceed this rating. For higher voltages, use an external interposing relay.

2. Why does the 1791-8BR not have internal fusing?
This module relies on external fuses for output protection. Internal protection only guards the module's internal circuitry. You must install external fuses for each output to protect field wiring and loads.

3. Can I connect two output channels in parallel for higher current?
No. Parallel output connections are not permitted. This practice lowers circuit resistance and may damage the relay contacts. Use one output per load or add an external power relay.

4. What input devices work best with the 1791-8BR?
Any 24VDC binary signaling device works well. For example, proximity switches, limit switches, pushbuttons, float switches, and auxiliary contacts are all compatible.

5. Is the 1791-8BR compatible with ControlLogix platforms?
Yes, but you need a RIO scanner module. Use a 1756-RIO or 1756-DHRIO module to bridge the RIO network. The 1791-8BR communicates only via legacy RIO protocol.