1791-8BC | Allen-Bradley Block I/O Combination Module - Petro Auto Tech

Product Overview

The 1791-8BC is a 16-Point combination discrete I/O module from the Block I/O series of Allen-Bradley. This module integrates eight 24 VDC discrete inputs and eight 24 VDC discrete outputs in a single, rackless unit. Designed for distributed I/O architectures, it supports Remote I/O (RIO) communication protocol and is compatible with PLC-2, PLC-3, 1785 PLC-5, 1747 SLC 500, and Logix platforms via a RIO communication module. The 1791-8BC is now discontinued and obsolete, making it a sought-after replacement part for existing systems.

Technical Specifications Analysis

The module operates on a 24V DC voltage supply with an external power range of 19.2–30V DC and a maximum current draw of 300 mA. It features eight input channels with an on-state voltage range of 10-30V DC and a nominal input signal of 24 VDC. The input on-state current is 11 mA @ 30V max, and the off-state current is 1.5 mA min. Outputs include eight channels with a voltage range of 10-30V DC. Output current ratings vary by mounting orientation: vertical mounting delivers 500 mA @ 60°C and 1A @ 30°C, while horizontal mounting provides 250 mA @ 60°C and 500 mA @ 30°C. The module includes individual channel LED indicators for status monitoring.

Application Scenarios & Usage Methods

This module is ideal for decentralized control in manufacturing lines, such as packaging, material handling, and assembly stations. Use it to connect sensors and actuators directly to a remote I/O network, reducing wiring and cabinet space. For example, in a conveyor system, connect eight 24V DC proximity sensors to the inputs and eight solenoid valves to the outputs. Configure the RIO network with terminating resistors: 150 ohms at 57.6K or 115.2K baud, and 82 ohms at 230.4K baud. The module communicates via 1771-CD or Belden 9463 cable.

Installation Guide & Packing List

Mount the module vertically or horizontally on a DIN rail in a clean, dry enclosure. Ensure the external power supply meets 19.2–30V DC and 300 mA minimum. Connect the RIO cable to terminals 6 (BLU), 7 (SHD), and 8 (CLR). Terminate the first and last nodes with the appropriate resistor. The package includes the 1791-8BC module only. No mounting hardware, cables, or resistors are included.

Purchasing Advice

Given the discontinued status, verify compatibility with your existing RIO network and controller. The module's output current varies with mounting orientation—select vertical for higher current at lower temperatures. Petro Auto Tech offers this module as a reliable source for legacy Allen-Bradley systems. Check stock availability and consider purchasing a spare for critical applications.

Repair & Maintenance Tips

Inspect the module regularly for dust or moisture buildup. Clean with a dry, lint-free cloth. Verify that the external power supply remains within 19.2–30V DC. Check LED indicators for proper channel status. If a channel fails, test the connected device and wiring before replacing the module. Store in a static-safe environment when not in use.

Frequently Asked Questions (FAQs)

Q: What is the input voltage range for the 1791-8BC?

A: The input signal range is 10-30 VDC, with a nominal input of 24 VDC.

Q: How many outputs does this module have?

A: The module has eight output channels, each with a voltage range of 10-30 VDC.

Q: What is the output current in vertical mounting at 60°C?

A: In vertical mounting at 60°C, each output can handle 500 mA.

Q: Is the 1791-8BC still in production?

A: No, the product lifecycle status is discontinued/obsolete.

Q: What communication protocol does this module support?

A: It supports Remote I/O (RIO) communication protocol.