Allen-Bradley 1791-8BC 16-Point Block I/O DC Module

Allen-Bradley 1791-8BC Combination DC I/O Module for Remote I/O Networks

Product Introduction

The Allen-Bradley 1791-8BC belongs to the legacy 1791 Block I/O family. This module combines eight discrete inputs and eight discrete outputs in one compact housing. It operates on 24VDC with an input voltage range of 10–30VDC. Therefore, this module is ideal for distributed I/O architectures where space is limited.

You can mount the 1791-8BC vertically or horizontally on a DIN rail. Its output current rating depends on the mounting orientation and temperature. For vertical mounting, it delivers 500mA at 60°C or 1A at 30°C. For horizontal mounting, it provides 250mA at 60°C or 500mA at 30°C.

This module supports Allen‑Bradley’s Remote I/O (RIO) protocol. Consequently, it works with PLC-2, PLC-3, PLC-5, and SLC 500 controllers. Additionally, you can connect it to CompactLogix or ControlLogix platforms using a RIO communication module.

Technical Specifications of the 1791-8BC

The 1791-8BC communicates via a three-wire RIO cable, such as 1771-CD or Belden 9463. Connection points are terminals 6 (BLU), 7 (SHD), and 8 (CLR). This module supports three baud rates: 57.6K, 115.2K, and 230.4K.

You must terminate the first and last node on the RIO network. Use a 150 ohm resistor for 57.6K and 115.2K baud rates. Use an 82 ohm resistor for 230.4K baud. Connect this resistor across terminals 6 (BLU) and 8 (CLR).

Input Specifications

• Number of inputs: 8 (one group)

• Nominal input voltage: 24VDC

• On-state voltage range: 10–30VDC

• On-state current: 2.5mA min at 10V, 11.0mA max at 30V

• Off-state voltage: 5VDC max

• Off-state current: 1.5mA min

Output Specifications

• Number of outputs: 8

• Output voltage range: 10–30VDC

• Output current (vertical): 500mA at 60°C, 1A at 30°C

• Output current (horizontal): 250mA at 60°C, 500mA at 30°C

Power and Environment

• External power supply: 19.2–30VDC

• Maximum current draw: 300mA

• Power dissipation: 11.8 watts

• Thermal dissipation: 41 BTU/hour

• Suitable wire: 14 gauge stranded max, 3/64 inch insulation max

Mechanical and Environmental Parameters

The 1791-8BC weighs 2.72 kg (6.00 lbs). Its rackless design saves panel space significantly. You can mount it on a standard DIN rail without any backplane.

Furthermore, this module includes individual LED indicators for each channel. An OK LED shows the module status. These visual indicators help you troubleshoot field wiring quickly.

However, note that output current derating applies at higher temperatures. Horizontal mounting reduces the current capacity compared to vertical mounting. Always check your ambient temperature before installation.

Installation Guidelines for the 1791-8BC

First, disconnect all power before handling the module. Then mount the 1791-8BC on a DIN rail. Ensure proper spacing around the module for heat dissipation.

Next, connect the external DC power supply (19.2–30VDC) to the power terminals. After that, wire your input devices, such as proximity sensors or pushbuttons, to the eight input channels. Then wire your output loads, including solenoids or indicator lights, to the eight output channels.

Finally, connect the RIO communication cable to terminals 6, 7, and 8. Set the baud rate and node address using the DIP switches. Remember to install terminating resistors at both ends of the RIO trunk line.

Packaging List

• 1 x Allen-Bradley 1791-8BC 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 I use the 1791-8BC with a modern ControlLogix system?
Yes, you can. However, you need a 1756-RIO or 1756-DHRIO module to bridge the RIO network. The 1791-8BC itself only speaks the legacy RIO protocol.

2. Why does output current change with mounting orientation?
Heat dissipation differs between vertical and horizontal mounting. Vertical mounting allows better airflow. Therefore, it supports higher output current. Horizontal mounting restricts airflow, so derating applies.

3. What happens if I don't use terminating resistors?
Communication errors will occur randomly. You may see data corruption and node dropouts. Always install a 150Ω or 82Ω resistor across terminals 6 and 8 at both cable ends.

4. Is the 1791-8BC compatible with 12VDC field devices?
The on-state voltage range starts at 10VDC. Therefore, 12VDC devices work properly. However, check that your field device provides at least 2.5mA minimum on-state current at 10V.

5. Is this module still available from Rockwell Automation?
No, the 1791-8BC is discontinued and obsolete. Rockwell no longer manufactures it. However, many third‑party suppliers offer refurbished units with technical support.