Allen-Bradley 1791-24B8 DC Block I/O 24 Inputs 8 Outputs

1791-24B8 DC Block I/O for Reliable Remote Control

The Allen-Bradley 1791-24B8 brings 32 total I/O points to your distributed control architecture. It has 24 inputs organized as three groups of 8 channels each. Additionally, it provides 8 output channels in one group. Both inputs and outputs operate on DC voltage from 10 to 30 Volts DC. Therefore, this module works perfectly with standard 24VDC sensors, actuators, and small solenoids. The built-in Remote I/O (RIO) interface connects directly to PLC-2, PLC-3, PLC-5, or SLC 500 scanners.

Technical Specifications of the 1791-24B8

This module requires an external power supply between 19.2 and 30 Volts DC at 300mA. The input on-state voltage range matches the same 10-30VDC as the outputs. Maximum on-state current reaches 11mA at 30 Volts DC. Off-state voltage stays below 5 Volts DC max, with off-state current at 1.5mA min. Input impedance is rated at 3.4 kOhm. For signal delay, you can select fast mode: 1ms ON and 5ms OFF. Alternatively, use slow mode: 1ms ON and 18ms OFF.

The 1791-24B8 has output ratings that depend on mounting orientation. For vertical mounting, each output handles 500mA to 1A at 30-60°C. For horizontal mounting, the range drops to 250-500mA at the same temperature. Surge current reaches 3 Amps for 50 milliseconds, recurring every 2 seconds. Output off-state current is 1mA with a voltage drop of 1 Volt. Current leakage stays below 0.5mA.

Power Dissipation and Thermal Management

The 1791-24B8 dissipates a maximum of 17.1 Watts of power. Its thermal dissipation reaches 58.1 BTU/hour. Consequently, you need adequate airflow inside your control cabinet. Do not block the ventilation slots on the module housing. For high-density installations, leave at least 50mm of space around the unit. This practice prevents overheating and extends component life.

Environmental Operating Conditions

This module works reliably in harsh industrial environments. The operational temperature range runs from 0°C to 60°C (32°F to 140°F). Relative humidity can vary between 5% and 95% non-condensing. Therefore, the 1791-24B8 suits unheated control rooms, outdoor enclosures, and washdown areas as long as condensation does not occur. Avoid direct sunlight and heat sources above 60°C.

Mechanical Dimensions and Mounting Options

The 1791-24B8 measures 176.5mm x 68.8mm x 98mm (6.95 x 2.7 x 3.85 inches). It weighs 2.25 lbs (1.02 kg). You can mount the unit vertically or horizontally on a DIN rail or directly on a panel. The compact size saves valuable cabinet space. Moreover, the built-in RIO adapter eliminates external communication modules. As a result, installation requires only power, I/O wiring, and a Belden 9463 blue hose cable.

Installation and Wiring Guidelines

First, mount the 1791-24B8 near your field devices. Second, connect external power between 19.2 and 30VDC at 300mA. Third, wire your inputs and outputs using shielded cables. Keep I/O wiring separate from communication lines. Fourth, connect the RIO network using Belden 9463 cable. Set the baud rate switch to 57.6K, 115.2K, or 230.4K to match your scanner. Finally, install a terminating resistor (82 or 150 ohm) on the last node of a daisy-chain network. Without this resistor, communication errors will occur.

Controller Compatibility

The 1791-24B8 works seamlessly with several Allen-Bradley controllers. For PLC-2, PLC-3, and PLC-5 families, use the embedded RIO scanner. For SLC 500 systems, add a 1747-SN Remote I/O scanner module. ControlLogix users require a 1756-RIO scanner. The 1791-24B8 then acts as an adapter on the RIO link. This setup allows you to expand I/O points without changing the main processor.

Why Choose the 1791-24B8 Over Other DC Blocks?

This module offers two input filter speeds. Fast mode (1ms/5ms) catches short pulses from high-speed sensors. Slow mode (1ms/18ms) rejects noise from mechanical switches. Furthermore, the output surge rating of 3A for 50ms handles inrush currents from lamps and small motors. Additionally, the wide 10-30VDC range accepts power from unregulated supplies. Therefore, the 1791-24B8 provides flexibility that fixed 24V modules cannot match.

Packaging List

• 1791-24B8 block I/O module (1 unit)

• Built-in RIO adapter and DC power supply

• Removable terminal block connectors

• Installation instructions quick sheet

Note: No certificates or warranty documents are included.

Shipping Method

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

Frequently Asked Questions (FAQ)

1. Can the 1791-24B8 sink or source current?
The inputs are designed for sinking (NPN) or sourcing (PNP) sensors depending on your common wiring. Check the manual for specific configurations. Most users connect PNP sensors.

2. What happens if I supply 32VDC to the 1791-24B8?
The absolute maximum rating is 30VDC. Exceeding this may damage the input circuits. Therefore, always regulate your power supply to 30VDC or less.

3. How many 1791-24B8 modules can I connect to one scanner?
Typically, up to 32 blocks at 57.6K baud. Higher baud rates reduce the maximum number. Refer to your scanner's user manual for exact limits.

4. Does the 1791-24B8 support output short-circuit protection?
Yes. The module includes internal current limiting. However, sustained shorts may still damage the output transistor. Use external fuses for critical circuits.

5. Can I replace a 1791-24B8 without powering down the RIO link?
No. Disconnect external power before removing the module. Hot swapping may damage the backplane connector or cause unpredictable behavior on the RIO network.