Allen-Bradley 1794-OB8 Flex I/O 8-Channel Output

Allen-Bradley 1794-OB8 8-Channel 24VDC Sourcing Output Module

Product Introduction

The Allen-Bradley 1794-OB8 is a compact and reliable digital output module for the Flex I/O system. This unit provides eight non-isolated, sourcing outputs with a nominal voltage of 24V DC. Its straightforward design offers a cost-effective solution for controlling indicator lights, relay coils, solenoids, and other 24V DC field devices. Additionally, the 1794-OB8 supports hot-swapping, which minimizes downtime during maintenance. This module is an ideal choice for smaller-scale control applications.

Technical Specifications

This output module delivers consistent electrical performance for standard automation tasks. Its specifications ensure compatibility with a wide range of 24V DC field devices.

Output Characteristics

• Number of Outputs: The 1794-OB8 provides 8 sourcing outputs in a single, non-isolated group.

• Voltage Range: The on-state voltage range is from 10V DC minimum to 31.2V DC maximum, with a nominal voltage of 24V DC.

• Current Ratings: Each channel supports a maximum on-state current of 500mA. The total output current rating for all 8 channels is 4.0A.

• Voltage Drop: The maximum on-state voltage drop is only 0.5V DC, ensuring efficient power delivery to the load.

• Signal Delay: The module offers a fast off-to-on signal delay of 0.5ms and an on-to-off delay of 1.0ms maximum.

• Off-State Leakage: The maximum off-state leakage current is a low 0.5mA, which prevents unwanted current flow when the output is off.

Power and Isolation

• FlexBus Current: The maximum FlexBus current drawn by the 1794-OB8 is 60mA at 5V DC.

• Power Dissipation: Under maximum load at 31.2V DC, the module dissipates up to 3.3W of power.

• Thermal Dissipation: Its thermal dissipation is rated at 11.2 BTU/hr under maximum load conditions.

• Isolation Voltage: The module provides a continuous isolation voltage of 50V between the field side and the system. However, there is no isolation between the individual output channels.

Fusing Considerations
The 1794-OB8 does not include internal fusing at the factory. Therefore, you should install external fusing for circuit protection if your application requires it. This approach allows you to select the appropriate fuse rating for your specific load conditions.

Mechanical and Environmental Parameters

Understanding the physical and environmental limits is essential for a successful installation. The 1794-OB8 is built to perform reliably in demanding industrial environments.

Physical Dimensions

• Dimensions (HxWxD): The module measures 46mm x 94mm x 53mm (1.8in x 3.7in x 2.1in).

• Weight: This unit weighs approximately 78 grams (2.75 oz).

• Installation: You can install the module by snapping it onto a DIN rail. It is inserted into a compatible terminal base unit.

• Hot Swapping: A key advantage of the Flex I/O system is that you can insert or remove the 1794-OB8 under power. Since the module has no direct wiring, this saves time and eliminates system restart inefficiencies.

Environmental Limits

• Operational Temperature: You can operate this unit in environments from 0 to 55°C (32 to 131°F). For extreme environments, XT-rated versions are also available.

• Storage Temperature: For storage, the acceptable range is -40 to 85°C (-40 to 185°F).

• Humidity: It functions correctly in conditions with 5 to 95% non-condensing relative humidity.

• Hazardous Location Rating: The 1794-OB8 is rated for safe operation in both European and North American hazardous locations, making it suitable for use in potentially explosive atmospheres.

Installation and Compatibility

Proper installation ensures the longevity and reliability of the Allen-Bradley 1794-OB8.

Terminal Base Requirements
This module must be mounted on a compatible terminal base unit. Specifically, it is designed for use with the 1794-TB3, 1794-TB3S, and 1794-TBKD base units. The wiring size and termination method depend on the specific terminal base you select.

Communication Adapters
The Flex I/O platform supports various communication adapters. Therefore, the 1794-OB8 can be used in systems with EtherNet/IP, ControlNet, DeviceNet, and Universal Remote I/O networks. This flexibility allows you to integrate it with Logix controllers like ControlLogix and CompactLogix, as well as legacy controllers such as PLC-5 and SLC 500.

Status Indication
Eight yellow LED indicators on the front panel show the status of each output channel. These indicators are logic-driven and provide immediate visual feedback for troubleshooting.

Packaging List

Your shipment for the Allen-Bradley 1794-OB8 will include the following item:

• (1) Allen-Bradley 1794-OB8 Flex I/O Digital Output Module

Frequently Asked Questions (FAQ)

1. What is the output current rating for each channel on the 1794-OB8?
Each channel on the 1794-OB8 supports a maximum continuous current of 500mA (0.5A). The total output current for all 8 channels is 4.0A.

2. Does the 1794-OB8 include internal fusing?
No, the 1794-OB8 does not have internal fusing. You should install external fuses for circuit protection as needed for your specific application.

3. What terminal bases are compatible with the 1794-OB8?
This module is compatible with the 1794-TB3, 1794-TB3S, and 1794-TBKD terminal base units.

4. Can I remove the 1794-OB8 while the system is powered on?
Yes, you can safely insert or remove the 1794-OB8 under power because it has no direct wiring connections. This feature prevents unnecessary system downtime.

5. What is the output signal delay for the 1794-OB8?
The off-to-on signal delay is 0.5ms maximum, and the on-to-off delay is 1.0ms maximum, ensuring fast and responsive output control.