Allen-Bradley 1756-OB8I – ControlLogix Isolated DC Output Module with 8 Individually Isolated Outputs

The Allen-Bradley 1756-OB8I is a ControlLogix isolated DC output module. Specifically, this module provides 8 individually isolated outputs. It operates at 12-24 Volts DC with a source output configuration. Furthermore, each output channel is fully isolated from the others. In addition, this module supports pilot duty applications. Consequently, it suits applications requiring high isolation between output channels.

Ordering Configuration and Product Identification

• Manufacturer: Rockwell Automation.

• Brand: Allen-Bradley.

• Part Number: 1756-OB8I.

• Series: ControlLogix.

• Module Type: Isolated DC Output Module.

• Number of Outputs: 8 individually isolated.

• Voltage Rating: 12-24 Volts DC.

• Operating Voltage Range: 10-30 Volts DC.

• Enclosure: None (open-style module).

• Connector Type: 36-pin removable terminal block housing.

• Fusing: Module not fused (use fused IFM for protection).

• Therefore, the Allen-Bradley 1756-OB8I offers complete channel-to-channel isolation for critical applications.

Output Current Ratings

• Maximum Current per Point: 2 Amps at 140°F (60°C).

• Maximum Current per Module: 16 Amps at 140°F (60°C).

• Minimum Load Current: 2 mA per point.

• Surge Current: 4 Amps for 10 ms per point.

• Surge Repeatability: Repeatable every second at 140°F.

• On-State Voltage Drop (Max): 0.5 Volts DC at 2 Amps.

• Off-State Leakage Current (Max): 0.5 mA per point.

• Pilot Duty: Available for inductive loads.

• Derating: Consider temperature for continuous operation.

• Thus, the 1756-OB8I delivers substantial current capacity with individual channel isolation.

Power Dissipation and Backplane Current

• Backplane Current (5.1 Volts): 165 milliamps.

• Backplane Current (24 Volts): 2 milliamps.

• Backplane Power (Total): 0.89 Watts.

• Power Dissipation (Max): 4.6 Watts at 140°F (60°C).

• Thermal Dissipation: 15.7 BTU per hour.

• Isolation Voltage (Continuous): 250 Volts.

• Routine Test: 1350 Volts AC for 2 seconds.

• Chassis Loading: Account for backplane current in system design.

• Consequently, the Allen-Bradley 1756-OB8I provides high isolation with moderate power consumption.

Removable Terminal Blocks (RTB)

• Compatible Terminal Block 1: 1756-TBCH.

• Compatible Terminal Block 2: 1756-TBS6H.

• RTB Type: 36-pin removable terminal block housing.

• Requirement: RTB must be installed before chassis mounting.

• Installation Order: Ground and install chassis first, then install module with RTB.

• Wiring Benefit: Easy module replacement without rewiring.

• Installation Speed: Faster than fixed terminal blocks.

• Field Wiring: Accepts standard copper conductors.

• Maintenance: Simple disconnect and reconnect.

• Therefore, the 1756-OB8I requires compatible RTB for proper installation.

Communication Formats

• Listen-Only Rack Optimization: Supported.

• Listen-Only Output Data: Supported.

• Scheduled Output Data: Supported.

• Output Data: Supported.

• Rack Optimization: Supported.

• Flexibility: Multiple formats for various system architectures.

• Compatibility: Works with standard ControlLogix communication.

• Configuration: Select format during module setup.

• Network Integration: Seamless with EtherNet/IP, ControlNet, DeviceNet.

• Thus, the 1756-OB8I supports versatile communication configurations.

Fault and Program Mode Behavior

• Per-Point Configuration: Yes, individually configurable.

• Configurable States: On, Off, or Hold Last State.

• Default State: Off (factory setting).

• Fault Mode: User-defined output behavior.

• Program Mode: User-defined output behavior.

• Flexibility: Each output can behave independently.

• Safety: Configure fail-safe states for critical loads.

• Application: Customize response to controller faults.

• Consequently, the Allen-Bradley 1756-OB8I provides flexible fault response configuration.

Installation Guide

First, verify that all system power sources are completely disconnected. Then, ground and install your ControlLogix chassis properly. After that, select your removable terminal block (1756-TBCH or 1756-TBS6H). Then, wire your field devices to the RTB terminals before installing the module. Connect the external DC power supply (10-30V DC) to the appropriate terminals. Ensure proper polarity for source outputs. Then, attach the wired RTB to the 1756-OB8I module. Next, slide the module into the chosen ControlLogix chassis slot. Push firmly until the backplane connector is fully seated. Then, apply backplane power by turning on the ControlLogix system. After that, download your output configuration to the module. Configure each output's fault and program mode behavior (On, Off, or Hold Last State). The default state is Off. Then, select the appropriate communication format (Rack Optimization, Scheduled Output Data, etc.). Finally, test each output point using your control program. For inductive loads, ensure proper surge suppression is installed. For output protection, consider using a fused IFM since the module is not fused internally. As a result, the Allen-Bradley 1756-OB8I will provide reliable isolated DC output control.

Packaging List

• One Allen-Bradley 1756-OB8I isolated DC output module

• One product identification label

• One installation and wiring guide

• Removable terminal block (1756-TBCH or 1756-TBS6H, order separately)

Common FAQs

Q1: How many outputs does the Allen-Bradley 1756-OB8I provide?
A: This module provides 8 individually isolated outputs. Specifically, each output channel is fully isolated from the others. Therefore, it suits applications requiring high channel-to-channel isolation.

Q2: What is the maximum current per output on the 1756-OB8I?
A: The maximum current per point is 2 Amps at 140°F (60°C). Surge current rating is 4 Amps for 10 ms per point. Therefore, it drives larger loads than typical 0.5A output modules.

Q3: What terminal blocks are compatible with the Allen-Bradley 1756-OB8I?
A: This module accepts 1756-TBCH or 1756-TBS6H removable terminal blocks. Both options are 36-pin RTB housings. Therefore, install the RTB before mounting the module in the chassis.

Q4: Can I configure fault behavior per output on the 1756-OB8I?
A: Yes, each output can be configured individually for Fault or Program mode. Available states are On, Off, or Hold Last State. The default state is Off. Consequently, fail-safe behavior is highly customizable.

Q5: What is the isolation voltage rating of the Allen-Bradley 1756-OB8I?
A: Continuous isolation voltage is 250 Volts. Routine testing is performed at 1350 Volts AC for 2 seconds. Therefore, it provides robust isolation for demanding applications.