Allen Bradley 1756-OB8 8 Point Digital Output Card

Allen Bradley 1756-OB8 8 Point Digital Output Card is an 8-point digital DC output module in the ControlLogix (Bulletin 1756) family, used to switch 10–30V DC field devices such as solenoids, relays, indicators, and contactors from a ControlLogix controller. Each of the 8 outputs delivers up to 2 A and connects through a 20-pin removable terminal block. The module mounts in a 1756 ControlLogix chassis and can operate in local or remote racks linked over ControlNet or EtherNet/IP networks, making it a common choice for discrete control, machine automation, and process applications.

Product Datasheet

• Brand: Allen-Bradley
• Model: 1756-OB8
• Series: ControlLogix
• Product Name: Digital DC Output Module
• Outputs: 8 points, 10–30V DC, 2 A per point
• Connector: 20-pin removable terminal block
• Mounting: 1756 ControlLogix chassis; local or remote racks over ControlNet / EtherNet/IP

Fault Troubleshooting and Common Questions

1. The module does not communicate with the controller. Check module seating in the chassis slot, backplane connections, system power, and the controller’s I/O configuration. Confirm the module is recognized in Studio 5000 Logix Designer and that the hardware revision is supported by the installed firmware.
2. Outputs do not switch on or off correctly. Inspect field wiring and terminal connections, verify the 10–30V DC load supply, and confirm the output channel is enabled and configured correctly in the controller program. Follow approved lockout/tagout procedures before working on output circuits.
3. Outputs appear active but field devices do not respond. Check the connected device, its supply voltage, and the 2 A per-point current rating. Verify common/return wiring and that the total load does not exceed the module’s capacity.
4. Can the 1756-OB8 replace another ControlLogix output module? Not without verification. Compare the full part number, point count, voltage range, current rating, connector layout, hardware revision, and firmware compatibility with the existing ControlLogix configuration.

Safety Precautions

• Isolate power and follow lockout/tagout procedures before removing or installing the module.
• Use electrostatic discharge protection when handling electronic assemblies.
• Verify terminal assignments and field wiring before energizing the system.
• Do not connect field signals until the electrical ratings have been confirmed.
• Use qualified personnel for troubleshooting and commissioning in operating control systems.

Frequently Asked Questions

Q1: What is the Allen Bradley 1756-OB8? The 1756-OB8 is an 8-point, 10–30V DC digital output module in the Allen-Bradley ControlLogix family, used to control discrete DC loads from a ControlLogix controller.

Q2: How many outputs does the 1756-OB8 have? It has 8 output points, each rated up to 2 A at 10–30V DC, wired through a 20-pin removable terminal block.

Q3: Where is the 1756-OB8 installed? It installs in a 1756 ControlLogix chassis and can be used in the local rack or in remote racks connected over ControlNet or EtherNet/IP.

Q4: What should be checked before purchase? Verify the model label, full part number, hardware revision, condition, connector configuration, and compatibility with the existing ControlLogix installation.