1756-OB8EI | Allen-Bradley | ControlLogix Digital Output Module

Configured for high-current digital output control in ControlLogix platforms, the Allen-Bradley 1756-OB8EI (1756-OB8EI Digital DC Sourcing Output Module) provides direct electrical switching through eight electronically-fused, individually isolated channels.

Suffix Breakdown & Model Matrix

The 1756-OB8EI module belongs to the ControlLogix I/O series, specifically engineered for sourcing output applications. The model configuration supports integration with 1756-TBCH or 1756-TBS6H removable terminal blocks (RTBs), which facilitate field wiring termination while allowing for module replacement without disturbing existing wiring harness connections.

Hardware Specifications

Firmware and Backplane Communication

The 1756-OB8EI utilizes the ControlLogix backplane bus communication to execute state changes with deterministic latency. The module is compatible with standard controller firmware revisions that support ControlLogix discrete I/O integration. Firmware flash compatibility is managed via the ControlFLASH utility, ensuring that the module maintains synchronized timing with the primary controller scan cycle. Users must verify that the backplane power budget accounts for the 250 mA current consumption at 5.1 VDC and the 2 mA at 24 VDC to ensure system stability during peak I/O loading.

Frequently Asked Questions

Q: Can this module be hot-swapped while the backplane is powered?

A: Yes, the 1756-OB8EI supports Removal and Insertion Under Power (RIUP). However, caution is required regarding the field-side 24 VDC power, as disconnecting the RTB will de-energize all connected field actuators immediately.

Q: How does the electronic fusing protect the module?

A: Each channel features integrated electronic overcurrent protection. Upon detecting a current exceeding the operational threshold, the channel transitions to a high-impedance state to prevent internal damage. The module status must be cleared via the software interface after the fault condition is addressed.

Field Installation Guidelines

• Terminal Block Attachment: Secure the 1756-TBCH or 1756-TBS6H terminal block to the module housing. Ensure the locking latch is fully engaged to prevent intermittent connection issues caused by cabinet vibration.
• Wiring and Isolation: Given that the 1756-OB8EI provides channel-to-channel isolation, ensure that field wiring is routed to prevent crosstalk between high-power actuator lines and sensitive control signals. Shielded cabling is recommended for long-distance runs to minimize inductive interference.
• Derating Requirements: When operating in high-temperature environments, strictly adhere to the linear derating curve. Exceeding the 10 A module-wide current limit at 60 deg C will lead to accelerated component degradation or thermal shutdown of the internal electronic fuses.
• Grounding: Verify that the chassis power supply is properly earth-grounded. The module common must be referenced correctly to the field power source to ensure proper DC sourcing functionality.