1756-OB8 Digital DC Output Module | Allen-Bradley

Configured for discrete signal output control in ControlLogix system platforms, the Allen-Bradley 1756-OB8 (1756-OB8 Digital DC Output Module) provides direct physical execution of low-voltage DC switching for external field devices.

Hardware Specifications

Profinet / EtherNet/IP Deterministic Networks

The 1756-OB8 integrates into the ControlLogix backplane to support deterministic I/O data exchanges. The module features two isolated groups of four outputs each, facilitating flexible power distribution for independent load circuits. Firmware-level compatibility ensures precise control over ON and OFF transition delays, measured at 1 ms and 2 ms maximum respectively. The hardware supports I/O density scaling through modular expansion within the chassis, while backplane power consumption is strictly regulated at 165 mA from the 5.1 VDC rail to maintain operational stability under varying load conditions.

Frequently Asked Questions

Q: Does the 1756-OB8 support module-level hot-swapping?

A: Yes, the module supports Removal and Insertion Under Power (RIUP). However, before removing the module from the chassis, ensure all field-side loads are de-energized to prevent arcing at the terminal block contacts.

Q: Are the outputs on the 1756-OB8 channel-to-channel isolated?

A: No, the outputs are grouped. The module features two output groups, with each group containing four points. While group-to-group isolation is provided, individual points within the same group share common electrical potential.

Field Installation Guidelines

1. Verify the ControlLogix chassis is properly grounded to the cabinet common to minimize electromagnetic interference (EMI).
2. Seat the 1756-TBNH or 1756-TBSH Removable Terminal Block (RTB) onto the module connector, ensuring the latch is fully engaged.
3. Terminate field wiring to the RTB; ensure 24 VDC supply leads are connected to the appropriate source terminals to provide power to the output groups.
4. Route signal cabling through segregated wire ducts to prevent crosstalk with high-voltage AC lines.
5. Check that the total module current does not exceed the 8 A maximum rating at 60 deg C, as exceeding this limit will trigger thermal protection or reduce operational lifespan.