Allen-Bradley 1756-OB8 ControlLogix Digital Output Module

1756-OB8 8-Channel DC Output Module by Rockwell Automation

Product Introduction

The 1756‑OB8 from Allen‑Bradley is a versatile digital output module designed for the ControlLogix controller system. It provides eight discrete output channels arranged in two groups of four channels each. Each channel delivers a nominal 24 VDC output with a wide operating range of 10 to 30 VDC. This module is ideal for energizing DC loads such as relays, solenoids, and indicator lamps. It can also control AC loads when used with interposing relays. The 1756‑OB8 occupies a single slot and works seamlessly in local or distributed I/O architectures.

Key Technical Specifications

The 1756‑OB8 offers reliable and high‑current digital outputs. Each channel supports up to 2 amperes at 60 °C, with a total module current of 8 amperes at the same temperature. The module features a fast signal delay of 1 ms maximum for ON transitions and 2 ms maximum for OFF transitions, ensuring responsive control in time‑critical applications.

• Number of Outputs: 8 (two groups of 4 channels)

• Output Voltage Range: 10–30 VDC (nominal 24 VDC)

• Current per Point: 2 A @ 60 °C

• Current per Module: 8 A @ 60 °C

• Signal Delay (ON): 1 ms max

• Signal Delay (OFF): 2 ms max

• Surge Current: 4 A for 10 ms, repeatable every 1 s at 60 °C

• Minimum Load Current: 2 mA per point

• Off‑State Leakage: 1 mA max per point

• On‑State Voltage Drop: 1.2 VDC max at 2 A

For power consumption, the 1756‑OB8 draws 165 mA at 5.1 VDC and only 2 mA at 24 VDC from the backplane. Its total backplane power is 0.89 W, with a maximum power dissipation of 2.5 W at 60 °C. Thermal dissipation is rated at 8.53 BTU/hr, making it energy‑efficient for dense control cabinets.

Isolation and Environmental Ratings

The 1756‑OB8 provides 250V continuous isolation between output channels and the backplane, ensuring reliable operation in electrically noisy environments. It operates within a temperature range of 0 to 60 °C and can withstand storage temperatures from ‑40 to 85 °C. The module also functions in 5–95% non‑condensing relative humidity. Its open‑style enclosure is suitable for panel mounting inside a protected cabinet.

Configuration and Fault Management

The 1756‑OB8 offers flexible configuration options through the Logix programming environment. You can set fault‑mode and program‑mode states per point, with choices of Hold Last State, On, or Off. The default setting is Off, which enhances safety during unexpected events. Additionally, the module supports multiple communication formats, including rack optimization, listen‑only, output data, and scheduled output data.

Installation and Mounting Guidelines

Installing the 1756‑OB8 is simple and follows standard ControlLogix procedures. The module fits into any available slot in a ControlLogix chassis, except the leftmost slot reserved for the processor or communication adapter.

• Step 1: Power down the chassis before installation.

• Step 2: Align the 1756‑OB8 with the card guides in your chosen slot.

• Step 3: Slide the module firmly into the backplane until the locking mechanism engages.

• Step 4: Attach a compatible removable terminal block (RTB). The 1756‑OB8 works with 1756‑TBNH (screw) and 1756‑TBSH (spring) RTBs.

• Step 5: Wire your output devices to the RTB according to the terminal labeling.

• Step 6: Power on the chassis and configure the module via your Logix software, setting parameters like RPI and fault states as needed.

The module supports hot‑swapping (insertion under power) when the system allows, reducing maintenance downtime.

Packaging List

Your shipment of the 1756‑OB8 includes the following items:

• One (1) Allen‑Bradley 1756‑OB8 ControlLogix digital output module

• Removable terminal block (RTB) – choose 1756‑TBNH or 1756‑TBSH at order

• Quick‑start installation guide

• Anti‑static protective packaging

Note: This package does not include calibration certificates, warranty documents, or software media.

Shipping and Delivery

We ship the 1756‑OB8 worldwide using trusted express carriers: FedEx, UPS, and DHL. Orders are processed within 24 business hours, and tracking numbers are provided promptly upon dispatch.

Frequently Asked Questions (FAQ)

1. What is the maximum current per point for the 1756‑OB8?
Each output point on the 1756‑OB8 can handle up to 2 amperes at 60 °C. The total module current is limited to 8 amperes at the same temperature.

2. Can the 1756‑OB8 control AC loads?
Yes, but you must use an interposing relay between the 1756‑OB8 output and the AC load. The module itself provides DC outputs only.

3. Which removable terminal blocks work with the 1756‑OB8?
The 1756‑OB8 is compatible with 1756‑TBNH (screw‑type) and 1756‑TBSH (spring‑clamp) 20‑pin terminal blocks.

4. How do I set the fault state for each output?
You can configure fault and program states per point using the Logix software. Options include Hold Last State, On, or Off, with Off as the default.

5. Is the 1756‑OB8 hot‑swappable?
Yes, the 1756‑OB8 supports insertion under power (hot‑swapping) when used in a compatible ControlLogix system, provided the chassis and backplane support this feature.