Allen-Bradley 1746-OB8 SLC 500 8-Point DC Sourcing Output Module

Description

3. Key Technical Specifications

4. Product Introduction

The Allen-Bradley 1746-OB8 is an 8-point DC sourcing discrete output module for the SLC 500 platform. It switches loads in the 10–50 V DC range and supplies up to 1 A continuous per point at 30 °C (derated to 0.5 A at 60 °C). All eight outputs share a single common and are optically isolated from the backplane.

This module is commonly used for driving solenoids, pilot lights, small relays, and other DC loads in existing SLC 500 installations. Field experience shows reliable performance when surge suppression is applied to inductive loads and total module current stays within the temperature-dependent limits. The unit draws only 135 mA from the 5 V backplane and requires no external 24 V supply for its logic.

1746-OB8

1746-OB8

5. Installation & Configuration Guide

Stage 1: Pre-Installation Preparation (5–8 minutes) ⚠️ Safety First: Notify operations of planned downtime. Place the process in a safe state. Lock out/tag out all power sources to the chassis and field devices. Wait at least 5 minutes for capacitors to discharge. Tools Required: Grounded ESD wrist strap, PH1 screwdriver, digital multimeter, wire labels, smartphone or camera for reference photos. Data Backup: Export the current RSLogix 500 project. Document the module slot number and any existing output addresses. Photograph the terminal block wiring and the old module label (including series letter).

Stage 2: Removing the Old Module (5–7 minutes)

1. Remove the front cover if present.
2. Label every field wire on the removable terminal block, then carefully disconnect them. Do not pull on the conductors.
3. Release the self-locking tabs on the top and bottom of the module and pull it straight out of the chassis. Avoid rocking the module to protect the backplane connectors.
4. Inspect the chassis backplane pins for bent contacts or debris. ⚠️ Keep the old module available until the new unit is verified and running under process conditions.

Stage 3: Installing the New Module (6–10 minutes)

1. Put on the ESD wrist strap and ground it. Confirm the replacement is exactly 1746-OB8.
2. Configuration Note: The 1746-OB8 has no DIP switches or jumpers. Output behavior is controlled solely by the ladder program addressing the output image bits.
3. Seat the new module firmly into the same chassis slot until the locking tabs click.
4. Reinstall the removable terminal block and reconnect the field wiring exactly as photographed. Observe correct polarity: positive supply to the VDC terminal(s), loads between each OUT terminal and DC common. Apply appropriate surge suppression (typically a 1N4004 diode reverse-wired across inductive loads). Self-Checklist: [ ] Wiring polarity correct [ ] Terminal block fully seated [ ] Locking tabs engaged [ ] Total module current within temperature limits

Stage 4: Power-On & Testing (8–12 minutes) Pre-Power Check: With a multimeter, verify no short circuits exist between the field supply and DC common or chassis ground. Power-On Steps:

1. Restore power to the chassis only (field devices still isolated if possible).
2. Observe the module status and individual output LEDs. Each output LED should illuminate when the corresponding output bit is commanded ON.
3. Connect the programming terminal, go online with RSLogix 500, and verify the module appears correctly in the I/O configuration.
4. Force or toggle individual output bits in the program and confirm field devices respond. Measure voltage drop and current on a few points under load.
5. Restore full field power and run a controlled process test. ⚠️ Troubleshooting Note: If an output LED is on but the field device does not energize, check field supply voltage, wiring polarity, and load current. Excessive heat or immediate trip of an external fuse usually indicates a shorted load or insufficient surge suppression. Derate total module current at elevated ambient temperatures to avoid thermal shutdown or shortened life.

6. Frequently Asked Questions (FAQ)

Q: Can the 1746-OB8 be hot-swapped under power? No. Inserting or removing the module with chassis power applied risks damaging the backplane connectors and the module. Always remove power first.

Q: Is the 1746-OB8 obsolete, and is stock genuinely new? Yes, the SLC 500 platform and this module have been discontinued by Rockwell. Available units are typically new surplus or carefully tested new-old-stock. Verify series letter and date code on the label before installation.

Q: What is the recommended direct replacement if 1746-OB8 stock is exhausted? Within the SLC 500 family the 1746-OB16 (16-point) or 1746-OBP8 (higher-current, narrower voltage range) are common alternatives, but wiring and addressing will change. For new designs Rockwell directs users to CompactLogix or ControlLogix DC output modules. Plan a program and terminal revision accordingly.

Q: Will I lose my programming logic when I replace the module? The processor retains the user program. Output addresses remain the same provided the module stays in the same slot. No configuration words need to be rewritten because the 1746-OB8 has no programmable parameters.

Q: Why is the price often lower than original Rockwell list prices? Because the module has been obsolete for years. New surplus and tested stock from reputable channels typically sell below original factory list. Condition, series, and included terminal block affect pricing.

Q: Does this module require external fusing or surge suppression? External fusing per group or per point is strongly recommended for short-circuit protection. For inductive loads (solenoids, contactor coils), install a reverse-biased diode (e.g., 1N4004) across the load. Failure to suppress inductive kickback is a frequent cause of premature output transistor failure.

Q: What are the main current and temperature limitations I must respect? At 30 °C ambient each point is rated 1.0 A and the module total is 8.0 A. At 60 °C these drop to 0.5 A per point and 4.0 A per module. Exceeding these values shortens life or causes thermal limiting. Always calculate total load current before installation.