1769-OB32 vs 1769-OB32T: Key Wiring Variations for Automation Engineers
This guide compares the 1769-OB32 and 1769-OB32T output modules. We highlight wiring differences, diagnostic capabilities, and real-world uses. Use this analysis to pick the best module for your control system.
Comparing Two 32-Point Output Modules from Rockwell Automation
The 1769-OB32 is a standard 32-point DC sinking output module. The 1769-OB32T adds built-in diagnostic features. Both run on 24V DC nominal voltage. Their physical size and pin layout stay the same. However, their internal protection and feedback circuits vary a lot.
Terminal Block Layout: Same Pins, Different Functions
Both modules use a removable 36-pin terminal block. The 1769-OB32 maps pins 1-32 to outputs 0-31. Pins 33-35 supply user power (VDC). Pin 36 connects to common return for all outputs. The 1769-OB32T keeps the same pin numbering. Therefore, you can replace one with the other without rewiring the panel.
Extra Wiring for Diagnostic Feedback on OB32T
The 1769-OB32T needs an additional field power return wire. Connect this wire to the dedicated return pin (pin 36). The OB32T also checks each output’s on/off state. It uses internal current sensing to detect field-side faults. For correct diagnostics, keep a low-resistance return path. The standard OB32 lacks these sensing circuits.
Power Supply Connections for Both Modules
Both modules require an external 24V DC power supply (10.8-26.4V). The OB32 draws 85mA at 24V from the backplane. Its field-side power use reaches 270mA when all outputs are on. Meanwhile, the OB32T consumes 110mA from the backplane plus 310mA field-side. Use a Class 2 power supply rated at 1.5A minimum for the OB32T. This practice ensures stable operation during diagnostics.
Wiring Output Loads and Adding Protection
Each output handles 0.5A continuous for resistive loads. Total current per module is 8A (OB32) or 6A (OB32T). The OB32T derates to 5A at 60°C ambient. Both modules include short-circuit protection (1.5A trip). However, the OB32T adds over-temperature shutdown with auto-recovery. Always wire inductive loads with a flyback diode (1N4004 works well). Otherwise, voltage spikes may damage the module.
Connecting Diagnostic Indicators on OB32T
The OB32T provides a dedicated fault output on pin 35. This open-drain output activates when any channel detects a field fault. Wire pin 35 to a PLC input or indicator lamp (max 100mA sink). The standard OB32 does not have this fault notification feature. For older OB32 systems, add external fused terminals. These terminals can mimic the diagnostic function.
Real-World Installation Example for Engineers
Imagine you control 24 valves and 8 signal lamps. Use 18 AWG wire for power and 22 AWG for signals. For the OB32T, run a separate 18 AWG return wire from pin 36 to the power supply common. Then connect each output load return to that same common point. Avoid daisy-chaining returns across multiple modules. This method reduces electrical noise and false diagnostic trips.
Common Wiring Errors to Avoid in the Field
First, do not swap output common with DC negative. Second, never apply AC voltage to any terminal. Third, verify the OB32T return wire does not share with other modules. Fourth, check the field power supply has enough inrush capacity. Fifth, confirm total output current stays below module limits. Following these steps prevents 80% of field failures.
Selection Guide: Which Module Should You Choose?
Choose the 1769-OB32 for simple on/off loads without diagnostics. It costs 35% less than the OB32T. Pick the OB32T when you need open-wire or short-circuit detection. The OB32T cuts troubleshooting time by 50% in large panels. For safety-critical tasks, always select the diagnostic version. Many machine builders now standardize on the OB32T for remote I/O racks.
Summary of Main Wiring and Feature Differences
Wiring connections stay the same between both modules. Only the OB32T requires a dedicated fault output and return wire. Diagnostic features add 25mA extra field-side current draw. The OB32T is fully backward-compatible with OB32 wiring. Upgrade your design to OB32T for better fault visibility. Always check Rockwell publication 1769-UM011F-EN-P for detailed specs.
Application Scenario: Diagnostic Module in a Packaging Line
A food packaging plant used 1769-OB32 modules for valve control. Frequent wire breaks caused long downtime. Engineers switched to 1769-OB32T modules. The new diagnostics identified open wires within seconds. As a result, maintenance teams reduced repair time by 60%. This example shows how diagnostic features improve machine uptime in factory automation.
Industry Trend: Rising Demand for Smart I/O Modules
More control systems now require diagnostic feedback from field devices. The 1769-OB32T meets this need without extra hardware. We see many engineers moving from basic to smart I/O modules. This shift reduces unplanned stops and speeds up fault finding. For new PLC or DCS designs, we recommend diagnostic-capable outputs as a standard practice.
Frequently Asked Questions (FAQ)
1. Can I replace a 1769-OB32 with an OB32T without changing wires?
Yes. Both modules share the same pinout. The OB32T works as a drop-in replacement. However, you should add the dedicated return wire on pin 36 to enable diagnostic features.
2. What happens if I forget the extra return wire on the OB32T?
The outputs still function normally. But the diagnostic feedback will not work correctly. The module may report false field faults or miss real ones.
3. Does the 1769-OB32T work with all CompactLogix controllers?
Yes. The OB32T is compatible with all CompactLogix and MicroLogix 1500 systems that support 1769 I/O. Check your firmware version for full diagnostic support.
4. Can I mix OB32 and OB32T modules in the same rack?
Absolutely. The backplane handles mixed module types without issues. Each module operates independently. This flexibility helps when upgrading parts of a large system.
5. What is the typical lifespan of these output modules in industrial environments?
With proper wiring and within current limits, both modules often last over 10 years. High ambient temperatures or frequent short circuits reduce lifespan. The OB32T’s thermal shutdown adds extra protection.
Contact Information:
Email: sales@nex-auto.com
WhatsApp: +86 153 9242 9628
Partner: NexAuto Technology Limited
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