Introduction: When the Plant Floor Changed
In the 1990s, plant engineers faced a quiet revolution. Thermocouples and millivolt sensors moved from standalone panel meters to modular control racks. The Allen-Bradley SLC 500 platform answered that shift. The 1746-NT4/B became a workhorse for temperature and low-level signal acquisition. It is a 4-channel thermocouple/millivolt analog input module. This module accepts J, K, T, E, R, S, B and N thermocouples plus +/-50 mV and +/-100 mV signals. Built-in cold-junction compensation (CJC) removes the need for external reference junctions. That design simplified wiring and improved measurement consistency. Today, the 1746-NT4/B is discontinued. SpareTechPLC helps you navigate replacement and migration options. This article explains what the module does, how it fits your system, and what to do next.
What Is the 1746-NT4/B?
The 1746-NT4/B is a thermocouple/millivolt input module for the SLC 500 / Bulletin 1746 series. It provides four input channels plus CJC. A 16-bit converter digitizes low-level signals with high resolution. The module draws 60 mA at 5V and 40 mA at 24V from the backplane. It mounts in a 1746 chassis such as the 1746-A4, -A7, -A10, or -A13. Those chassis offer 4, 7, 10, or 13 slots and are panel-mount only. The 1746 chassis backplane supplies 5.1V DC at 10A and 24V DC at 2.88A to all modules. Field-side load power requires an external supply. Removable terminal blocks simplify field wiring. Bulletin 1492 pre-wired interface cables are available for faster installation.
Key Specifications That Matter
Input Flexibility
Four channels support eight thermocouple types plus two millivolt ranges. This flexibility lets one module handle multiple sensor types across a process area. You reduce spare part counts and simplify panel design.
16-Bit Resolution
A 16-bit converter provides fine resolution for low-level signals. That matters when you measure small millivolt changes from thermocouples. Better resolution means tighter process control and fewer false alarms.
Power Draw
The module draws 60 mA at 5V and 40 mA at 24V. These values help you calculate backplane loading. They also confirm compatibility with the 1746 chassis power budget.
Cold-Junction Compensation
Built-in CJC removes the need for external reference junctions. That saves wiring time and reduces error sources in temperature measurement.
Installation and Setup
The 1746-NT4/B is not hot-swappable. Do not install, remove, or wire this module with power applied. Always follow lockout/tagout procedures. Slot 0 is reserved for the processor or a 1747-ASB adapter. Install the module in any other available slot. Use removable terminal blocks for field wiring. For faster commissioning, consider Bulletin 1492 pre-wired interface cables. The chassis is panel-mount only. Ensure adequate clearance for airflow. The family operating temperature range is 0 to 60 C. Storage range is -40 to 85 C. Humidity should stay between 5 and 95 percent, non-condensing.
Compatibility and Integration
The 1746-NT4/B works within the SLC 500 platform. It communicates through the 1746 backplane. The backplane supplies 5.1V DC at 10A and 24V DC at 2.88A. Field-side load power needs an external supply. The module fits 1746-A4, -A7, -A10, and -A13 chassis. These chassis provide 4, 7, 10, or 13 slots. All are panel-mount. If you use a 1747-ASB adapter, keep slot 0 reserved for it. The module is not hot-swappable. Plan your installation during scheduled downtime.
Maintenance and Reliability
Thermocouple modules have no moving parts. Wear points are minimal. Focus on environmental factors. Keep the panel within 0 to 60 C. Avoid condensation. Check terminal connections periodically for corrosion or looseness. Verify CJC performance during scheduled calibrations. Because the 1746-NT4/B is discontinued, spare units may become scarce. Stocking a spare module reduces downtime risk. Contact SpareTechPLC to check current availability.
Replacement and Upgrade Path
Rockwell Automation lists the Bulletin 1746 I/O as discontinued. The recommended migration path is Compact 5000 I/O or ControlLogix analog, such as the 1756-IF8. That move modernizes your control system. It also improves diagnostics and integration with current platforms. Before migrating, document your existing wiring and channel assignments. Verify that your new platform supports the thermocouple types and millivolt ranges you use. Plan for new terminal blocks and cabling. If you need a direct replacement, verify current lifecycle status and availability with SpareTechPLC before ordering.
Selection Guide: What to Verify Before Buying
Before purchasing a 1746-NT4/B, confirm the following:
- Your chassis model and available slots (avoid slot 0 if using a processor or 1747-ASB adapter).
- Your sensor types (J, K, T, E, R, S, B, N) and signal ranges (+/-50 mV, +/-100 mV).
- Backplane power budget (60 mA at 5V, 40 mA at 24V).
- External field-side load power supply requirements.
- Environmental conditions (0 to 60 C operating, 5-95% RH non-condensing).
SpareTechPLC is a reliable supplier for discontinued Allen-Bradley parts. Contact them to confirm stock and replacement options.
Frequently Asked Questions
Is the 1746-NT4/B still available?
The 1746-NT4/B is discontinued. Availability depends on remaining stock. Contact SpareTechPLC to verify current inventory.
What thermocouple types does the 1746-NT4/B support?
It supports J, K, T, E, R, S, B, and N thermocouples. It also accepts +/-50 mV and +/-100 mV signals.
Can I hot-swap the 1746-NT4/B?
No. Do not install, remove, or wire this module with power applied. Always follow lockout/tagout procedures.
What is the replacement for the 1746-NT4/B?
Rockwell recommends migrating to Compact 5000 I/O or ControlLogix analog, such as the 1756-IF8.
How many channels does the 1746-NT4/B have?
It has four thermocouple/millivolt input channels plus cold-junction compensation (CJC).
