Allen-Bradley 1746-NT8 SLC 500 Thermocouple Module

Description

Key Technical Specifications

Rockwell documentation specifies eight channels, 120 mA at 5 V DC, 70 mA at 24 V DC, sigma-delta conversion, programmable filtering, and the stated isolation characteristics. The supported thermocouple and mV ranges are also documented in SLC 500 technical literature.

Product Introduction

The Allen-Bradley 1746-NT8 is an eight-channel thermocouple/mV analog input module for SLC 500 control systems. It accepts B, E, J, K, N, R, S, and T thermocouples plus ±50 mV and ±100 mV signals, with sigma-delta conversion and programmable digital filtering.

The module provides automatic calibration and input isolation suited to temperature monitoring and low-level analog measurement. Because Rockwell Automation discontinued the product in September 2024, replacement projects should verify chassis compatibility, signal configuration, application software, and the proposed engineering replacement before procurement.

1746-NT8

1746-NT8

Installation & Configuration Guide

Stage 1 — Pre-Installation and Safety

1. Confirm the catalog number: Verify the module label reads 1746-NT8 before installation.
2. Check the SLC chassis: The module can occupy an I/O slot, but Slot 0 is reserved for the processor.
3. Record the existing configuration: Back up the SLC processor program and document the module slot, channel assignments, thermocouple types, scaling, filtering, and alarm limits.
4. Verify power capacity: Allow for approximately 120 mA at 5 V DC and 70 mA at 24 V DC from the chassis backplane.
5. Estimated time: 15–30 minutes for an existing, documented installation.

WARNING: Do not treat the 1746-NT8 as a generic analog input card. Its channel configuration, thermocouple type, scaling, and input-image addressing must match the SLC program.

Stage 2 — Removal of Existing Module

1. Place the controlled process in a safe state.
2. Follow the plant’s electrical isolation/LOTO procedure.
3. Record all terminal wiring and channel assignments before disconnecting the removable terminal block.
4. Disconnect the terminal block and secure field wiring to prevent accidental shorts.
5. Release the module from the chassis and remove it without stressing the backplane connector.
6. Inspect the replacement module for damaged terminals, cracked housing, corrosion, or contamination.
7. Estimated time: 15–30 minutes.

WARNING: Thermocouple polarity matters. Reversing a thermocouple pair can produce incorrect temperature readings even when the channel appears electrically healthy.

Stage 3 — Installation and Configuration

1. Insert the 1746-NT8 into the assigned SLC 500 I/O slot.
2. Secure the module according to the SLC 500 chassis installation procedure.
3. Reconnect the terminal block using the documented channel wiring.
4. Confirm that each channel uses the intended thermocouple type or mV range.
5. Restore the SLC program and verify the module configuration words associated with the selected slot.
6. Confirm scaling and engineering-unit conversion in the ladder logic.
7. Check input filtering settings against the process response requirements.
8. Estimated time: 30–60 minutes, excluding program modifications.

The module uses configuration data through the SLC I/O architecture, so simply inserting the hardware does not guarantee that every channel will use the intended measurement type.

Stage 4 — Power-Up and Testing

1. Restore power according to the plant commissioning procedure.
2. Confirm the module status indication is normal.
3. Verify each connected thermocouple channel independently.
4. Compare PLC temperature values against a calibrated reference instrument where practical.
5. Check for open-circuit indications and unexpected out-of-range values.
6. Confirm HMI values, alarms, interlocks, and historian values.
7. Monitor the process through at least one representative operating cycle.
8. Estimated time: 30–90 minutes depending on the number of channels and process criticality.

Commissioning note: Do not validate only the raw input value. Verify the complete signal path: sensor → 1746-NT8 → SLC input data → scaling → HMI/SCADA → alarm/interlock logic.

Frequently Asked Questions (FAQ)

1. What is the Allen-Bradley 1746-NT8 used for?

The 1746-NT8 is an 8-channel thermocouple/mV analog input module for Allen-Bradley SLC 500 systems. It is primarily used to acquire temperature signals from thermocouples and other low-level millivolt sources.

2. Which thermocouple types does the 1746-NT8 support?

The documented supported types are B, E, J, K, N, R, S, and T. The module also accepts ±50 mV and ±100 mV analog signals.

3. Is the 1746-NT8 still manufactured by Rockwell Automation?

No. Rockwell Automation lists the 1746-NT8 as discontinued effective September 30, 2024. Rockwell identifies the replacement category as Engineering Replacement, so buyers should not assume that a newer module can be installed without engineering changes.

4. Can I replace a 1746-NT8 with a 1746-NT4?

Not as a direct substitution. The NT4 has four channels while the NT8 has eight. Even where configuration concepts are similar, the channel count, I/O addressing, and application requirements must be reviewed before substitution. Rockwell lists the NT4 and NT8 as separate SLC 500 modules.

5. Does the 1746-NT8 require external signal conditioning for thermocouples?

For its supported thermocouple types, the module is designed to accept the thermocouple signals directly. It incorporates thermocouple measurement circuitry and automatic calibration.

6. What should I verify when purchasing a surplus 1746-NT8?

Verify the exact catalog number, series/revision, terminal block condition, connector condition, module labeling, and physical signs of corrosion or repair. For a production-critical application, request a functional test covering all eight channels rather than relying only on a power-on test.

7. Is the 1746-NT8 suitable for a new control-system design?

Generally, it is a legacy-system replacement part, not a preferred choice for a new architecture, because the SLC 500 platform and this module are discontinued. For an existing SLC installation, however, a tested 1746-NT8 can be appropriate when maintaining the installed architecture is preferable to a larger migration project. Rockwell continues to provide SLC 500 technical documentation and migration resources.