1746-NT8 Allen-Bradley Thermocouple Input Module

Allen-Bradley 1746-NT8 SLC 500 Thermocouple/mV Input Module

Product Introduction

The Allen-Bradley 1746-NT8 is a high-density Thermocouple/mV Analog Input Module for the SLC 500 control platform. This module provides eight isolated input channels for direct connection to a wide range of thermocouple sensors, including types J, K, T, E, S, R, B, and N. Beyond temperature measurement, the 1746-NT8 also accepts ±50 mV and ±100 mV analog signals, making it suitable for various process monitoring applications.

Its sigma-delta A/D conversion and programmable input filtering deliver stable, accurate readings even in electrically noisy environments. Additionally, the module features two cold-junction compensation (CJC) sensors with an accuracy of ±1.72°C, ensuring precise thermocouple measurements. For convenient diagnostics, the 1746-NT8 includes configurable open-circuit detection with Zero, Upscale, and Downscale settings, plus nine green LED indicators for module and channel status.

Technical Specifications and Performance

The 1746-NT8 offers robust technical capabilities for precise temperature measurement across eight channels. It provides a 16-bit resolution for input conversion and 14-bit resolution for output data, ensuring detailed and accurate readings. The module's update time ranges from 356 milliseconds to 4.05 seconds, allowing you to balance speed and filtering based on your application requirements.

Furthermore, it uses sigma-delta modulation for analog-to-digital conversion, which effectively rejects high-frequency noise. The step response is rated at 60 ms in and 2.5 ms out, meaning the module reacts swiftly to input signal changes. For input filtering, the 1746-NT8 incorporates a programmable digital low-pass filter with selectable frequencies.

Its normal mode rejection between terminals exceeds 100 dB at 50-60 Hz, and its common mode rejection between inputs and ground also exceeds 100 dB at 50-60 Hz. Consequently, this module maintains exceptional signal integrity even in electrically harsh environments. The module also performs automatic calibration at power-up and approximately every 2 minutes thereafter, ensuring consistent measurement accuracy without manual intervention.

Supported Thermocouple Types and mV Inputs

This module supports an extensive range of thermocouple types, providing great flexibility for diverse temperature sensing needs. Specifically, it works with J, K, T, E, S, R, B, and N thermocouples. Each type covers different temperature ranges and accuracy characteristics, so you can select the most suitable sensor for your process.

Beyond thermocouple inputs, the 1746-NT8 accepts direct millivolt signals of ±50 mV and ±100 mV. Therefore, you can use this module not only for temperature but also for measuring other process variables that produce low-level voltage outputs, such as pressure transducers or strain gauges. The input impedance exceeds 10 MΩ, and any input devices connected to this module must have an impedance of less than 100 Ω to meet the specified accuracy level.

Each channel includes open-circuit, under-range, over-range, and channel error detection. You can configure the module's response to an open circuit as Zero (forces the reading to 0), Downscale (forces the reading to the lowest value), or Upscale (forces the reading to the maximum value). These settings help you quickly diagnose channel status and implement fail-safe operations based on your process requirements. Monitoring systems such as HMI and SCADA can access these error alarms through assigned bits for effective process oversight.

Wiring, CJC, and Cable Recommendations

Proper wiring ensures optimal performance from the Allen-Bradley 1746-NT8. For thermocouple inputs, you must use shielded twisted thermocouple extension wire to maintain signal integrity and measurement accuracy. For millivolt inputs, Belden 8761 cable or an equivalent shielded wire works well.

The module uses the 1746-RT34 removable terminal block (RTB) for connecting input devices, which includes two cold-junction compensation (CJC) sensors. These CJC sensors automatically compensate for temperature variations at the terminal block, ensuring accurate thermocouple readings. For best results, position the module away from heat sources and maintain stable ambient temperatures around the terminal block.

Also, ground the cable shield at one end only to prevent ground loops. The 1746-NT8 provides input overvoltage protection at ±30V DC and 600W pulsed for 1ms, safeguarding the module from accidental voltage spikes. Additionally, it offers 12.5V DC circuit isolation tested at 500V DC for 1 minute between inputs and chassis ground, and between inputs and the backplane.

Mechanical, Environmental, and Power Parameters

The 1746-NT8 is engineered for reliable operation in demanding industrial settings. It operates within an ambient temperature range of 0°C to 60°C and can withstand storage between -40°C and 85°C. The module also tolerates a non-condensing relative humidity of 5% to 95%.

For electrical isolation, it provides 12.5V DC circuit isolation tested at 500V DC for 1 minute between inputs and the chassis ground, as well as between inputs and the backplane. The module draws 120 mA from the 5V DC backplane supply and 70 mA from the 24V DC supply. Its total backplane power consumption reaches a maximum of 2.28 W (0.6 W at 5V DC and 1.68 W at 24V DC).

This efficient power design reduces thermal buildup inside the chassis and enhances overall system reliability. The 1746-NT8 has received UL and CUL approvals, confirming its compliance with recognized safety standards. The module's front panel features nine green LED status indicators: one for module health and one for each of the eight channels, enabling quick visual diagnostics.

Installation and Compatibility Guide

Installing the Allen-Bradley 1746-NT8 is simple and straightforward. You can place it in any slot of an SLC 500 chassis, except for Slot 0, which is reserved for the processor or an adapter module. This module is compatible with both fixed and modular SLC 500 hardware styles.

In a fixed system, it typically installs in a 1746-A2 expansion chassis. Meanwhile, in a modular system, it fits into any local or remote I/O chassis. The module occupies a single slot and draws its power directly from the backplane, so no external power supply is required. The 1746-NT8 is compatible with SLC 5/01, 5/02, 5/03, 5/04, and 5/05 processors.

The module offers three data formats for input signals: engineering units, Scaled for PID (Proportional-Integral-Derivative), and proportional counts. Scaled for PID is the most compatible data format for thermocouple and millivolt input signals, making it the preferred choice for most applications. During installation, match the sensor wiring to the correct channel terminals and configure the input type, filter frequency, and open-circuit response settings in your SLC 500 program for optimal performance.

Packaging List

• 1 x Allen-Bradley 1746-NT8 Thermocouple/mV Input Module

• 1 x Allen-Bradley 1746-RT34 Removable Terminal Block (RTB) with CJC sensors

• 1 x Installation Instructions Manual

Shipping

We ship this product worldwide using FedEx, UPS, or DHL to ensure fast and secure delivery.

Frequently Asked Questions (FAQ)

1. Can I install the 1746-NT8 in Slot 0 of my SLC 500 chassis?
No. The 1746-NT8 can occupy any slot except Slot 0. Slot 0 is exclusively reserved for the SLC 500 processor or a communication adapter module such as Remote I/O, DH+, or Ethernet.

2. What thermocouple types does the 1746-NT8 support?
This module supports J, K, T, E, S, R, B, and N thermocouple types. It also accepts ±50 mV and ±100 mV analog input signals.

3. What is the accuracy of the CJC sensors on the 1746-NT8?
The 1746-NT8 includes two cold-junction compensation sensors with an accuracy of ±1.72°C. These sensors automatically compensate for temperature variations at the terminal block, ensuring accurate thermocouple readings.

4. How does the 1746-NT8 respond to an open thermocouple circuit?
You can configure the module's response as Zero (forces reading to 0), Downscale (forces reading to the lowest value), or Upscale (forces reading to the maximum value). Choose the setting that best supports your process fail-safe strategy.

5. Does the 1746-NT8 require an external power supply?
No. The 1746-NT8 draws its operating power directly from the SLC 500 backplane. It consumes 120 mA at 5V DC and 70 mA at 24V DC, with a total power consumption of 2.28 W maximum.