Allen-Bradley 1746-NR8 RTD Module Input SLC 500

Product Overview

The Allen-Bradley 1746-NR8 is a high-performance RTD (Resistance Temperature Detector) and resistance analog input module for the SLC 500 PLC platform. Equipped with eight independent input channels, this module is engineered for precision temperature monitoring in complex industrial processes. By utilizing advanced Sigma-Delta Modulation for A/D conversion, it achieves an exceptional temperature resolution of 0.01°C. Whether monitoring thermal variations in food processing or managing resistance ranges for heavy machinery, the 1746-NR8 provides the high-density I/O capability required to maintain tight process control.

Technical Configuration

The 1746-NR8 is designed for maximum versatility within an SLC 500 chassis:

• Channel Count: 8 high-accuracy input channels for RTD or direct resistance signals.
• Input Ranges: Supports multiple resistance ranges (0–150 Ω, 0–300 Ω, 0–500 Ω, and 0–1000 Ω).
• Conversion Technology: Sigma-Delta Modulation ensures superior noise rejection and measurement linearity.
• Update Rates: Fully adjustable from 6 ms up to 2 seconds to match application dynamics.
• Slot Compatibility: Installs into any available I/O slot of an SLC 500 chassis (except Slot 0).

Technical Specifications

Installation Guide

To integrate the 1746-NR8 into your SLC 500 system, verify that your power supply has sufficient current capacity for the module's 5V and 24V DC backplane requirements.

Wiring and Shielding

RTD signals are highly sensitive to electrical noise. For optimal precision, use high-quality shielded twisted-pair cable for all sensor connections. Ground the cable shield at the module end only to prevent ground loops. The module supports 2, 3, or 4-wire RTD configurations; however, 3 or 4-wire setups are highly recommended to compensate for lead-wire resistance, which is essential for maintaining the rated 0.01°C resolution. Ensure the module is firmly seated and the chassis lever is locked before applying power.

Engineering Advantages

The 1746-NR8 stands out for its "Precision Performance" profile, offering a step response as fast as 60 ms for dynamic thermal tracking. Despite its advanced circuitry, the module maintains a lightweight profile with a net weight of approximately 0.28 kg, minimizing the physical stress on the backplane connectors. Its wide operating temperature range (0–60°C) makes it resilient in non-conditioned industrial environments. Furthermore, the adjustable update time allows engineers to prioritize either high-speed data acquisition for fast-moving processes or heavy filtering for high-stability measurements in noisy environments.

Technical FAQs

Q1: Can I mix different RTD types on the same module?
A1: Yes, the 1746-NR8 allows you to configure each channel individually through the SLC 500 programming software, supporting various RTD types and resistance ranges simultaneously.

Q2: Why does the module use Sigma-Delta Modulation?
A2: This modulation technique provides excellent high-frequency noise rejection and high resolution, which is critical when dealing with the low-voltage signals produced by RTD sensors.

Q3: Is lead-wire compensation automatic?
A3: The module automatically compensates for lead-wire resistance when using 3-wire or 4-wire sensor configurations, ensuring that only the sensor's resistance is measured.