Introduction to the Allen-Bradley 1746-NR8
When your SLC 500 system needs reliable temperature or resistance data, the Allen-Bradley 1746-NR8 delivers. This RTD/resistance analog input module packs eight channels into a single slot, supporting a wide range of RTD sensors and direct resistance inputs. It is a direct replacement for aging modules and a smart upgrade for expanding your existing control architecture.
Key Technical Specifications
The 1746-NR8 operates on backplane power, drawing 100 mA at 5 VDC and 55 mA at 24 VDC, with a maximum power consumption of 1.82 W. Its sigma-delta A/D conversion method ensures stable, noise-resistant readings. The module accepts Platinum (385) 100, 200, 500, and 1000; Platinum (3816) 100, 200, 500, and 1000; Copper (426) 10; Nickel (618) 120; Nickel (672) 120; and Nickel Iron (518) 604 RTD sensors. Direct resistance ranges include 0–150 Ohms, 0–500 Ohms, 0–1000 Ohms, and 0–300 Ohms. Update times range from 6 milliseconds to 2 seconds, and resolution is 0.01 degrees Celsius. The module supports 2-wire, 3-wire, and 4-wire RTD configurations, and it occupies any slot except Slot 0 in the I/O chassis.
Comparison with Similar Modules
| Feature | 1746-NR8 | Typical 4-channel RTD module |
|---|---|---|
| Channels | 8 | 4 |
| RTD types | Platinum, Copper, Nickel, Nickel Iron | Limited to Platinum/Nickel |
| Resistance input | 0–150, 0–500, 0–1000, 0–300 Ohms | 0–500 Ohms only |
| Update time | 6 ms to 2 s | Fixed 100 ms |
| Backplane current | 100 mA @ 5 VDC, 55 mA @ 24 VDC | 80 mA @ 5 VDC |
Why Choose the 1746-NR8 for Your SLC 500 System?
This module is exclusively for SLC 500 fixed and modular hardware styles. It does not require an external power supply, simplifying wiring and reducing cabinet space. The removable terminal block (1746-RT-35) allows quick maintenance without disturbing field wiring. With auto-calibration at power-up and programmable calibration every 5 minutes, the 1746-NR8 maintains accuracy over long production runs. Its low-pass digital filter and programmable input filters reject electrical noise common in industrial environments.
Applications and Use Cases
Temperature monitoring in food processing, chemical reactors, and HVAC systems benefits from the 1746-NR8’s RTD compatibility. For mechanical applications, the resistance input mode measures displacement or movement by tracking resistance changes in linear position sensors. The module’s 2-wire, 3-wire, and 4-wire support lets you match sensor wiring to your accuracy needs—use 4-wire for critical temperature loops and 2-wire for less demanding monitoring.
Installation and Wiring Best Practices
Install the 1746-NR8 in any slot except Slot 0. For 2-wire RTDs, use Belden 9501 shielded cable or equivalent, and avoid long cable runs to maintain accuracy. If long runs are unavoidable, use a lower gauge wire such as AWG #16 instead of AWG #24 to reduce resistance. For 3-wire or 4-wire RTDs, Belden 9533 or Belden 83503 cable is recommended. The removable terminal block accepts single wires only; do not double up conductors.
Maintenance and Reliability
The 1746-NR8 has no moving parts, so wear is minimal. Periodically inspect terminal connections for tightness and check the LED indicators for channel status and module operation. Keep the module within its operating temperature range of 0 to 60 degrees Celsius and storage range of -40 to 85 degrees Celsius. Non-condensing relative humidity should stay between 5 and 95%. Following these guidelines ensures years of dependable service.
Compatibility and Integration
The 1746-NR8 integrates seamlessly with SLC 500 processors and I/O chassis. It communicates via the backplane, so no special communication protocols are needed. The module works with fixed and modular SLC 500 styles, making it a versatile choice for both new installations and retrofits. For systems using other Allen-Bradley controllers, verify backplane compatibility before ordering.
Selection Guide
Before purchasing, confirm your SLC 500 chassis has an available slot (not Slot 0) and that your power supply can handle the additional 100 mA at 5 VDC and 55 mA at 24 VDC. The 1746-RT-35 terminal block is required and may be ordered separately. SpareTechPLC stocks genuine Allen-Bradley 1746-NR8 modules and can help you verify compatibility with your existing system.
Frequently Asked Questions
Q: Can I use the 1746-NR8 with a 2-wire RTD?
A: Yes, the module supports 2-wire, 3-wire, and 4-wire RTDs. For 2-wire, use Belden 9501 shielded cable or equivalent and keep cable runs short for best accuracy.
Q: What is the update time for all eight channels?
A: Update time ranges from 6 milliseconds to 2 seconds, depending on configuration.
Q: Does the 1746-NR8 require an external power supply?
A: No, it is powered by the SLC 500 backplane.
Q: What terminal block does the 1746-NR8 use?
A: It uses the Allen-Bradley 1746-RT-35 removable terminal block.
Q: Can I use the 1746-NR8 for position measurement?
A: Yes, the resistance input mode (0–150, 0–500, 0–1000, 0–300 Ohms) can measure displacement or movement in mechanical devices.
