Description
Product Introduction
The Allen-Bradley 1746-NT8 is an 8-channel high-density thermocouple input module engineered for the SLC 500 PLC platform. Designed to convert low-voltage millivolt signals directly from temperature sensors without requiring external transmitters, this card slots directly into any standard 1746 I/O rack slot.
Unlike basic 4-channel analog inputs, the 1746-NT8 utilizes on-board Cold Junction Compensation (CJC) sensors integrated directly into the removable terminal block. This provides low-drift temperature measurement across harsh industrial environments while offering configurable filtering to eliminate 50Hz/60Hz line noise interference on long sensor wire runs.
Key Technical Specifications
| Parameter | Value |
| Manufacturer | Allen-Bradley / Rockwell Automation |
| Catalog Number | 1746-NT8 |
| Module Type | Thermocouple / Millivolt Input Module |
| Number of Channels | 8 Individually Configurable Channels |
| Thermocouple Types | J, K, T, E, R, S, B, N |
| Millivolt Input Ranges | ±50 mV DC, ±100 mV DC |
| Backplane Current (5V DC) | 85 mA |
| Backplane Current (24V DC) | 50 mA |
| Resolution | 16-bit (0.1°C or 0.1°F per LSB) |
| Isolation Voltage | 500V DC channel-to-backplane isolation |
Application Scenarios & The “Trench” Experience
When an analog temperature input card dies on a heat-treat furnace or plastic extrusion line running an older SLC 504 or 505 processor, production halts instantly. Burners lock out, zones drop offline, and thermal runaway alarms start blaring across the factory floor. Replacing a failed 1746-NT8 gets your thermal loop control back online fast without having to rewrite RSLogix 500 ladder logic or rip out the whole rack to upgrade to CompactLogix.
- Industrial Heat Treat Furnaces: Multi-Zone Temperature Control – Reads multiple type-K thermocouples across furnace zones to drive precise PID heating loops.
- Plastics Injection Molding: Barrel & Die Temperature Monitoring – Collects real-time thermal telemetry from high-density heater bands.
- Chemical & Petrochemical Processing: Reactor Vessel Thermal Logging – Tracks process fluid temperatures across multiple vessel jacket zones.
Field Case Study: A metal forging plant lost channel 3 through channel 6 on their 1746-NT8 card during a severe power surge, causing a furnace temperature controller fault right before a major production run. Upgrading the entire PLC system was quoted at three weeks of lead time and $40,000 in downtime. By pulling a fully tested 1746-NT8 module from tested surplus stock, the site technician swapped the card, transferred the removable terminal block, and re-energized the furnace in under 30 minutes.
1746-NT8
1746-NT8
Transparency SOP: Quality Assurance & Testing
We don’t just ship boxes; we test them on actual Allen-Bradley 1746 backplanes. Every 1746-NT8 card passes our strict 5-step validation before leaving the warehouse:
- Inbound Inspection: Visual and optical check for damaged terminal pins, cracked casing plastics, thermal stress marks, or altered jumpers.
- Backplane Power-On Self-Test: Insertion into an 1746-A7 chassis with an SLC 5/05 CPU to confirm backplane communications and verify module LED status initialization.
- Millivolt Calibration Test: Precision DC mV signal generator injection across all 8 channels to check A/D conversion accuracy and reading drift.
- CJC Sensor Verification: Verification of the removable terminal block’s onboard Cold Junction Compensation thermistors under thermal chamber conditions.
- ESD Safe Cleaning & Packaging: Solvent cleaning of backplane edge connectors, followed by sealing in anti-static ESD bags with desiccant packaging.
The Veteran’s Tech Trap Guide
- ⚠️ Don’t Mix Up Terminal Blocks: The 1746-NT8 uses a specific removable terminal block with built-in CJC thermistors. If you accidentally swap it with a standard 1746-16-point discrete terminal block from another module, your temperature readings will drift wildly out of calibration!
- ❗ Configure Channel DIP Switches / Software Words First: Before dropping the card into your rack, double-check your input type configuration words in your RSLogix 500 data table setup (
M0andM1files). A mismatch between ladder logic configuration data and sensor wire type (e.g., set for Type J but wired for Type K) will give you wildly wrong readings. - ⚠️ Use Thermocouple Extension Wire Only: Never connect standard copper wire between your junction box and the 1746-NT8 terminal block. Using plain copper wire creates uncompensated cold junctions at the terminal strip, ruining system measurement accuracy.
Frequently Asked Questions
Q: Can I replace a 1746-NT4 card with a 1746-NT8 card?
A: Yes, but keep in mind the 1746-NT8 provides 8 channels instead of 4, and the memory mapping/I/O configuration word layout in RSLogix 500 differs between the two cards. You will need to modify your program data files to handle the 8-channel addressing.
Q: Is the 1746-NT8 card hot-swappable?
A: No. SLC 500 chassis backplanes do not support RIUP (Removal and Insertion Under Power). You MUST power down the 1746 rack power supply before pulling or inserting the card to prevent damaging the backplane bus.
Q: Does the 1746-NT8 require an external 24V DC power supply?
A: No external power wire runs to the card terminals. The card receives its operating logic power directly from the 1746 rack backplane power supply (drawing 85mA at 5V DC and 50mA at 24V DC).
Q: Are these units original Allen-Bradley parts?
A: Yes, our inventory consists of authentic Allen-Bradley hardware sourced from factory surplus reserves, plant upgrades, and backup inventory. Every unit undergoes live chassis testing before shipping.
Q: What warranty coverage is included?
A: Every 1746-NT8 module comes backed by our 12-month full replacement warranty. If a module fails under normal operating conditions within one year, we send a replacement right away.
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