Description
- Model: SCXI-1326
- Part Number: 777687-26
- Brand: National Instruments (NI)
- Series: SCXI (Signal Conditioning eXtensions for Instrumentation)
- Core Function: High-voltage signal termination and routing for switch modules
- Product Type: Terminal Block
- Key Specs: 300 V / 2 A Rating Screw-Terminal Connectivity Integrated Cold-Junction Compensation (CJC)
- Compatibility: Specifically designed for the NI SCXI-1127 and NI SCXI-1128 switch modules.
- Maximum Voltage: 300 Vrms, CAT II
- Maximum Current: 2 A per channel
- Number of Terminals: 48 screw terminals (supporting 32×1, 16×2, or 8×4 configurations depending on the module).
- CJC Sensor: Includes an onboard thermistor for Cold-Junction Compensation, essential for high-accuracy thermocouple measurements.
- Wiring: Accepts wire gauges from 14 to 26 AWG.
- Connection Type: Direct plug-in to the front of the SCXI module.
- Safety: Includes high-voltage safety interlocks that prevent the switch module from operating if the terminal block is removed.
NI SCXI-1326
NI SCXI-1326
NI SCXI-1326
Application Scenarios & Pain Points
The NI SCXI-1326 is the physical interface for National Instruments’ high-voltage switching modules. While the switch module (like the 1127) does the mechanical relay work, the 1326 provides the secure, shielded environment for your field wiring.
A common pain point with the SCXI-1326 is thermistor failure or drift. Because it houses the CJC sensor, if that small thermistor becomes damaged or poorly calibrated, all thermocouple readings through the module will be offset by several degrees. Another frequent issue is arcing in high-humidity environments; if the terminal block is not kept clean and dry, 300V signals can jump between the tight spacing of the screw terminals, permanently damaging the terminal block’s PCB.
Typical Application Scenarios:
- Automated Test Equipment (ATE) Switching high-voltage signals between multiple units under test (UUT) in production lines.
- Large-Scale Temperature Monitoring Using the onboard CJC to route dozens of thermocouples into a single DAQ system for kiln or reactor monitoring.
- Power Supply Testing Scanning through various high-voltage output rails during burn-in testing.
- Battery Stack Measurement Switching between individual cells in high-voltage EV battery packs.
Case Study: The “Safety Relay” Lockout
Background: A technician at an aerospace facility reported that their SCXI-1127 switch module had “died.” It wasn’t responding to LabVIEW commands, and the “Active” LED wouldn’t light up, even though NI-MAX said the module was present.
Problem: We inspected the hardware and found that the module was fine. The issue was the SCXI-1326 terminal block. One of the safety interlock pins on the back of the terminal block was bent. The SCXI system is designed to “lock out” high-voltage switches if it doesn’t detect a properly seated terminal block to prevent the risk of electric shock.
Solution: Straightened the bent pin and reseated the SCXI-1326.
Result:
- Immediate Recovery: The switch module initialized instantly once the interlock circuit was completed.
- Safety Maintained: The hardware protection worked exactly as intended.
- Engineer’s Insight: “If your switch module is ‘unresponsive,’ check your terminal block seating first. The SCXI-1326 isn’t just a plastic box; it’s a safety component that talks to the module’s backplane.”
Compatible Replacement Models
The SCXI-1326 is specialized for high-voltage switching.
| Original Model | Replacement Model | Compatibility | Note |
|---|---|---|---|
| SCXI-1326 | 777687-26 | ✅ Exact Match | Standard HV terminal block. |
| SCXI-1326 | SCXI-1331 | ❌ Incompatible | 1331 is for the 1129 Matrix; different connector. |
| SCXI-1326 | SCXI-1320 | ❌ Incompatible | 1320 is for analog input modules (1100/1102). |
Troubleshooting Quick Reference
| Symptom | Possible Cause | Relation | Quick Check | Action |
|---|---|---|---|---|
| Switch Won’t Close | Interlock Not Set | ✅ High | Ensure the 1326 is fully screwed into the module. | Reseat and tighten thumb screws. |
| Inaccurate Temp Data | CJC Fail | ✅ High | Check if the CJC temperature matches ambient. | Replace thermistor or the 1326 block. |
| Cross-Talk / Noise | Dirty Terminals | ⚠️ Med | Inspect terminals for carbon tracking or dust. | Clean with isopropyl alcohol. |
| Intermittent Signal | Loose Wire | ✅ High | Tug on individual wires to check for loose screws. | Re-terminate and tighten to spec. |
❗ Pro Tip: The “HV” Clearance
When wiring the SCXI-1326 for 300V applications, ensure you strip only enough insulation to secure the wire in the terminal. Excess bare wire outside the screw terminal significantly increases the risk of high-voltage arcing, especially in systems prone to vibration.
Handling & Installation:
- Safety: Always disconnect high-voltage sources before opening the 1326 cover.
- CJC Placement: If using thermocouples, ensure the wires are in close thermal contact with the onboard thermistor for the most accurate compensation.
- Strain Relief: Use the internal cable tie-down points. Heavy bundles of wire can pull on the terminals and eventually cause the PCB traces inside the 1326 to crack.
