Key Technical Specifications

• Product Model: SCXI-1127
• Manufacturer: National Instruments
• Relay Type: Armature Relay
• Channel Configurations: 1×32, 2×16, 4×8, 1×64 (1-wire)
• Max Switching Voltage: 300 Vrms or 300 VDC
• Max Switching Current: 1 A at 30 VDC, 200 mA at 250 Vrms
• Contact Resistance: 100 mΩ (Initial)
• Contact Material: Gold-clad silver alloy
• Thermal EMF: <3 µV (Differential)
• Bandwidth: 10 MHz (Worst-case channel, 50 Ω termination)

 

Product Introduction

I’ve wired up enough SCXI-1127s to know that when you need to switch 300V signals with low thermal EMF, there aren’t many modules that handle it as cleanly. This is a high-density armature relay multiplexer designed to sit in an SCXI chassis and route signals directly to a DMM like the PCI-4070. It’s a workhorse for thermocouple, RTD, and general high-voltage scanning tasks.Engineers stick with this module because of its versatility. You can configure it via software as a 1×32, 2×16, or even a 1×64 single-wire mux. The gold-clad silver alloy contacts give you that critical low thermal EMF (<3 µV) and a solid 100 MHz bandwidth when terminated properly. Just be careful with the 1-wire mode; it uses internal relays like HLSELECT and 1WIRE that can confuse people if they try to control them manually without the right NI-SWITCH driver version.

Quality SOP & Tech Pitfalls

We don’t just ship these blind. Every SCXI-1127 goes through a strict SOP before it leaves the lab. We start with a visual inspection under magnification to check for burnt relays or lifted traces. Then, it goes on a live SCXI test chassis where we run an NI-SWITCH self-test and verify channel-to-channel isolation using a Fluke 115. We check the contact resistance and ensure the HVAB backplane adapter is seated correctly before sealing it in anti-static packaging.Here is where people mess up: The scanning trigger modes. I’ve seen guys wire up a DMM and an SCXI-1127, set it to synchronous scanning, and wonder why it skips channels. They forgot to configure the DMM’s Measurement Complete (MC) signal to output externally, or they didn’t wire the AUX trigger cable correctly. Also, if you’re expanding the multiplexer to 1×64 using the HVAB backplane adapter, make sure you close the AB0 and AB2 switches. Leaving them open on power-up is a guaranteed way to get zero readings.

 

Installation & Configuration Guide

1. Pre-Installation: ⚠️ High Voltage Warning. Ensure the SCXI chassis power is OFF. Take a photo of the existing SCXI-1127’s DIP switches and HVAB adapter jumpers. Verify your DMM and SCXI chassis are properly grounded.
2. Removal: Disconnect the SCXI-13xx terminal blocks. Unscrew the front panel thumbscrews and gently pull the module straight out of the backplane. Avoid bending the pins.
3. Installation: Copy the DIP switch and HVAB jumper settings exactly. Slide the new SCXI-1127 into the rightmost slot (or designated slot) of the SCXI chassis. Secure the thumbscrews and reconnect the terminal blocks.
4. Power-On & Testing: Power on the SCXI chassis. Open NI MAX to auto-detect the module and run a self-test. Use the NI-SWITCH Soft Front Panel to manually close a channel and verify continuity with a multimeter before running your LabVIEW scan list.

Compatible Replacement Models

• ✅ Drop-in Replacement: SCXI-1128 (This is the solid-state relay version. Same pinout and form factor, but handles different current/voltage specs. Verify your load requirements before swapping.)
• ⚠️ Software Compatible: SCXI-1130 (High-density reed relay matrix. Fits the same SCXI chassis and uses the same NI-SWITCH API, but requires rewiring the terminal blocks due to different pinouts.)
• ❌ Hardware Mod Required: PXI-2532 (This is a PXI format switch. It will not fit in an SCXI chassis. You would need a PXI chassis, a new backplane, and likely a rewritten LabVIEW application.)

 

Frequently Asked Questions (FAQ)

Can I use this SCXI-1127 to switch 240V AC mains power?
Yes, but be careful. The module is rated for 300 Vrms and 200 mA at 250 Vrms. If your AC load draws more than 200mA, you will weld the armature contacts shut. Always calculate your load current first.Do I need to buy a separate DMM to use this?
The SCXI-1127 is just a switch; it doesn’t measure anything. You need a DMM (like the PCI-4070) or a DAQ device to read the signals. If you’re using a PXI-1010 chassis, put the DMM in slot 8 to use the internal PXI/SCXI trigger lines for synchronous scanning.Why is my 1×64 configuration not working?
If you’re trying to run it as a 1×64 single-wire mux, you have to close the internal 1WIRE and HLSELECT relays. Older versions of NI-DAQ didn’t support this well. Make sure you’re using NI-SWITCH 2.0 or later, and configure the topology as “1127/Independent” in your code.Is it safe to hot-swap this module in the SCXI chassis?
Absolutely not. SCXI modules are not designed for hot-swapping. Pulling or inserting this card while the chassis is powered will likely arc the backplane pins and fry the module or the chassis controller. Always kill the power first.What terminal block do I need for 4-wire RTD measurements?
You need the SCXI-1331 terminal block. It provides the separate excitation (A) and sense (B) pairs required for 4-wire configurations. Standard 2-wire blocks won’t give you the accuracy you need for precision resistance measurements.