Keysight / Signametrics SM4042 40 Channel PCI Relay Multiplexers

The Keysight/Signametrics SM4042 is a 40-channel PCI relay multiplexer engineered for automated testing and precision data acquisition. It delivers the signal integrity required for 6½ or 7½ digit multimeter accuracy through isolated architecture, optical isolation, and thermal EMF control below 1 µV. The card reconfigures on-the-fly across multiple topologies: 40-channel 2-wire, 20-channel 4-wire, 10-channel 6-wire, matrix configurations, single-ended 80:1, or universal independent relay control. Break-before-make switching with variable actuation timing ensures reliable signal routing in millivolt and milliohm measurement ranges.

## Technical Specifications

• **Channels:** 40
• **Thermal EMF:** < 1 µV
• **Measurement Resolution:** Supports full accuracy for 6½ or 7½ digit DMMs
• **Switching:** Break-before-make with variable actuation time and delay
• **Isolation:** Optical isolation; isolated power and controls
• **Signal Characteristics:** Very low noise, low voltage errors

## Key Features

• **Configuration Modes:** 40-channel 2-wire, 20-channel 4-wire, 10-channel 6-wire, two groups of 20:1, four groups of 10:1, single-ended 80:1, universal mode
• **Scanning Operations:** Triggered and auto-scan with sync output
• **Diagnostics:** Shorts/opens testing, stuck relay detection, contact resistance measurement, bounce and settle time analysis
• **Form Factor:** PCI plug-in card

## Typical Applications

Automated test systems requiring precision signal routing for resistance and voltage measurement in low-level acquisition. Matrix switching for multi-channel data collection. Thermocouple and sensor multiplexing with isothermal terminal blocks.

## Compatibility & Integration

• **Host Interface:** PCI
• **Operating Systems:** Windows, Linux
• **Programming Languages:** Visual Basic, C++, C#, LabVIEW, LabWindows
• **Optional Modules:** SM40 (screw terminal), SM40T (isothermal with temperature sensor), SM40L (loopback test)
• **Status:** Obsolete as of July 31, 2011. Keysight recommends the 34972 as replacement.