How to Control a Keysight DMM with TofuPilot

Connect a Keysight 34461A or 34465A DMM to Python using PyVISA, send SCPI commands to measure DC voltage, AC voltage, resistance, and current, then log results automatically to TofuPilot via an OpenHTF plug.

Prerequisites

Keysight 34461A or 34465A DMM (or compatible 344xxA series)
Python 3.8+
OpenHTF and TofuPilot installed
Keysight IO Libraries Suite (for USB) or network access (for Ethernet)

terminal

pip install pyvisa pyvisa-py tofupilot openhtf

Step 1: Connect to the Instrument

Keysight DMMs support two primary interfaces: USB-TMC and Ethernet (LXI).

Interface	VISA Resource String	Use Case
USB-TMC	`USB0::0x0957::0x1A07::MY12345678::INSTR`	Single bench, direct PC connection
Ethernet (VXI-11)	`TCPIP0::192.168.1.100::inst0::INSTR`	Networked test systems
Ethernet (HiSLIP)	`TCPIP0::192.168.1.100::hislip0::INSTR`	Higher throughput, recommended for 34465A

Find your instrument's resource string:

find_instruments.py

import pyvisa

rm = pyvisa.ResourceManager()
print(rm.list_resources())

Open a connection and verify identity:

connect.py

import pyvisa

rm = pyvisa.ResourceManager()
dmm = rm.open_resource("USB0::0x0957::0x1A07::MY12345678::INSTR")
dmm.timeout = 5000  # ms
print(dmm.query("*IDN?"))
# Keysight Technologies,34461A,MY12345678,A.02.14-02.40-02.14-00.49-01-01

Step 2: Configure SCPI Measurements

Always reset to a known state before configuring.

scpi_basics.py

dmm.write("*RST")
dmm.write("*CLS")
print(dmm.query("SYST:ERR?"))
# +0,"No error"

DC Voltage

measure_dc_voltage.py

# Auto-range DC voltage
dmm.write("CONF:VOLT:DC AUTO,DEF")
reading = float(dmm.query("READ?"))
print(f"DC Voltage: {reading:.6f} V")

# Manual range: 10 V range
dmm.write("CONF:VOLT:DC 10,DEF")
reading = float(dmm.query("READ?"))

AC Voltage

measure_ac_voltage.py

dmm.write("CONF:VOLT:AC AUTO,DEF")
reading = float(dmm.query("READ?"))
print(f"AC Voltage (RMS): {reading:.6f} V")

Resistance (2-wire and 4-wire)

measure_resistance.py

# 2-wire resistance, auto-range
dmm.write("CONF:RES AUTO,DEF")
reading = float(dmm.query("READ?"))
print(f"2-wire resistance: {reading:.4f} Ohm")

# 4-wire resistance (eliminates lead resistance)
dmm.write("CONF:FRES AUTO,DEF")
reading = float(dmm.query("READ?"))
print(f"4-wire resistance: {reading:.4f} Ohm")

DC Current

measure_current.py

dmm.write("CONF:CURR:DC AUTO,DEF")
reading = float(dmm.query("READ?"))
print(f"DC Current: {reading:.6f} A")

SCPI Command Reference

Measurement	CONF Command	Range Values
DC Voltage	`CONF:VOLT:DC {range},DEF`	0.1, 1, 10, 100, 1000 V
AC Voltage	`CONF:VOLT:AC {range},DEF`	0.1, 1, 10, 100, 750 V
2-wire Resistance	`CONF:RES {range},DEF`	100, 1k, 10k, 100k, 1M, 10M, 100M Ohm
4-wire Resistance	`CONF:FRES {range},DEF`	Use for less than 100 Ohm
DC Current	`CONF:CURR:DC {range},DEF`	0.0001, 0.001, 0.01, 0.1, 1, 3 A
AC Current	`CONF:CURR:AC {range},DEF`	0.001, 0.01, 0.1, 1, 3 A

Step 3: Tune Speed vs. Accuracy with NPLC

NPLC (Number of Power Line Cycles) controls integration time. Higher NPLC averages more noise but takes longer.

nplc_config.py

dmm.write("CONF:VOLT:DC 10,DEF")
dmm.write("VOLT:DC:NPLC 1")    # balanced (default)
dmm.write("VOLT:DC:NPLC 10")   # high accuracy, slower
dmm.write("VOLT:DC:NPLC 0.02") # fast, more noise

NPLC	Integration Time (60 Hz)	Readings/sec	Use Case
0.02	333 us	~50	Fast production screening
0.2	3.3 ms	~15	Good speed/noise balance
1	16.7 ms	~5	Standard accuracy
10	167 ms	~0.6	High accuracy measurements
100	1.67 s	~0.1	Calibration-grade

Manual range eliminates the auto-range delay (~20-50 ms) and is recommended for production test throughput.

Step 4: Build a Keysight DMM OpenHTF Plug

keysight_dmm_plug.py

import pyvisa
import openhtf as htf


class KeysightDMM(htf.plugs.BasePlug):
    """OpenHTF plug for Keysight 344xxA series DMMs."""

    RESOURCE = "USB0::0x0957::0x1A07::MY12345678::INSTR"
    NPLC = 1.0

    def setUp(self):
        self._rm = pyvisa.ResourceManager()
        self._dmm = self._rm.open_resource(self.RESOURCE)
        self._dmm.timeout = 5000
        self._dmm.write("*RST")
        self._dmm.write("*CLS")

    def tearDown(self):
        if self._dmm:
            self._dmm.close()
        self._rm.close()

    def measure_dc_voltage(self, range_v: float = None) -> float:
        range_str = str(range_v) if range_v else "AUTO"
        self._dmm.write(f"CONF:VOLT:DC {range_str},DEF")
        self._dmm.write(f"VOLT:DC:NPLC {self.NPLC}")
        return float(self._dmm.query("READ?"))

    def measure_ac_voltage(self, range_v: float = None) -> float:
        range_str = str(range_v) if range_v else "AUTO"
        self._dmm.write(f"CONF:VOLT:AC {range_str},DEF")
        return float(self._dmm.query("READ?"))

    def measure_resistance(self, four_wire: bool = False, range_ohm: float = None) -> float:
        cmd = "FRES" if four_wire else "RES"
        range_str = str(range_ohm) if range_ohm else "AUTO"
        self._dmm.write(f"CONF:{cmd} {range_str},DEF")
        self._dmm.write(f"{cmd}:NPLC {self.NPLC}")
        return float(self._dmm.query("READ?"))

    def measure_dc_current(self, range_a: float = None) -> float:
        range_str = str(range_a) if range_a else "AUTO"
        self._dmm.write(f"CONF:CURR:DC {range_str},DEF")
        self._dmm.write(f"CURR:DC:NPLC {self.NPLC}")
        return float(self._dmm.query("READ?"))

Step 5: Write Production Tests with TofuPilot

test_power_supply_board.py

import openhtf as htf
from openhtf.util import units
from tofupilot.openhtf import TofuPilot
from keysight_dmm_plug import KeysightDMM


@htf.plug(dmm=KeysightDMM)
@htf.measures(
    htf.Measurement("output_voltage_3v3")
    .in_range(minimum=3.267, maximum=3.333)
    .with_units(units.VOLT),
    htf.Measurement("output_voltage_5v0")
    .in_range(minimum=4.900, maximum=5.100)
    .with_units(units.VOLT),
)
def test_output_voltages(test, dmm):
    """Verify regulated output voltages are within tolerance."""
    test.measurements.output_voltage_3v3 = dmm.measure_dc_voltage(range_v=10)
    test.measurements.output_voltage_5v0 = dmm.measure_dc_voltage(range_v=10)


@htf.plug(dmm=KeysightDMM)
@htf.measures(
    htf.Measurement("sense_resistor_value")
    .in_range(minimum=0.099, maximum=0.101)
    .with_units(units.OHM),
)
def test_sense_resistor(test, dmm):
    """Verify current sense resistor with 4-wire measurement."""
    r = dmm.measure_resistance(four_wire=True, range_ohm=100)
    test.measurements.sense_resistor_value = r


@htf.plug(dmm=KeysightDMM)
@htf.measures(
    htf.Measurement("quiescent_current")
    .in_range(maximum=0.005)
    .with_units(units.AMPERE),
)
def test_quiescent_current(test, dmm):
    """Verify quiescent current does not exceed 5 mA."""
    test.measurements.quiescent_current = dmm.measure_dc_current(range_a=0.01)


@htf.plug(dmm=KeysightDMM)
@htf.measures(
    htf.Measurement("ac_ripple")
    .in_range(maximum=0.050)
    .with_units(units.VOLT),
)
def test_output_ripple(test, dmm):
    """Verify AC ripple on the 5 V output."""
    test.measurements.ac_ripple = dmm.measure_ac_voltage(range_v=1)


def main():
    test = htf.Test(
        test_output_voltages,
        test_sense_resistor,
        test_quiescent_current,
        test_output_ripple,
        test_name="Power Supply Board Functional Test",
    )

    with TofuPilot(test):
        test.execute(test_start=lambda: input("Enter DUT serial number: ").strip())


if __name__ == "__main__":
    main()

Step 6: Fast Multi-Measurement Mode

For high-throughput lines, use SAMP:COUN for burst sampling.

fast_sampling.py

import pyvisa
import numpy as np

rm = pyvisa.ResourceManager()
dmm = rm.open_resource("USB0::0x0957::0x1A07::MY12345678::INSTR")
dmm.timeout = 10000

dmm.write("*RST")
dmm.write("CONF:VOLT:DC 10,DEF")
dmm.write("VOLT:DC:NPLC 0.2")
dmm.write("SAMP:COUN 10")
dmm.write("TRIG:SOUR IMM")
dmm.write("TRIG:DEL 0")

dmm.write("INIT")
raw = dmm.query("FETC?")

readings = [float(x) for x in raw.split(",")]
print(f"Mean: {np.mean(readings):.6f} V")
print(f"Std:  {np.std(readings):.6f} V")

Troubleshooting

Symptom	Likely Cause	Fix
`VI_ERROR_RSRC_NFOUND`	Wrong resource string or driver not installed	Run `rm.list_resources()`, install Keysight IO Libraries
`+1,"Hardware error"`	Overload on input terminals	Check probe connections, reduce input signal
Reading returns `+9.9E+37`	Signal exceeds selected range	Use auto-range or increase manual range
Readings drift over 30 seconds	DMM not warmed up	Allow 30-minute warm-up for less than 10 ppm accuracy
USB device not found on Linux	Missing udev rules	Add `SUBSYSTEM=="usb", ATTRS{idVendor}=="0957", MODE="0666"`
HiSLIP connection refused	HiSLIP not enabled	Enable via front panel: Utilities > I/O Config > LAN > HiSLIP
`NPLC` command returns error	Wrong function prefix	NPLC is per-function: `VOLT:DC:NPLC`, `RES:NPLC`
Slow throughput over LAN	VXI-11 overhead	Switch to HiSLIP for 10x faster transactions
4-wire reading matches 2-wire	Wrong terminals	Confirm SENSE HI/LO connected to separate terminals