---
seo_title: "Control a Digital Multimeter with Python - instro"
myst:
  html_meta:
    description: "Control digital multimeters from Python with one API for Keysight, Agilent, and Keithley: set functions and ranges, measure voltage, current, and resistance."
---

# Digital Multimeter (DMM)

{.lead}
Using InstroDMM for SCPI-based digital multimeters

`InstroDMM` provides a unified interface for digital multimeters. This class is initialized with a vendor-specific driver (`Agilent34401A`, `Keysight34461A`, `Keithley2400`, …), and provides the vendor-agnostic API (`set_measurement_function`, `read`, `set_range`, …).

## Creating an InstroDMM

```python
from instro.dmm.drivers import Agilent34401A
from instro.dmm import InstroDMM

dmm = InstroDMM(
    name="myDMM",
    driver=Agilent34401A("ASRL3::INSTR"),
)
```

## Supported Vendors

:::{driver-cards} dmm
:::

- **Virtual**: no hardware or server; readings simulated in-process (`VirtualDMM`, see [Virtual instruments](/library/simulated-instruments.md#virtual-instruments))
- **Simulated**: no hardware required; pairs with the bundled [DMM simulator](/library/simulated-instruments.md#dmm-simulator) (`SimulatedDMM`)

If your vendor or model is not listed, see [Custom Driver Development](/library/custom-instruments.md#digital-multimeter-dmm), or open a [Driver Request](https://github.com/nominal-io/instro/issues) issue on GitHub.

## Example

More examples found in [Examples](/examples/dmm/index.md)

## Details

The following presents details about the `InstroDMM`. Specific driver details can be found on [their pages](#supported-vendors).

### Driver Composition

An `InstroDMM` is built from a concrete driver:

```
InstroDMM("name", driver=Agilent34401A("ASRL3::INSTR"))
```

- **`Agilent34401A`** owns the connection setup and vendor-specific command mapping.
- **`InstroDMM`** owns the category-level workflow: measurements, commands, publishers, the background daemon.

### Lifecycle

The typical InstroDMM workflow:

1. **Construct**: instantiate the vendor driver and pass it to `InstroDMM`.
2. **`open()`**: establishes the VISA connection.
3. **Configure measurement**: set the measurement function (e.g. DC voltage), and optionally resolution, aperture, and range.
4. **`read()`**: trigger a measurement and get the value. The `read_<function>()` shorthands (e.g. `read_dc_voltage()`) fold steps 3 and 4 into one call.
5. **`close()`**: disconnect from hardware.

Optionally, you can use background fetching via `start()` / `stop()`, but it is an atypical use case for a DMM. See [Two ways to get data](/using-instro.md#two-ways-to-get-data) for more.

### Measurement Functions

`InstroDMM` supports these measurement functions (availability depends on the driver):

- **DC Voltage**
- **AC Voltage**
- **DC Current**
- **AC Current**
- **2-Wire Resistance**
- **4-Wire Resistance**

You must call `set_measurement_function()` before configuring resolution, aperture, range, or calling `read()`.

### Single-call reads

When you only need a value and don't need to tune resolution, aperture, or range, use the per-function shorthand instead of the two-step sequence:

```python
voltage = dmm.read_dc_voltage()  # selects DC voltage if needed, then reads
```

One exists per measurement function: `read_dc_voltage()`, `read_ac_voltage()`, `read_dc_current()`, `read_ac_current()`, `read_resistance()` (2-wire), and `read_four_wire_resistance()`.

Each selects the function only when it isn't already active, so repeated calls in the same function cost a single measurement and publish `set_measurement_function.cmd` once. They publish exactly the same channels as `set_measurement_function()` + `read()`.

### Init from a Config File

`InstroDMM` can also be constructed directly from a JSON config file, which removes the need to write any Python setup code:

```python
from instro.dmm import InstroDMM

dmm = InstroDMM(config="bench_dmm.json")
```

Where `bench_dmm.json` contains:

```json
{
  "version": 1,
  "instrument": "InstroDMM",
  "device": {
    "name": "bench_dmm",
    "description": "Bench multimeter",
    "manufacturer": "Keysight",
    "model": "34461A"
  },
  "driver": {
    "name": "Keysight34461A",
    "connection_type": "visa",
    "visa": {
      "visa_resource": "USB0::0x2A8D::0x1301::MY12345678::INSTR"
    }
  },
  "measurement": {
    "function": "DC_VOLTAGE",
    "aperture_nplc": 10,
    "range": 10.0
  },
  "timing": {
    "poll_interval": 1.0
  },
  "publishers": [
    {
      "type": "NominalCorePublisher",
      "dataset_rid": "<dataset_rid>"
    }
  ]
}
```

To try a config with no hardware, use the virtual driver: `"driver": {"connection_type": "virtual", "name": "VirtualDMM"}`.

See [Config Files](/library/config-files.md) for the full field reference.

## Custom Driver Development

For more information on writing a custom driver, see [Custom Driver Development](/library/custom-instruments.md#digital-multimeter-dmm).