> ## Documentation Index
> Fetch the complete documentation index at: https://instro.nominal.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Digital Multimeter (DMM)

> 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 theme={null}
from instro.dmm.drivers import Agilent34401A
from instro.dmm import InstroDMM

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

## Supported Vendors

<Columns cols={2}>
  <Card title="Agilent/HP/Keysight 34401A" horizontal href="/dmm/Agilent34401A">
    <img src="https://mintlify.s3.us-west-1.amazonaws.com/nominal/snippets/drivers/dmm/agilent-hp-keysight-34401a/image.png" style={{ width: "100%", height: "100px", objectFit: "contain" }} noZoom alt="Agilent/HP/Keysight 34401A" />
  </Card>

  <Card title="Keysight 34461A" horizontal href="/dmm/Keysight34461A">
    <img src="https://mintlify.s3.us-west-1.amazonaws.com/nominal/snippets/drivers/dmm/keysight-34461a/image.png" style={{ width: "100%", height: "100px", objectFit: "contain" }} noZoom alt="Keysight 34461A" />
  </Card>

  <Card title="Keithley 2400" horizontal href="/dmm/Keithley2400">
    <img src="https://mintlify.s3.us-west-1.amazonaws.com/nominal/snippets/drivers/dmm/keithley-2400/image.png" style={{ width: "100%", height: "100px", objectFit: "contain" }} noZoom alt="Keithley 2400" />
  </Card>
</Columns>

* **Simulated**: no hardware required; pairs with the bundled [DMM simulator](/library/simulated-instruments#dmm-simulator) (`SimulatedDMM`)

If your vendor or model is not listed, see [Custom Driver Development](/library/custom-instruments#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)

## 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#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 theme={null}
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 theme={null}
from instro.dmm import InstroDMM

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

Where `bench_dmm.json` contains:

```json theme={null}
{
  "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>"
    }
  ]
}
```

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

## Custom Driver Development

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