Simulated Instruments

Overview

instro ships local SCPI simulators for select instrument categories, so you can develop and test without owning the physical hardware. Each simulator runs as a TCP server and pairs with a Simulated<Category> driver:

from instro.psu import InstroPSU
from instro.psu.drivers import SimulatedPSU

with InstroPSU(
    name="bench_psu",
    driver=SimulatedPSU("TCPIP0::127.0.0.1::5025::SOCKET"),
    num_channels=1,
) as psu:
    psu.set_voltage(5.0, channel=1)

PSU simulator

The PSU simulator is a local SCPI server for development, tests, and demos. It listens on a TCP socket, exposes a programmable power-supply command set, and includes a terminal UI for editing channel loads, probe resistance, and channel limits while your code is running.

Quickstart

Start the simulator in one terminal:

python -m instro.psu.scpi_sim_server

It binds to 127.0.0.1:5025 by default. If you are working from a source checkout, run the same command through uv:

uv run python -m instro.psu.scpi_sim_server

Connect from another terminal or script with the SimulatedPSU driver. This small Python client opens an InstroPSU, configures channel 1, and reads voltage and current measurements:

from instro.psu import InstroPSU
from instro.psu.drivers import SimulatedPSU


with InstroPSU(
    name="bench_psu",
    driver=SimulatedPSU("TCPIP0::127.0.0.1::5025::SOCKET"),
    num_channels=2,
) as psu:
    psu.set_voltage(5.0, channel=1)
    psu.set_current_limit(1.0, channel=1)
    psu.output_enable(True, channel=1)

    voltage = psu.get_voltage(channel=1)
    current = psu.get_current(channel=1)
    print(f"V: {voltage.latest:.3f} V")
    print(f"I: {current.latest:.3f} A")

    psu.output_enable(False, channel=1)

Use CLI flags to change the bind address, port, or initial channel count:

python -m instro.psu.scpi_sim_server --host 127.0.0.1 --port 5025 --channels 2

Usage

The simulator runs a terminal UI while the TCP server listens in the background. The UI is organized by panel header.

PSU simulator terminal UI showing the PSU panel, channel panels, and SCPI log

Filled labels are selectable actions. Use the arrow keys to move between controls. Focused actions use a light highlight. Press Enter to select an action, Escape to close an edit prompt, and q to quit.

PSU

The ⟢ NOMINAL PSU panel shows the simulator identity and VISA resource string. Use the resource string when driver code connects to the simulator.

Channel

Each CHANNEL panel shows the live state for one output. The panel is split into status, voltage, and current sections.

Header	Functionality
STATUS	Shows output mode, output state, sense mode, and protection trips. Select REMOVE to delete the channel from the simulator.
VOLTAGE	Shows terminal voltage, programmed voltage, voltage limit, OVP state, and OVP level. Select V LIMIT to edit the channel voltage ceiling.
CURRENT	Shows output current, programmed current limit, current limit, OCP state, and OCP level. Select I LIMIT to edit the channel current ceiling.

Changing V LIMIT or I LIMIT runs *RST. The output turns off, programmed voltage and current return to zero, protection state turns off, and OVP/OCP levels return to the configured V/I limits.

Probe

The PROBE panel shows sense lead resistance. Select PROBE R to edit the probe resistance in ohms.

Load

The LOAD panel shows the external load model. Select R to edit load resistance in ohms. Select EMF to edit the series electromotive force in volts.

+ channel

The + CHANNEL panel adds a new output channel with the next available channel ID.

SCPI log

The SCPI LOG panel records incoming commands, responses, and queued errors. It is read-only.

Programming guide

Brackets mark optional SCPI path components. For example, [SOURce:]VOLTage[:LEVel] accepts VOLT, VOLT:LEV, SOUR:VOLT, and SOUR:VOLT:LEV. Angle brackets mark command arguments. They are notation only; do not include the angle brackets in the command.

<NRf+> is a positive flexible numeric value, such as 0, 1.25, or 1e-3.
<literal|literal> is one literal value from the listed options.

The pipe character (|) means OR. For example, <LOC|REM> accepts LOC or REM, and <NRf+|MIN|MAX> accepts one numeric value, MIN, or MAX. Do not send the pipe character in the command. MIN maps to 0. MAX maps to V LIMIT for voltage and OVP commands, and I LIMIT for current and OCP commands. Do not send MIN or MAX to queries.

Defaults and reset

On startup and after *RST, each channel uses the reset operating state:

Setting	Reset value
Programmed voltage	`0`
Current limit	`0`
Output state	`OFF`
OVP state	`OFF`
OCP state	`OFF`
OVP level	`MAX`
OCP level	`MAX`

*RST preserves simulator configuration, including channel count, channel IDs, load resistance, series EMF, probe resistance, sense mode, and configured channel limits.

Error queue

Errors are queued and returned one at a time by SYSTem:ERRor?. The simulator can emit these error codes:

Code	Message	Cause
`0`	`No error`	The queue is empty
`-108`	`Parameter not allowed`	A query received an argument
`-109`	`Missing parameter`	A command was sent without a required argument
`-113`	`Undefined header`	The command header is not supported
`-114`	`Header suffix out of range`	The command targets a channel that does not exist
`-141`	`Invalid character data`	A numeric argument could not be parsed
`-222`	`Data out of range`	A numeric input is outside its accepted range
`-224`	`Illegal parameter value`	A literal argument is not accepted for that command
`301`	`PV Above OVP`	Programmed voltage is above the OVP level
`303`	`PC Above OCP`	Current limit is above the OCP level
`304`	`OVP Below PV`	OVP level is below programmed voltage
`305`	`OCP Below PC`	OCP level is below current limit
`323`	`Overcurrent protection tripped`	OCP tripped while enabled
`324`	`Overvoltage protection tripped`	OVP tripped while enabled

Common commands

`*IDN?`
Query simulator identity	Response: String
`*RST`
Reset channel operating state and clear errors	Response: None
`*CLS`
Clear the error queue	Response: None

MEASure subsystem

`MEASure:CURRent?`
Measure output current	Response: Number
`MEASure:VOLTage?`
Measure output voltage	Response: Number

OUTPut subsystem

`OUTPut[:STATe] <0\|1\|OFF\|ON>`
Enable or disable output	Response: None
`OUTPut[:STATe]?`
Query output state	Response: `0` or `1`
`OUTPut:PROTection:CLEar`
Clear latched OVP and OCP faults	Response: None
`OUTPut:PROTection:TRIPped?`
Query latched OVP or OCP fault state	Response: `0` or `1`

SOURce subsystem

`[SOURce:]CURRent[:LEVel][:IMMediate][:AMPLitude] <NRf+\|MIN\|MAX>`
Set current limit	Response: None
`[SOURce:]CURRent[:LEVel][:IMMediate][:AMPLitude]?`
Query current limit	Response: Number
`[SOURce:]CURRent:PROTection[:LEVel] <NRf+\|MIN\|MAX>`
Set OCP level	Response: None
`[SOURce:]CURRent:PROTection[:LEVel]?`
Query OCP level	Response: Number
`[SOURce:]CURRent:PROTection:STATe <0\|1\|OFF\|ON>`
Enable or disable OCP	Response: None
`[SOURce:]CURRent:PROTection:STATe?`
Query OCP state	Response: `0` or `1`
`[SOURce:]VOLTage[:LEVel][:IMMediate][:AMPLitude] <NRf+\|MIN\|MAX>`
Set programmed voltage	Response: None
`[SOURce:]VOLTage[:LEVel][:IMMediate][:AMPLitude]?`
Query programmed voltage	Response: Number
`[SOURce:]VOLTage:PROTection[:LEVel] <NRf+\|MIN\|MAX>`
Set OVP level	Response: None
`[SOURce:]VOLTage:PROTection[:LEVel]?`
Query OVP level	Response: Number
`[SOURce:]VOLTage:PROTection:STATe <0\|1\|OFF\|ON>`
Enable or disable OVP	Response: None
`[SOURce:]VOLTage:PROTection:STATe?`
Query OVP state	Response: `0` or `1`

SYSTem subsystem

`SYSTem:ERRor?`
Pop one error from the error queue	Response: `code,“message”`
`SYSTem:SENSe[:STATe] <LOC\|REM>`
Select local or remote sense	Response: None
`SYSTem:SENSe[:STATe]?`
Query sense mode	Response: `LOC` or `REM`

DMM simulator

The DMM simulator is a local SCPI server for development, tests, and demos. It listens on a TCP socket and exposes a digital-multimeter command set: function switching plus the five primary measurements (DC/AC voltage, DC/AC current, and two-wire resistance). Each function measures a configurable stimulus value returned with Gaussian noise, so repeated reads look like a real bench instrument. Unlike the PSU simulator, the DMM simulator is headless: there is no terminal UI. Set stimulus values with CLI flags at startup, or through the Python API when embedding the simulator in tests.

Quickstart

Start the simulator in one terminal:

python -m instro.dmm.scpi_sim_server

It binds to 127.0.0.1:5026 by default. If you are working from a source checkout, run the same command through uv:

uv run python -m instro.dmm.scpi_sim_server

Connect from another terminal or script with the SimulatedDMM driver. This small Python client opens an InstroDMM, switches functions, and reads measurements:

from instro.dmm import InstroDMM
from instro.dmm.drivers import SimulatedDMM


with InstroDMM(
    name="bench_dmm",
    driver=SimulatedDMM("TCPIP0::127.0.0.1::5026::SOCKET"),
) as dmm:
    print(f"V: {dmm.read_dc_voltage().latest:.4f} V")
    print(f"R: {dmm.read_resistance().latest:.1f} ohm")

Use CLI flags to change the bind address, port, or the stimulus each function measures:

python -m instro.dmm.scpi_sim_server --port 5026 --dc-voltage 3.3 --resistance 4700

Stimulus model

The stimulus is the simulator’s stand-in for the physical quantity on the probes (the DMM analog of the PSU simulator’s load model). Each measurement function has one stimulus value, and every measurement returns that value with Gaussian noise (±0.5% at 3 standard deviations).

Function	CLI flag	Default
DC voltage	`--dc-voltage`	`1.25` V
AC voltage	`--ac-voltage`	`0.5` V
DC current	`--dc-current`	`0.1` A
AC current	`--ac-current`	`0.05` A
Resistance	`--resistance`	`1000` Ω

When embedding the simulator in Python (for example in tests), construct the server in-process and set stimulus values at runtime:

from instro.dmm.scpi_sim_server import (
    FUNCTION_RESISTANCE,
    SimulatedDMM,
    SimulatedDMMServer,
)

sim = SimulatedDMM()
server = SimulatedDMMServer(sim, port=0)  # port 0 = pick a free port
server.start()
print(f"TCPIP0::127.0.0.1::{server.port}::SOCKET")

sim.set_stimulus(FUNCTION_RESISTANCE, 4700.0)  # change what the "probes" see

# ... drive it with SimulatedDMM / InstroDMM ...

server.shutdown()

Stimulus values persist across *RST: reset returns the instrument state (active function, error queue) to defaults, but the simulated physical world stays put.

Programming guide

Brackets mark optional SCPI path components. For example, [SENSe:]FUNCtion accepts FUNC, FUNCTION, and SENS:FUNC. Keywords match in short or long form, in any case.

Command	Functionality
`*IDN?`	Returns `NOMINAL,SIMULATED_DMM,000001,1.0`.
`*RST`	Resets the active function to DC voltage and clears the error queue. Stimulus values are preserved.
`*CLS`	Clears the error queue.
`SYSTem:ERRor?`	Pops the oldest queued error, or `0,"No error"`.
`[SENSe:]FUNCtion "<function>"`	Sets the active function: `VOLT:DC`, `VOLT:AC`, `CURR:DC`, `CURR:AC`, or `RES`. Bare `VOLT`/`CURR` select DC. Quotes optional.
`[SENSe:]FUNCtion?`	Returns the active function, quoted (e.g. `"VOLT:DC"`).
`CONFigure:VOLTage[:DC]`	Sets the active function to DC voltage. Same pattern for `:AC`, `CURRent[:DC]`, `CURRent:AC`, and `RESistance`.
`MEASure:VOLTage[:DC]?`	Switches to the function and returns a noisy measurement of its stimulus. Same pattern for the other four functions.
`READ?`	Returns a noisy measurement using the active function.

Unknown headers queue -113,"Undefined header"; an unknown function name queues -224,"Illegal parameter value". The SimulatedDMM driver checks SYST:ERR? after every transaction and raises on any queued error.

​Overview

​PSU simulator

​Quickstart

​Usage

​PSU

​Channel

​Probe

​Load

​+ channel

​SCPI log

​Programming guide

​Defaults and reset

​Error queue

​Common commands

​MEASure subsystem

​OUTPut subsystem

​SOURce subsystem

​SYSTem subsystem

​DMM simulator

​Quickstart

​Stimulus model

​Programming guide

Overview

PSU simulator

Quickstart

Usage

PSU

Channel

Probe

Load

+ channel

SCPI log

Programming guide

Defaults and reset

Error queue

Common commands

MEASure subsystem

OUTPut subsystem

SOURce subsystem

SYSTem subsystem

DMM simulator

Quickstart

Stimulus model

Programming guide