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A simulator for real hardware which is accessible via TCP, UDP or serial line

Project description

Instrument Simulator

Pypi python versions Pypi version Pypi status License

A simulator for real hardware. This project provides a server able to spawn multiple simulated devices and serve requests concurrently.

This project provides only the required infrastructure to launch a server from a configuration file (YAML, TOML or json) and a means to register third-party device plugins through the python entry point mechanism.

So far, the project provides transports for TCP, UDP and serial line. Support for new transports (ex: USB, GPIB or SPI) is being implemented on a need basis.

PRs are welcome!


(TL;DR: pip install sinstruments[all])

From within your favorite python environment:

$ pip install sinstruments

Additionally, if you want to write YAML configuration files in YAML:

$ pip install sinstruments[yaml]

...or, for TOML based configuration:

$ pip install sinstruments[toml]


Once installed the server can be run with:

$ sinstruments-server -c <config file name>

The configuration file describes which devices the server should instantiate along with a series of options like the transport(s) each device listens for requests on.


Imagine you need to simulate 2 GE Pace 5000 reachable through a TCP port each and a CryoCon 24C accessible through the serial line.

First, make sure the dependencies are installed with:

$ pip install gepace[simulator] cryoncon[simulator]

Now we can prepare a YAML configuration called simulator.yml:

- class: Pace
  name: pace-1
  - type: tcp
    url: :5000
- class: Pace
  name: pace-2
  - type: tcp
    url: :5001
- class: CryoCon
  name: cryocon-1
  - type: serial
    url: /tmp/cryocon-1

We are now ready to launch the server:

$ sinstruments-server -c simulator.yml

That's it! You should now be able to connect to any of the Pace devices through TCP or the CryoCon using the local emulated serial line.

Let's try connecting to the first Pace with the nc (aka netcat) linux command line tool and ask for the well known *IDN? SCPI command:

$ nc localhost 5000

Device catalog

This is a summary of the known third-party instrumentation libraries which provide their own simulators.

If you wrote a publicly available device feel free complete the above list by creating a PR.

Hint: sinstruments-server ls shows a list of available plugins.


The configuration file can be a YAML, TOML or JSON file as long as it translates to a dictionary with the description given below.

In this chapter we will use YAML as a reference example.

The file should contain at least a top-level key called devices. The value needs to be a list of device descriptions:

  - class: Pace
    name: pace-1
    - type: tcp
      url: :5000

Each device description must contain:

  • class: Each third-party plugin should describe which text identify itself
  • name: a unique name. Each device must be given a unique name at your choice
  • transports: a list of transports from where the device is accessible. Most devices provide only one transport.
    • type: Each transport must define its type (supported are tcp, udp, serial)
    • url: the url where the device is listening on

Any other options given to each device are passed directly to the specific plugin object at runtime. Each plugin should describe which additional options it supports and how to use them.


For TCP and UDP transports, the url has the <host>:<port> format.

An empty host (like in the above example) is interpreted as (which means listen on all network interfaces). If host is or localhost the device will only be accessible from the machine where the simulator is running.

A port value of 0 means ask the OS to assign a free port (useful for running a test suite). Otherwise must be a valid TCP or UDP port.

Serial line

The url represents a special file which is created by the simulator to simulate a serial line accessible like a /dev/ttyS0 linux serial line file.

This feature is only available in linux and systems for which the pseudo terminal pty is implemented in python.

The url is optional. The simulator will always create a non deterministic name like /dev/pts/4 and it will log this information in case you need to access. This feature is most useful when running a test suite.

You are free to choose any url path you like (ex: /dev/ttyRP10) as long as you are sure the simulator has permissions to create the symbolic file.

Simulating communication delays

For any of the transports (TCP, UDP and serial line) is is possible to do basic simulation of the communication channel speed by providing an additional baudrate parameter to the configuration. Example:

- class: CryoCon
  name: cryocon-1
  - type: serial
    url: /tmp/cryocon-1
    baudrate: 9600

Back door

The simulator provides a gevent back door python console which you can activate if you want to access a running simulator process remotely. To activate this feature simply add to the top-level of the configuration the following:

backdoor: ["localhost": 10001]
  - ...

You are free to choose any other TCP port and bind address. Be aware that this backdoor provides no authentication and makes no attempt to limit what remote users can do. Anyone that can access the server can take any action that the running python process can. Thus, while you may bind to any interface, for security purposes it is recommended that you bind to one only accessible to the local machine, e.g.,


Once the backdoor is configured and the server is running, in a another terminal, connect with:

$ nc 10001
Welcome to Simulator server console.
You can access me through the 'server()' function. Have fun!
>>> print(server())

Develop a new simulator

Writting a new device is simple. Let's imagine you want to simulate a SCPI oscilloscope. The only thing you need to do is write a class inheriting from BaseDevice and implement the handle_message(self, message) where you should handle the different commands supported by your device:

# myproject/

from sinstruments.simulator import BaseDevice

class Oscilloscope(BaseDevice):

    def handle_message(self, message):"received request %r", message)
        message = message.strip().decode()
        if message == "*IDN?":
            return b"ACME Inc,O-3000,23l032,3.5A"
        elif message == "*RST":
  "Resetting myself!")

Don't forget to always return bytes! The simulator doesn't make any guesses on how to encode str

Assuming this file is part of a python package called myproject, the second thing to do is register your simulator plugin in your

        "sinstruments.device": [

You should now be able to launch your simulator by writing a configuration file:

# oscilo.yml

- class: Oscilloscope
  name: oscilo-1
  - type: tcp
    url: :5000

Now launch the server with

$ sinstruments-server -c oscillo.yml

and you should be able to connect with:

$ nc localhost 5000
ACME Inc,O-3000,23l032,3.5A

Configuring message terminator

By default the eol is set to \n. You can change it to any character with:

class Oscilloscope(BaseDevice):

    newline = b"\r"

Request with multiple answers

If your device implements a protocol which answers with multiple (potentially delayed) answers to a single request, you can support this by converting the handle_message() into a generator:

class Oscilloscope(BaseDevice):

    def handle_message(self, message):"received request %r", message)
        message = message.strip().decode()
        if message == "*IDN?":
            yield b"ACME Inc,O-3000,23l032,3.5A"
        elif message == "*RST":
  "Resetting myself!")
        elif message == "GIVE:ME 10":
            for i in range(1, 11):
                yield f"Here's {i}\n".encode()

Don't forget to always yield bytes! The simulator doesn't make any guesses on how to encode str

Support for specific configuration options

If your simulated device requires additional configuration, it can be supplied through the same YAML file.

Let's say you want to be able to configure if your device is in CONTROL mode at startup. Additionally, if no initial value is configured, it should default to 'OFF'.

First lets add this to our configuration example:

# oscilo.yml

- class: Oscilloscope
  name: oscilo-1
  control: ON
  - type: tcp
    url: :5000

Then, we re-implement our Oscilloscope __init__() to intercept this new parameter and we handle it in handle_message():

class Oscilloscope(BaseDevice):

    def __init__(self, name, **opts):
        self._control = opts.pop("control", "OFF").upper()
        super().__init__(name, **opts)

    def handle_message(self, message):
        elif message == "CONTROL":
            return f"CONTROL {self._control}\n".encode()

You are free to add as many options as you want as long as they don't conflict with the reserved keys name, class and transports.

Writing a specific message protocol

Some instruments implement protocols that are not suitably managed by a EOL based message protocol.

The simulator allows you to write your own message protocol. Here is an example:

from sinstruments.simulator import MessageProtocol

class FixSizeProtocol(MessageProtocol):

    Size = 32

    def read_messages(self):
        transport = self.transport
        buff = b''
        while True:
            buff +=, size=4096)
            if not buff:
            for i in range(0, len(buff), self.Size):
                message = buff[i:i+self.Size]
                if len(message) < self.Size:
                    buff = message
                yield message

class Oscilloscope(BaseDevice):

    protocol = FixSizeProtocol


Pytest fixture

If you are developing a python library that provides access to an instrument accessible through socket or serial line and you wrote a simulator for it, you might be interested in testing your library against the simulator.

sinstruments provides a pair of pytest helpers that spawn a simulator in a separate thread.


The first usage is simply using the server_context helper. There is actually nothing pytest speficic about this helper so you could imagine using it in other scenarios as well.

Here is an example:

import pytest

from sinstruments.pytest import server_context

cfg = {
    "devices": [{
        "name": "oscillo-1",
        "class": "Oscilloscope",
        "transports": [
            {"type": "tcp", "url": "localhost:0"}

def test_oscilloscope_id():
    with server_context(cfg) as server:
        # put here code to perform your tests that need to communicate with
        # the simulator. In this example an oscilloscope client
        addr = server.devices["oscillo-1"].transports[0].address
        oscillo = Oscilloscope(addr)
        assert oscillo.idn().startswith("ACME Inc,O-3000")

You might notice that in the configuration we use port 0. This is telling the simulator to listen on any free port provided by the OS.

The actual test retrieves the current address assigned by the OS and uses it in the test.

As you can see, the tests are not dependent of the availability of one specific port which makes them portable.

Here is a suggestion on how you could write your own fixture using the server_context helper. The aim was to reduce the amount of boilerplate code you need to write your test:

def oscillo_server():
    with server_context(config) as server:
        server.oscillo1 = server.devices["oscillo-1"]
        server.oscillo1.addr = server.oscillo1.transports[0].address
        yield server

def test_oscilloscope_current(oscillo_server):
    oscillo = Oscilloscope(oscillo_server.oscillo1.addr)
    assert .05 < oscillo.current() < 0.01


A second helper is the server fixture. This fixture depends on an existing config feature that must be present in your module. Here is an example following the previous code:

from sinstruments.pytest import server

def config()
    yield cfg

def test_oscilloscope_voltage(server):
    addr = server.devices["oscillo-1"].transports[0].address
    oscillo = Oscilloscope(addr)
    assert 5 < oscillo.voltage() < 10

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