Accessing an instrument remotely

Overview

Questions

• How can we access the instruments remotely?

Objectives

• Learn to connect between QMI contexts and control instruments over contexts

Create an instrument ‘server’

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QMI makes it easy to access a QMI instrument driver instance that exists in another QMI context in another Python program. The power of QMI is that these programs may be even running on different computers. The Python program that contains the instrument instance must, though, be accessible via the network. This can be achieved by extending the QMI configuration file. The new file will look as follows:

JSON

{
    # Log level for messages to the console.
    "logging": {
        "console_loglevel": "INFO"
        # "logfile": "log.log"
    },
    # Directory to write main log file.
    "log_dir": "~/qmi_course/log",
    "contexts": {
        # Testing remote instrument access.
        "instr_server": {
            "host": "127.0.0.1",
            "tcp_server_port": 40001
        }
    }
}

Start the instrument ‘server’ with the following lines:

PYTHON

from qmi.instruments.dummy.noisy_sine_generator import NoisySineGenerator
qmi.start("instr_server")
nsg = qmi.make_instrument("nsg", NoisySineGenerator)

Because the name of the context, “instr_server”, matches the name specified in the configuration file, QMI opens a TCP server port for this context. Check that the reading of configuration file succeeded by calling qmi.context().get_context_config(). In the response string, the host and tcp_server_port values should be the same as in the configuration file. If this is not the case, stop the context, and start it again providing the qmi.conf file path with the config_file parameter. Then try again to confirm the host and port. Other QMI contexts from other Python programs can now connect to this port to access the sine generator instrument.

Accessing the instrument as a ‘client’

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To try this, leave the instrument server session running and start another Python session in a separate terminal window (but using the same qmi.conf file) to create a ‘client’:

PYTHON

import qmi
qmi.start("random_client")
qmi.context().connect_to_peer("instr_server")
nsg = qmi.get_instrument("instr_server.nsg")
nsg.get_sample()

NOTE: With Windows, the connect_to_peer might require explicit input of the context address. You can check the address by calling qmi.show_network_contexts(). Then give the whole address as second parameter in the call with peer_address=<the_address>. If the connecting now went without an exception, everything should be now OK. You can confirm this by calling again qmi.show_network_contexts()and see that the ‘connected’ column has now changed to ‘yes’. Then proceed to get the instrument and a sample.

NOTE 2: peer_address=”127.0.0.1:40001” also works as the ‘localhost’ address changes into the IPv4 address in the background. This exercise demonstrated how the second Python program is able to access the NoisySineGenerator instance proxy that exists within the first Python program (and QMI context within it). To do this, the QMI context of the second program connects to the “instr_server” context via TCP. Behind the scenes, the two contexts exchange messages through this connection to arrange for the method get_sample() to be called in the real instrument instance through the proxy, and the answer to be sent back to the calling proxy in the second program.

Key Points

• Instruments to be accessed remotely should be defined in qmi.conf - both on the ‘server’ and ‘client’ side
• Connect to another QMI context using qmi.context().connect_to_peer("<context_name>", peer_address="<ho.st.i.p:port>")
• Obtain remote instrument control with qmi.get_instrument("<context_name>.<instrument_name>")