Accessing an instrument remotely

Estimated time: 15 minutes

Overview

Questions

• How can we access the instruments remotely?

Objectives

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

Accessing a remote instrument

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QMI makes it easy to access an instrument instance that exists in another Python program. The programs may even run on different computers. The Python program that contains the instrument instance must 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"
    },
    "contexts": {
        # Testing remote instrument access.
        "instr_server": {
            "host": "127.0.0.1",
            "tcp_server_port": 40001
        }
    }
}

Note that JSON is very picky about the use of commas. There must be a comma between multiple elements in the same group, but there may not be a comma after the last element of a group.

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 Python programs can now connect to this port to access the sine generator.

To try this, leave the instrument server session running and start another Python session in a separate terminal window:

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 also requires 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
• 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>")