Executing Custom Commands

To communicate with SHDLC devices, usually you won’t use this base driver directly since it doesn’t provide device specific commands. Instead, you use a dedicated Python driver created for a specific device type, like sensirion-shdlc-sensorbridge for the Sensirion SEK-SensorBridge.

But if there is no Python driver available (yet) for a particular SHDLC device, this base driver could still be used to communicate with that device. There are two different ways how to execute custom SHDLC commands with this driver.

Using ShdlcCommand Directly

The simplest way is to pass an ShdlcCommand object to the execute() method of the device:

from sensirion_shdlc_driver import ShdlcSerialPort, ShdlcConnection, ShdlcDevice
from sensirion_shdlc_driver.command import ShdlcCommand
from struct import unpack

with ShdlcSerialPort(port='COM1', baudrate=115200) as port:
    device = ShdlcDevice(ShdlcConnection(port), slave_address=0)

    # Execute custom command
    raw_response = device.execute(ShdlcCommand(
        id=0xD2,  # The command ID as specified in the device documentation
        data=b"\x01",  # The payload data to send
        max_response_time=0.2,  # Maximum response time in Seconds
    ))
    print("Raw Response: {}".format(raw_response))

    # Convert raw response to proper data types
    uint32, uint8 = unpack('>IB', raw_response)
    print("Response: {}, {}".format(uint32, uint8))

For more options and detailed information, please take a look at the documentation of ShdlcCommand.

Deriving from ShdlcCommand

Another, more powerful way to implement custom commands is to create separate classes for each command. Especially if the raw response bytes need to be converted to other data types this is handy since it will make the execute() method returning the converted response.

from sensirion_shdlc_driver import ShdlcSerialPort, ShdlcConnection, ShdlcDevice
from sensirion_shdlc_driver.command import ShdlcCommand
from struct import pack, unpack

class MyCustomCommand(ShdlcCommand):
    def __init__(self, bool_parameter):
        super(MyCustomCommand, self).__init__(
            id=0xD2,  # Command ID as specified in the device documentation
            data=pack(">B", bool_parameter),  # Payload data
            max_response_time=0.2,  # Maximum response time in Seconds
        )

    def interpret_response(self, data):
        # Convert the received raw bytes to the proper data types
        uint32, uint8 = unpack('>IB', data)
        return uint32, uint8


with ShdlcSerialPort(port='COM1', baudrate=115200) as port:
    device = ShdlcDevice(ShdlcConnection(port), slave_address=0)
    uint32, uint8 = device.execute(MyCustomCommand(bool_parameter=True))
    print("Response: {}, {}".format(uint32, uint8))

Creating a Device Class

To create a more convenient API, it might make sense to wrap the custom commands in a new device class. You have two options to create a device class:

• Inherit from ShdlcDeviceBase and add methods for all the supported SHDLC commands of the device, including common commands. This is the preferred way since it leads to a device class which perfectly fits the particular device, but it requires additional effort to also implement all the common SHDLC commands. To reduce effort, you can start by copying the needed methods from ShdlcDevice.
• Inherit from ShdlcDevice and add only the device-specific commands since the common commands are automatically available since the base class already provides them. This is simpler, but has the downside that you even inherit methods which the particular device might not support, or parameters like response time or post processing time might be wrong since they are device specific.

No matter which way you choose, implementing the device class works exactly the same way:

from sensirion_shdlc_driver import ShdlcSerialPort, ShdlcConnection, ShdlcDeviceBase
from sensirion_shdlc_driver.command import ShdlcCommand
from struct import pack, unpack

class MyCustomCommand(ShdlcCommand):
    def __init__(self, bool_parameter):
        super(MyCustomCommand, self).__init__(
            id=0xD2,  # Command ID as specified in the device documentation
            data=pack(">B", bool_parameter),  # Payload data
            max_response_time=0.2,  # Maximum response time in Seconds
        )

    def interpret_response(self, data):
        # Convert the received raw bytes to the proper data types
        uint32, uint8 = unpack('>IB', data)
        return uint32, uint8


class MyCustomShdlcDevice(ShdlcDeviceBase):
    def __init__(self, connection, slave_address):
        super(MyCustomShdlcDevice, self).__init__(connection, slave_address)

    def my_custom_command(self, bool_parameter):
        return self.execute(MyCustomCommand(bool_parameter))


with ShdlcSerialPort(port='COM1', baudrate=115200) as port:
    device = MyCustomShdlcDevice(ShdlcConnection(port), slave_address=0)
    uint32, uint8 = device.my_custom_command(bool_parameter=True)
    print("Response: {}, {}".format(uint32, uint8))