# Buggy code
def run_async_server():
    # ----------------------------------------------------------------------- #
    # initialize your data store
    # ----------------------------------------------------------------------- #
    # The datastores only respond to the addresses that they are initialized to
    # Therefore, if you initialize a DataBlock to addresses from 0x00 to 0xFF,
    # a request to 0x100 will respond with an invalid address exception.
    # This is because many devices exhibit this kind of behavior (but not all)
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #
    # Continuing, you can choose to use a sequential or a sparse DataBlock in
    # your data context.  The difference is that the sequential has no gaps in
    # the data while the sparse can. Once again, there are devices that exhibit
    # both forms of behavior::
    #
    #     block = ModbusSparseDataBlock({0x00: 0, 0x05: 1})
    #     block = ModbusSequentialDataBlock(0x00, [0]*5)
    #
    # Alternately, you can use the factory methods to initialize the DataBlocks
    # or simply do not pass them to have them initialized to 0x00 on the full
    # address range::
    #
    #     store = ModbusSlaveContext(di = ModbusSequentialDataBlock.create())
    #     store = ModbusSlaveContext()
    #
    # Finally, you are allowed to use the same DataBlock reference for every
    # table or you you may use a seperate DataBlock for each table.
    # This depends if you would like functions to be able to access and modify
    # the same data or not::
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #     store = ModbusSlaveContext(di=block, co=block, hr=block, ir=block)
    #
    # The server then makes use of a server context that allows the server to
    # respond with different slave contexts for different unit ids. By default
    # it will return the same context for every unit id supplied (broadcast
    # mode).
    # However, this can be overloaded by setting the single flag to False
    # and then supplying a dictionary of unit id to context mapping::
    #
    #     slaves  = {
    #         0x01: ModbusSlaveContext(...),
    #         0x02: ModbusSlaveContext(...),
    #         0x03: ModbusSlaveContext(...),
    #     }
    #     context = ModbusServerContext(slaves=slaves, single=False)
    #
    # The slave context can also be initialized in zero_mode which means that a
    # request to address(0-7) will map to the address (0-7). The default is
    # False which is based on section 4.4 of the specification, so address(0-7)
    # will map to (1-8)::
    #
    #     store = ModbusSlaveContext(..., zero_mode=True)
    # ----------------------------------------------------------------------- #
    store = ModbusSlaveContext(
        di=ModbusSequentialDataBlock(0, [17] * 100),
        co=ModbusSequentialDataBlock(0, [17] * 100),
        hr=ModbusSequentialDataBlock(0, [17] * 100),
        ir=ModbusSequentialDataBlock(0, [17] * 100),
    )
    context = ModbusServerContext(slaves=store, single=True)

    # ----------------------------------------------------------------------- #
    # initialize the server information
    # ----------------------------------------------------------------------- #
    # If you don't set this or any fields, they are defaulted to empty strings.
    # ----------------------------------------------------------------------- #
    identity = ModbusDeviceIdentification()
    identity.VendorName = "Pymodbus"
    identity.ProductCode = "PM"
    identity.VendorUrl = "http://github.com/bashwork/pymodbus/"
    identity.ProductName = "Pymodbus Server"
    identity.ModelName = "Pymodbus Server"
    identity.MajorMinorRevision = "1.5"

    # ----------------------------------------------------------------------- #
    # run the server you want
    # ----------------------------------------------------------------------- #

    # TCP Server

    StartTcpServer(context, identity=identity, address=("localhost", 5020))


# Traceback (AttributeError)
client = ModbusSerialClient(port='notexist', method='rtu')
client.connect()
Traceback (most recent call last):
File "<input>", line 1, in <module>
File "lib/python3.6/site-packages/pymodbus/client/sync.py", line 476, in connect
self.socket.interCharTimeout = self.inter_char_timeout
AttributeError: 'NoneType' object has no attribute 'interCharTimeout'

# Fixed code
def run_async_server():
    # ----------------------------------------------------------------------- #
    # initialize your data store
    # ----------------------------------------------------------------------- #
    # The datastores only respond to the addresses that they are initialized to
    # Therefore, if you initialize a DataBlock to addresses from 0x00 to 0xFF,
    # a request to 0x100 will respond with an invalid address exception.
    # This is because many devices exhibit this kind of behavior (but not all)
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #
    # Continuing, you can choose to use a sequential or a sparse DataBlock in
    # your data context.  The difference is that the sequential has no gaps in
    # the data while the sparse can. Once again, there are devices that exhibit
    # both forms of behavior::
    #
    #     block = ModbusSparseDataBlock({0x00: 0, 0x05: 1})
    #     block = ModbusSequentialDataBlock(0x00, [0]*5)
    #
    # Alternately, you can use the factory methods to initialize the DataBlocks
    # or simply do not pass them to have them initialized to 0x00 on the full
    # address range::
    #
    #     store = ModbusSlaveContext(di = ModbusSequentialDataBlock.create())
    #     store = ModbusSlaveContext()
    #
    # Finally, you are allowed to use the same DataBlock reference for every
    # table or you you may use a seperate DataBlock for each table.
    # This depends if you would like functions to be able to access and modify
    # the same data or not::
    #
    #     block = ModbusSequentialDataBlock(0x00, [0]*0xff)
    #     store = ModbusSlaveContext(di=block, co=block, hr=block, ir=block)
    #
    # The server then makes use of a server context that allows the server to
    # respond with different slave contexts for different unit ids. By default
    # it will return the same context for every unit id supplied (broadcast
    # mode).
    # However, this can be overloaded by setting the single flag to False
    # and then supplying a dictionary of unit id to context mapping::
    #
    #     slaves  = {
    #         0x01: ModbusSlaveContext(...),
    #         0x02: ModbusSlaveContext(...),
    #         0x03: ModbusSlaveContext(...),
    #     }
    #     context = ModbusServerContext(slaves=slaves, single=False)
    #
    # The slave context can also be initialized in zero_mode which means that a
    # request to address(0-7) will map to the address (0-7). The default is
    # False which is based on section 4.4 of the specification, so address(0-7)
    # will map to (1-8)::
    #
    #     store = ModbusSlaveContext(..., zero_mode=True)
    # ----------------------------------------------------------------------- #
    store = ModbusSlaveContext(
        di=ModbusSequentialDataBlock(0, [17] * 100),
        co=ModbusSequentialDataBlock(0, [17] * 100),
        hr=ModbusSequentialDataBlock(0, [17] * 100),
        ir=ModbusSequentialDataBlock(0, [17] * 100),
    )
    store.register(
        CustomModbusRequest.function_code,
        "cm",
        ModbusSequentialDataBlock(0, [17] * 100),
    )
    context = ModbusServerContext(slaves=store, single=True)

    # ----------------------------------------------------------------------- #
    # initialize the server information
    # ----------------------------------------------------------------------- #
    # If you don't set this or any fields, they are defaulted to empty strings.
    # ----------------------------------------------------------------------- #
    identity = ModbusDeviceIdentification()
    identity.VendorName = "Pymodbus"
    identity.ProductCode = "PM"
    identity.VendorUrl = "http://github.com/bashwork/pymodbus/"
    identity.ProductName = "Pymodbus Server"
    identity.ModelName = "Pymodbus Server"
    identity.MajorMinorRevision = "2.2.0"

    # ----------------------------------------------------------------------- #
    # run the server you want
    # ----------------------------------------------------------------------- #

    # TCP Server

    StartTcpServer(
        context,
        identity=identity,
        address=("localhost", 5020),
        custom_functions=[CustomModbusRequest],
    )