Modbus requests and responses contain an Application Data Unit (ADU) which contains a Protocol Data Unit (PDU). The ADU is an envelope containing a message, the PDU is the message itself. Modbus requests can be sent via two communication layers, RTU or TCP/IP. The ADU for these layers differs. But the PDU, the message, always has the same structure, regardless of the way it's transported.
.. automodule:: umodbus.functions
.. automodule:: umodbus.client.tcp
.. automodule:: umodbus.client.serial.rtu