Table of Contents Introduction: Why Has Modbus Survived for 40 Years? The Historical Background and Development of Modbus 2.1 Birth Background 2.2 Evolution 2.3 Why Has Modbus Not Been Phased Out? Modbus Protocol Architecture 3.1 Basic Architecture 3.2 Data Model Three Major Modbus Variants: RTU / ASCII / TCP 4.1 Modbus RTU (Most Classic and Common) 4.2 Modbus ASCII (Early Version) 4.3 Modbus TCP (Ethernet Version) Modbus Frame Structure and Function Code Details 5.1 Modbus R