Modbus is a simple, widely-used industrial master/slave (client/server) protocol for reading and writing device data, organized around four register/bit tables. The data model: coils are single read/write bits (typically outputs like relays), discrete inputs are read-only bits (input states like switches), holding registers are 16-bit read/write words (setpoints, configuration), and input registers are 16-bit read-only words (measurements like temperature or current). The master issues a function code, for example read holding registers (0x03), write single register (0x06), write multiple registers (0x10), targeting a slave address, and the slave responds only when addressed. It comes in variants sharing this model: Modbus RTU (compact binary frames over a serial line, usually RS-485 for multi-drop), Modbus ASCII (legacy text form), and Modbus TCP (the same model over Ethernet/IP). Its appeal is simplicity and ubiquity, almost every PLC, drive, meter, and industrial sensor speaks it, so it's the lingua franca for industrial data access even though it's old and minimal.
Communication Protocols · Interview question
What is Modbus and what's its data model?
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Modbus & 1-Wire
Two field protocols beyond the big three: Modbus (industrial register access over RS-485/TCP, master/slave with CRC) and 1-Wire (a single line carrying power and data to addressable sensors).