[1] Modbus is a fieldbus, an industrial computer networking protocol, originally designed to use via programmable logic controllers (PLCs), but became a de facto standard communication protocol for communication between industrial electronic devices various buses and networks.
One of the oldest and most reliable industrial communication protocols and developed by Modicon (now Schneider Electric) in 1979, Modbus works on a Request-Response model (Master-Slave or Client-Server).
Etymology The name "Modbus" is combined of 2 elements:
"Mod" is from Modicon, the company that invented it. (Modicon itself stands for Modular Industrial Control).
"Bus" refers to a computer bus, which is a shared communication pathway or line used to connect and transfer data between multiple electronic components or devices.
[1] Modbus is popular in industrial environments as it's openly allocated and royalty-free. It was developed for industrial applications, is easy to deploy and maintain compared to other standards, and places few restrictions on the format of the data to be transmitted.
[1] Modbus uses serial communication lines, Ethernet, or the Internet protocol suite as a transport layer and supports communication to and from many devices connected to the same cable or Ethernet network. E.g., there can be a device measuring temperature and another device to measure humidity connected to the same cable, both communicating measurements to the same computer, via Modbus.
By working on a Request-Response/Master-Slave model, it assigns the role of a client or master and the server (slave).
The Client (Master): A network often has only one client (e.g., a central PLC or a computer running monitoring software). The client is the only device allowed to start a conversation. It asks for data or sends commands.
The Server(s) (Slave(s): 247 server devices max can connect to the network (e.g., temperature sensors, motor drives). They only speak when spoken to by the client and can't talk to each other.
Data storage
A Modbus device organizes its data into 4 distinct tables based on whether the data is a simple binary switch or a numerical value, and whether it's read-only or read-write:
Note: Since register is 16 bits (2 bytes) and holds an integer 0-65,535, a device must send big decimal value (like a 32-bit float), which will combine 2 adjacent registers.
The most common and highly efficient type for short-to-medium distance, device-to-device communication, Modbus RTU uses binary encoding and transmits over serial interfaces like RS-485 or RS-232.
Wires: It uses serial cables, commonly RS-485 (2 wires twisted together).
Format: Data is sent in raw, compact binary format.
Upsides: Very cheap, very electric noise-resistant in factories, works in long distances (up to 1,200 m).
Format: Instead of raw binary, it translates data into readable ASCII characters.
Upsides: It is slower than RTU, but it's easier for humans to troubleshoot because you can read the raw text streams. It is rarely used in modern setups.
The Wire: Uses standard Ethernet cables (RJ45) and standard network switches.
The Format: It wraps the standard Modbus data packet inside a standard TCP/IP network packet.
Why it's used: It is blazing fast, allows multiple clients on the same network, and lets you connect industrial equipment directly to local IT networks or the internet.