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Modbus

It is an application-layer messaging protocol, positioned at level 7 of the OSI model. It provides client/server communication between devices connected on different types of buses or networks. MODBUS is a reliable technology that is used by millions of automation devices to communicate.

Today, support for the simple and elegant structure of MODBUS continues to grow. The Internet community can access MODBUS at a reserved system port 502 on the TCP/IP stack. MODBUS is a request/reply protocol and offers services specified by function codes. MODBUS function codes are elements of MODBUS request/reply PDUs. This protocol specification document describes the function codes used within the framework of MODBUS transactions.

Connections
Use modbus with the most popular connection types, like RS485, TCP and UDP. The last two can be transferred with a simple LAN cable. Unlike RS485 which requires at least two wires. Check the RS485 website for further details. Collect some information to select the connection you need.
Read about the most basic connection types
Frame format
Traditional Modbus messages can be placed in RTU or ASCII frames and can be packaged with a TCP/IP interface, so it can be sent over the internet. Both RTU and ASCII contains a Checksum, so when converting to TCP/IP, it doesn't require a checksum calculation or in other words, CRC.
Choose your preferred frame format
Devices / Masters
Master devices are designed to control all slave devices on the Modbus. Most Master devices can be placed between your computer and the slaves. In most cases your computer is the Master device. In this scenario communication goes through the COM port of the computer.
Learn about the most common master devices
Devices / Slaves
On a pair of Modbus wires, slaves can be addressed from 1 to 247. Each slave replies to Function Codes from the Master. Most slaves can measure data or interact with the environment, which is stored in registers. Function Codes can read/write registers. For example read temperature or switch relays.
See the most popular slaves
Appendix
Here you can find all extra information you need for communicating with the master and slaves. In case your computer is the master, you can directly call functions and get a response from the slaves. In any other cases you have to control the master device through your machine.
Find all extra information you need

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