MODBUS is an application layer messaging protocol, positioned at level 7 of the OSI model,
that provides client/server communication between devices connected on different types of
buses or networks.
The industry’s serial de facto standard since 1979, MODBUS continues to enable 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. The objective of this
document is to describe the function codes used within the framework of MODBUS
transactions.
MODBUS is an application layer messaging protocol for client/server communication between
devices connected on different types of buses or networks.
It is currently implemented using:

•  TCP/IP over Ethernet. See MODBUS Messaging Implementation Guide V1.0a.
•  Asynchronous serial transmission over a variety of media (wire : EIA/TIA-232-E, EIA-422, EIA/TIA-485-A; fiber, radio, etc.)
• MODBUS PLUS, a high speed token passing network.