Common Standard Protocol of All Manufacturers
MODBUS is an application layer messaging protocol located at level 7 of the OSI model. It provides client / server communication between devices connected to different buses or networks.
MODBUS, the de facto industrial serial standard since 1979, continues to enable millions of automation devices to communicate. Today, the support continues to grow for MODBUS's simple and elegant structure. The Internet community can access MODBUS from a dedicated system port 502 in the TCP/IP stack.
MODBUS is a request/response protocol and provides services specified by function codes. The MODBUS function codes are elements of the MODBUS request/response PDUs. This protocol specification document describes the function codes used within MODBUS operations.