# MODBUS This is an excerpt from the modbus.org website of the MODBUS organization. [For more modbus.org](http://modbus.org/) 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. For details about the Modbus protocol, please visit [modbus.org](/ins/en-1/communication-protocols/modbus/modbus-rtu-over-udp.md). {% file src="/files/-LkfStMdCbRMyTq8X7qB" %} --- # Agent Instructions: Querying This Documentation If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question. Perform an HTTP GET request on the current page URL with the `ask` query parameter: ``` GET https://inscada.gitbook.io/ins/en-1/communication-protocols/modbus.md?ask= ``` The question should be specific, self-contained, and written in natural language. The response will contain a direct answer to the question and relevant excerpts and sources from the documentation. Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.