The objective of this document is to present the MODBUS messaging service over TCP/IP , in order to provide reference information that helps software developers to implement this service. The encoding of the MODBUS function codes is not described in this document, for this information please read the MODBUS Application Protocol Specification [1]. This document gives accurate and comprehensive description of a MODBUS messaging service implementation. Its purpose is to facilitate the interoperability between the devices using the MODBUS messaging service. This document comprises mainly three parts: ? An overview of the MODBUS over TCP/IP protocol ? A functional description of a MODBUS client, server and gateway implementation. ? An implementation guideline that proposes the object model of an MODBUS implementation example.