Field-buses constitute a fundamental element in the development of processes automation. They facilitate the exchange of data among programmable logical controllers, distributed processes and the supervision system. In this article the requirements and the necessary methodology for the implementation on a microcontroller of the Modbus protocol are presented. In order to evaluate this methodology, the protocol was implemented in the 16 bits HCS12 microcontroller. A simple application was programmed and the microcontroller was connected to a net where the master is a LabView program running in a PC. The correct performance was verified by means of a Modbus Sniffer programmed in LabView.