Abstract
This chapter focuses on Modbus® protocol, its structures, and frames. Modbus® is a transmission protocol developed by Gould Modicon (now Schneider Electric) for process control systems. It is a simple, flexible, publicly published protocol, which allows devices to exchange discrete and analog data. End users are aware that specifying Modbus® as the required interface between subsystems is a way to achieve multi-vendor integration with the most purchasing options and at the lowest cost. Small equipment makers are also aware that they must offer Modbus® with EIA-232 and/or EIA-485 to sell their equipment to system integrators for use in larger projects. Certain characteristics of the Modbus® protocol are fixed, such as frame format, frame sequences, handling of communications errors and exception conditions and the functions performed. Other characteristics are selectable. These are transmission medium, transmission characteristics and transmission mode, namely, RTU or American Standard Code II (ASCII). The remote terminal unit (RTU) mode is the preferred Modbus® mode. The ASCII transmission mode has a typical message that is about twice the length of the equivalent RTU message.
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