Messages - Fuji Electric FRENIC-AQUA series User Manual

This chapter describes the Modbus RTU protocol, as well as the host side procedure for using this
protocol and error processing.
The Modbus RTU protocol was a set of specifications developed in the United States. In this chapter,
the terms in the specifications are accompanied by English ones as much as possible.

3.1 Messages ................................................................................................................................. 3-1

3.1.1 Message formats.............................................................................................................. 3-1
3.1.2 Message types ................................................................................................................. 3-1
3.1.3 Message frames .............................................................................................................. 3-2
3.1.4 Message categories......................................................................................................... 3-4
3.1.5 Communications examples............................................................................................ 3-12
3.2 Host Side Procedures ............................................................................................................ 3-13
3.2.1 Inverter's response time................................................................................................. 3-13
3.2.2 Timeout processing........................................................................................................ 3-14
3.2.3 Receiving preparation complete time and message timing from the host ..................... 3-15
3.2.4 Frame synchronization method...................................................................................... 3-15
3.3 Communications Errors.......................................................................................................... 3-16
3.3.1 Categories of communications errors ............................................................................ 3-16
3.3.2 Operations in case of errors........................................................................................... 3-17
3.4 CRC-16 .................................................................................................................................. 3-20
3.4.1 Overview of the CRC-16 ................................................................................................ 3-20
3.4.2 Algorithm ........................................................................................................................ 3-20
3.4.3 Calculation example....................................................................................................... 3-22
3.4.4 Frame length calculation................................................................................................ 3-23
Modbus RTU PROTOCOL
Table of Contents
CHAPTER 3