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Modbus RTU Communication .
ECW500 | 59
7 MODBUS RTU COMMUNICATION
The Modbus protocol was originally developed in 1979. Currently, it is an open protocol widely used by several
manufacturers in different kinds of equipment. The Modbus-RTU communication of the ECW500 was
developed based on the following documents:
▪ MODBUS Protocol Reference Guide Rev. J, MODICON, June 1996.
▪ MODBUS Application Protocol Specification, MODBUS.ORG, May 8th 2002.
▪ MODBUS over Serial Line, MODBUS.ORG, December 2nd 2002.
These documents define the format of messages used by the elements that are part of the Modbus network,
the services (or functions) that can be provided by the network and how these elements exchange data.
7.1 STRUCTURE OF THE MESSAGES IN THE RTU MODE
The Modbus-RTU network uses the master-slave system for exchanging messages. It allows up to 247 slaves,
but only one master. All communication begins with the master making a request to a slave and the slave
responds to the master what was requested. In both telegrams (question and answer), the structure used is the
same: Address, Function Code, Data and CRC. Only the data field can have variable length, depending on what
is being requested.
Master (request telegram):
Address Function Requested Data CRC
1 byte 1 byte N bytes 2 bytes
Slave (response telegram):
Address Function Requested Data CRC
1 byte 1 byte N bytes 2 bytes
7.2 MODBUS FUNCTIONS AVAILABLE ON THE ECW500
The MODBUS functions available on the ECW500 are listed below. Detailed information to use these functions
is found in the MODBUS protocol specification.
■ F0x01: Read Coils.
■ F0x02: Read Discrete Inputs.
■
F0x03: Read Holding Registers.
■ F0x04: Read Input Registers.
■ F0x05: Write Single Coils.
■
F0x0F: Write Multiple Coils.
■ F0x10: Write Multiple Registers.
■ F0x2B: Read Device Identification.
7.2.1 Parameter Types of the ECW500
The parameter types of the ECW500 are organized by Modbus address range, as described below.
Range 0 a 20 Commands for the ECW500. These are the commands available to change the state of the
equipment. It has the same effect as the digital input commands. They are boolean write registers, and the
reading returns false until the command is executed.
Range 10.000 a 10.018
Status flags of the ECW500. These are the flags to indicate the state of the
equipment. They are boolean read-only registers that return true or false. The writing has no effect.
Range 30.002 a 31.346
Configuration parameters. These are the variables for configurating the regulator.
They consist in 4 registers of 8 bits, or 2 registers Modbus of 16 bits, which result in 32 bits of data with the
floating point type of single precision standard IEEE 754. Parameters 31214 and 31216 (words of status and
command) are special and must be read/written as integer 32 bits. A further description of these words is
presented in Chapter 9.