Introduction Of Modbus-Rtu Communication Protocol; Overview - Siemens SICAM P35 Manual

6.2

Introduction of MODBUS-RTU Communication Protocol

6.2.1

Overview

Type of Protocol
This is MODBUS RTU protocol applicable to real-time communication of embedded electric energy meter. This
protocol defines the data exchange between terminal unit (slave) and Master, which is realized in the form of
RTU (remote terminal unit) of MODBUS. Asynchronous master-slave half-duplex communication is used.
Communication is initiated from master and slave responds after receiving request from master.
Communication response time < 0.2 sec.
Physical Layer
• Transmission interface: RS485
• Communication address: 0 to 247
• Communication baud rate: 600 bps to 38,400 bps
• Communication media: Shield Twisted Pair
Data Link Layer (DLL)
• Transmission manner: asynchronous master-slave half-duplex
• Data frame format: 1 start bit, 8-digit data, even parity check bit, 1 stop bit
• Data packet format:
Address
8 bits
Data packet transmission sequence always uses identical address, function code, data and check code. Each
data packet should be transmitted as a continuous bit stream. When data packet from the master reaches
slave, slave matching the address field in data packet will receive and check the data. If no error, slave will
execute the request in data packet and send the responsive date packet to the master. The data packet sent
back by slave contains slave Address, Function executed, Data generated from function execution and CRC.
• Address
Address is at the beginning of data packet and consists of an 8-bit data which means slave address specified
by the master. Each slave address in the bus is unique. Valid address range is from 0 to 247. After the master
sends data packet, only slave matching the inquire address of the master will respond.
• Function
It describes which function is executed by slave. Meanings of all function codes are given below.
Code
03 H
10 H
• Data
It contains data required for executing specific function by slave or data acquired when slave responds inquiry
of master. Content of these data may be address code or data.
• CRC
Check code is 16-bit check data formed in data transmission for CRC of master and slave. Various
interferences exist in communication, so data transmitted during communication may change. CRC will avoid
data distortion during responsive transmission of master and slave and improves security and efficiency of
system. For formation rule of check code, see Annex 1.
SICAM P35 P36, 3-phase Multi-Function Power Meter, User Manual
E50417-H8940-C562-4, Release Date 06.2018
Function
8 bits
Definition
Read data
Preset multiple registers
6.2 Introduction of MODBUS-RTU Communication Protocol
Data
n*8 bits
Read current binary value of one or more variables
Overwrite values of several variables using specific binary value
Communication
CRC
16 bits
Function
39