پروتکل مدباس - Modbus

Download : MODBUS FARSI.pdf
 Modbus Farsi - Translate by Mehran

I. Introduction

Modbus is a communications protocol based on master-slave architecture.
Modbus allows remote administration of a network of devices, for example in a system which measures torque and angle of tightening and then communicates the results to a remote computer or to a PLC.

Modbus normally is used over serial lines (RS-485 and RS-232) or Ethernet (Modbus TCP). There are four types of Modbus protocol:

            1 - Modbus ASCII
            2 - Modbus RTU
 
            3 - Modbus Plus                

            4 - Modbus TCP/IP

II. The Modbus protocol

Diagram basic architecture with Modbus master and slaves
:


Master is usually a PC or a PLC.

A simple example with Modbus protocol between a PC and a Rexroth ErgoSpin.
ErgoSpin Rexroth,Selected by IRAN PLC, because it is very easy to setup.
And fieldbus are very reliable with Rexroth ErgoSpin.


III.  Exchange master and slaves

Note :
Addressing : Two Modbus slaves can not have the same address.
Modbus slaves can not communicate together.


- Exchange frames between master to one slave

The master sending a message frame and the addressed slave responds with another message frame.


- Exchange frames between master to one slave

The master broadcasts a message to all slaves on the network, the slaves execute the order of the message without sending a response.


IV. The Modbus message structure


The master sending a message frame and the addressed slave responds with another message frame.
Every message frame in Modbus  includes a function code and data bytes.
The error-checking in the 
message frame is LRC in ASCII and CRC in RTU.
Every message contains the slave address, for protocol ASCII is 2 characters and 1 byte for protocol RTU.




V. Frame Format


- Modbus RTU Frame Format :

  Name  Length  Function
 Start  3.5c idle  Times of silence
 Address  8 bits  Station Address
 Function  8 bits  Function codes
 Data  n * 8 bits  Data + length
 CRC Check  16 bits  Error checks
 End  3.5c idle  Times of silence

- Modbus ASCII Frame Format :

  Name  Length  Function
 Start  1 char
 ASCII value is 3A hex
 Address  2 chars
 Station Address
 Function  2 chars
 Function codes
 Data  n chars
 Data + length
 LRC Check  2 chars
 Error checks
 End  2 chars
 ASCII values of 0D & 0A hex


VI. Modbus Supported Function Codes

Common Modbus function codes


Code Description
01 Read coil status
02 Read input status
03 Read holding registers
04 Read input registers
05 Force single coil
06 Preset single register
07 Read exception status
15 Force multiple coils
16 Preset multiple registers
17 Report slave ID



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IRAN PLC,
Nov 21, 2014, 9:07 AM