Modbus Protocol part 1

Recently i used Schneider PLC to learn ladder programming and i saw that PLC has Modbus communication.I did some research about this communication protocol and thought you also may have interest about this industrially used communication protocol.

Modbus is a serial communication protocol developed by Modicon.😮😮

Well, What is serial communication and what is a Protocol ???

                                            Serial communication is process of sending/receiving data ,one bit at a time sequentially(or in an order).This definition simply gives the definition for parallel communication,which is two or more bits at a time simultaneously along multiple channels.

Parallel vs Serial communication (From wikipedia)

                                         Communication protocol is a system of rules that is applied to communication process/system to transmit data along any communication channel.

These rules are used to define ...  

• Recovery methods. (due to Errors occur in the communication)
• Data type used in the communication.
• Address formats for data exchange.
• Flow control of data.
• Sequence control of data,Etc.

                                                     

The common language used by all Modicon devices(eg. PLC,VSD) is Modbus protocol.This protocol defines the message structure which is used to communicate between controllers,monitoring devices,Etc and therefore this is a messaging protocol.There can be seen so many protocols used in communication such as file transfer[FTP],Transmitting structured text(Hypertext)[HTTP],Etc.So now we know that Modbus is serial messaging protocol used in Modicon devices.

                                            Modbus is Application layer protocol positioned at level 7 of the OSI model.😮😮😮 Hell ! what is that ???

                                            The ISO(International Organization of Standardization)-OSI(Open System Interconnection) model is 7 layered  architecture defined for a communication system.

OSI model (from studytonight.com)

Now I will give very rough idea about the OSI model.

• The physical layer converts the digital/analog bits into electrical signal or optical signals.It defines transmission medium(Copper cables,Bluetooth.Etc).

• The Data link layer synchronizes the information which is to be transmitted over the physical layer.

• The Network layer routes the signal through different channels from one node to other and It acts as a network controller. It manages the Subnet traffic.

• The Transport layer decides if data transmission should be on parallel path or single path and the functions such as Multiplexing, Segmenting or Splitting on the data are done by this layer.

•  The Session layer manages and synchronize the conversation between two different applications.

• The Presentation layer takes care that the data is sent in such a way that the receiver will understand the information (data) and will be able to use the data.

• The Application layer is responsible for interacting with various software applications and implement  communication components for all of them. Mail services, directory services, network resource etc are services provided by application layer.

Each layer has set of defined protocols used for different applications.

                                                       So,   Modbus protocol is application layer messaging protocol which provides Client/Server communications between Modicon devices.

In the next post i will go more detail into technical side of Modbus protocol.

If you have any doubt please leave a message.Thanks.