Difference Between Master-Slave and Client-Server Configuration

  

2 Oct 2020.

In this post, we will clarify the difference between master-slave protocol and client-server protocol. (Click on the images for a more zoomed view)

Hello programmers. In industrial communication, we must be well-versed with all the basic terms which are used so that it will become easier for you to identify the correct configuration. Many a times, we come across two words which have the same meaning; but are slightly different in functionalities. Today, we will come across two such protocols which are widely used in communication and often confuses a designer in distinguishing. The first one is the master-slave and the second one is the client-server. Referring the above image, we will see whether the PLC is a master or slave in master-slave configuration; and whether it is a client or server in client-server configuration.

Two devices can communicate if they can exchange information and understand each other. First, master/slave communication originated in industrial automation whereas client/server communication originated in IT industry. Both these terms go together in communication; we just must identify the proper type of device. There are many protocols used; but people often end up getting confused in both these terms. That’s because; they think both are same protocols with same meaning.

Master-Slave vs. Client-Server:

What Is Master-Slave Architecture?

Master-slave system is the one where the master communicates with one or more slave devices. This is the general answer when a question arises as to what master and slave concept is. Basically, you can think of it is as the dominating type one. The master is the one which will initiate communication and control the slave devices timely. The slaves must respond to the master for data transfer. Each slave has it’s own identification and the master will identify it through that. The master will continuously poll for data in the network. This is the difference between a master and slave.

Client-Server Communication Protocol:

Now, the client-server protocol is the same as master-slave. Many ask what is the difference between client and server? Here, the client is the one which requests for data and the server will have to respond to the client for data transfer. So, you can think of it as the democratic one. The server will patiently wait for the client to request data. Once it gets the request, it will respond accordingly. Once the request is over, communication won’t happen, and the server will just wait for a request from client. This is the difference between client and server.

Basically, a master oversees the network and the one which starts communication. No one can talk without master’s permission. But there is no such control protocol in client-server. It’s just a simple game of request and response. Look at the above-mentioned data; you must have found in both the terms; data transfer is happening but the major difference being of control. A client can only access the data and not modify the data of server, whereas the master has a major role in influencing the data from slaves. Both the terms just go similar in terms of data request and response.

Now, catching up the matching terms, you can say that a master is equal to client, whereas a slave is equal to server. The master requests for data and the slave responds to it with the data.

So, in our topic, PLC is the master and sensor is the slave, if you look from master-slave point. A PLC is the one which controls the network and slave devices for data transfer. The sensor will get polling request from the PLC in a periodic time interval and the sensor will send the data accordingly. If you look in client-server scope, the PLC is a client, and the sensor is a server. The PLC will request for data and the sensor will respond by sending data to it.  

In general, if you have some devices and when you want to distinguish; it’s important to first understand the concept of that individual device. Once you catch up the terms, it will be easier for you to design the network.

I have covered the general theory related to these protocols. I have also not attempted to cover every type of features deeply; you can learn it easily in the software help file or internet. I have just given you an insight of this type of protocols. Learn the basics and explore a new type of study in this type of automation.

 

Thank you, guys; I hope you enjoyed reading the practices normally used for this type of study in industrial automation.