OPC Server - Client

Hello,

I'm a bit confused about OPC

I have a little experience with OPC and PROFIBUS. There we had one Master and one/more Slaves

When I read e.g. Kepware's web site they talk about Server and Client. I have a feeling that this is not the same thing as Master/Slave but I can't connect the dots...

Can someone PLEASE explain to me how this works!

// Anders

Hello,

An OPC server provides data to an OPC client. In, for example MODBUS, the master request data from the slave.

Good luck,

Mark
http://www.peakhmi.com/

Does that mean that both the Master and the Slave are Clients?

// Anders

I haven't worked with OPC so much but this is the way i got it,
correct me if i got it wrong

OPC-Server: Read data on PLC.
OPC-Client: read a OPC-Server,
an example. to store the OPC-Server data to SQL-database

I think we are confusing terms from different network purposes.

'ason' definitions are appropriate give a SCADA type situation with a central program (server) gathering requested data from one or more PLCs and providing it as requested to perhaps multiple requesting units (Clients).

The 'master'/'slave' terminology is more appropriate for a central unit (Master) giving commands to one ore more dependent (slave) units.

Now, a server program could request data from both the 'master' and the 'slave' units.

By the way, there has been discussion for the 'master/slave' terminology to be phased out given the current trend toward 'political correctness'. One substitute has been 'controller unit' and 'controlled unit', or perhaps 'leader' and 'follower'.

Hello,

Yes and no.

In MODBUS the master issues a read or write request and the slave responds. The slave never initiates the communication. A master does not respond to read or write request. It generates request.

In OPC, the client issues commands to the server. The server responds to the command. The command could be a simple read or write.

The client could also command the server to notify the client if X occurs. Every time X occurs the server sends a message to the client. So, who is the master now?

OPC servers cannot communicate with OPC servers and OPC clients cannot communicate with OPC clients.

In DNP3 these are named Master and Outstation.

Good luck,

Mark
http://www.peakhmi.com/

Hi, in Kepserver you can load on or more drivers that can be used to communicaiton to devices. The drivers, depending upon which ones are used, can be configured as Masters or slaves. When configured as a master it will poll a slave device.When configured as a Slave we act like any other slave device and wait for a master to write data to us or request data from us.

So the drivers are running in the server. OPC DA or UA clients connect to the server and request data from it. These data requests are what actually triggers the drivers to poll the devices for data, if there is no client connected to the server it does not do anything. OPC is a client/server architecture model.

Hope this makes sense.