OPC is a protocol developed for create a common way to transfer data and command from different PLCs.
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More info in Italian language are here.
The basic principle of OPC specification is a client-server model in which a any process (client) based on OPC can access any data source (server) with OPC interfaces.
• OPC server enables hardware vendors to offer their customers services that allow any client to access their equipment
• OPC client application controls the devices and manage the relevant data using the standard methods of access to an object OPC