Open Connectivity PROTOCOL

Programmable Logic Controller is a mini computer used to automate manufacturing processes. There are multiple machines like Robots, machines in Packaging lines; machines in assembly lines use PLCs to automate the manufacturing. In the Automotive industry, Vehicle production is divided into Stamping, Assembly and Painting processes. Each of the above processes requires extensive use of PLCs. There should be a mechanism for the communication of process control data to IT system / application. OPC – Open Connectivity provides published industrial standard for system interconnectivity with PLCs.

OPC at a glance

OPC is a collection of software interfaces for exchanging data between IT applications and PLCs.  This software has been designed according to the rules of Microsoft COM technology. OPC Server (COM Server) can provide data about the PLCs. OPC Client (COM client) can access the offered data. Legacy expansion for OPC is OLE for Process Control. You can find more information about OPC at the following link: http://www.opcfoundation.org/

Why is OPC needed?

Proprietary solution from each vendor might require multiple drivers to be installed for the information exchange from PLCs. Multiple device drivers create too many data requests for the same data from multiple devices. Implementation of these drivers cost more money and time. OPC is the popular standard to avoid the necessity of having too many device drivers. OPC Drivers are readily available from different vendors.

OPC Components

·         OPC Data Access (DA): Provides access to real time process data. Using OPC DA, most 
           recent values of temperatures, pressures, etc. can be found.

·         OPC Historical Data Access (HDA): Provide analysis to historical process data, which are
           typically stored in database or Remote terminal unit (RTU)

·         OPC Alarm and Events (A&E) is used to exchange process alarms and events.

·         OPC Data Exchange (DX) defines how OPC Servers exchange data with other OPC
           Servers

OPC Foundation also has developed an OPC compliance test suite to certify different vendors on following OPC standards

Different OPC Servers

Link Master

LinkMaster OPC is provided by Kepware Company. This links the data between OPC Servers. For example, RSLinx (Rockwell Automation) OPC Server might be used for connectivity to Allen Bradley PLCs.  LinkMaster can be used to group and add multiple OPC Servers data. LinkMaster also supports scaling of link data item – means scaling of raw data to be converted into engineering units for OPC Client applications.

RSLinx

Rockwell Automation has developed RSLinx - an OPC Server. Lite version of RSLinx doesn’t contain OPC Server implementation. There are two ways by which applications can be connected to RSLinx: i) OPC Client (DLL) to be loaded on to the client machine to communicate with the server. ii) DDE (Dynamic Data Exchange) client is used to connect to RSLinx DDE Server.

KEPServerEX

KEPServerEX provides interfaces and connectivity for OPC DA, AE and Unified Architecture (UA) as well. Unified architecture was introduced by OPC Foundation to avoid the dependency with Microsoft COM connectivity for Client Server connection. UA provides secure way of accessing information from Client to Server without COM technology. This also supports Windows 7 as the OS.

Siemens SIMATIC NET

SIMATIC NET is the industrial communication offered by Siemens Company. Siemens also provides the whole suite of OPC Interfaces for this product for accessing PLC data.