The OLE for Process Control (OPC) is an open standard for sharing data between applications dispositivosde computer field and based on Microsoft's OLE. Allows applications written leery and chelate process value data to be shared easily in a computer network.

In this article we will see how is the architecture of OPC and what are its main advantages for communicating control and automation systems.

In the example shown four devices with which we want to communicate and three applications. As used requires access to four "drivers" in each application, see the necessity of twelve different approaches.

Figure 3.6 Drivers Owners

With OPC, OLE for Process Control is only necessary that each application is an OS | o OPC client to access each application, making it more efficient) to transfer data. The development of OPC servers is done independently, Customers are only enough to be customers.

Figure 3.7 OPC Servers and Clients

In control systems the value is passed, the quality of data and time stamp.

OPC interfaces can take the raw data of the E / S physical to SCADA or DCS, or may carry information from the DCS or SCADA systems otrafe applications.

When a server mplementar the main consideration is the frequency of data transfer via shared communication paths to physical devices, process controllers comb.

To improve the transfer rate used by exception reporting, ie sending the data value only when it changes.

Figure 3.8 Location of PCO

The server is composed of multiple objects: the server, the group and item. The server object maintains information! server is a container for groups of objects. The group object maintains information about itself and provides mechanisms to contain and organize the items.

OPC groups allow clients to organize data. The customer can configure and speed at which the OPC server should provide the data. Access is always ja items across groups. The items are the address where data is located.

To access the data, similar to DDE, the client application must specify:

• OPC Server Name data

• The name of the machine that hosts ("host") data to OPC server.

• The name of the item according to the manufacturer's specific OPC server.

Figure 3.9 OPC Group

To address an item depends on the server can be:

"TIC101.PV"

Or when you have low level access to the device, you may need to include information such as accounts, engineering units, scaling and conditioning, for example:

"COM1.STATION: 42.REG: 40001, 0.4095, -100.0, +1234.0"

OPC servers must consolidate and optimize the access requirements of several customers to provide efficient communication for the physical device. PARF inputs (readings), the data delivered by the device are taken to a "buffer" for the distribution or collection asynchronous synchronous multiple OPC clients. For outputs (writes) the data server updates the physical device on behalf of OPC clients.

Customers must have the skills of timing and "signaling". Synchronization is the client's ability to read or write values ​​and attributes urge single transaction. For example when you download a recipe for Batcfh process, ensure that all values ​​in the group are sent out together, and that recipe is not ja start executing until all values ​​are reached.

The data read from different physical devices are very difficult to synchronize. "Signage" is the client's ability to control the order in which data are written ios. For example, if a recipe to download using a Flag "Recipe Download Complete", you must ensure that this flag is written at the end of the data.

OPC BENEFITS

Include:

• Plug & Play connectivity between software and hardware from different manufacturers. Components can be easily integrated in large systems and business automation.

• Cost of hardware and software using a single standard. The OPC server drivers replace the I / S owners of each device. OPC client applications such as HMI or SCADA software can interoperate with any OPC server. Software vendors can concentrate on developing new features, instead of dedicating to program a lot of drivers.

• The technology can be used beyond E / S hardware more complex system of business control and DCS, SCADA, HMI, beat control, maintenance and other applications requiring data exchanged in real time.

QUALITY FLAGS

Represent the state of quality data value of the item.

The least significant 8 bits of the word quality is defined in three areas: quality and status substatus limit, in the following arrangement.

QQSSSSLL

The most significant 8 bits are available for vendor specific use.

Table 3.1 Field Quality