OPC Data Access 2.04 接口规范

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"OPC DA 2.04是OPC数据访问接口的规范，用于定义开发者构建OPC Data Access客户端和服务器的接口标准。该规范旨在通过分析和设计过程，创建一个标准化接口，以促进多供应商之间的无缝协作，确保不同厂商开发的服务器和客户端能够顺利互操作。该文档适用于Windows 95、Windows NT 4.0或更高版本的操作系统，以MS-Word格式发布，由OPC Foundation编写并发布，版本号为2.04，发布日期为2000年9月5日。" OPC DA（OPC Data Access）是OPC（OLE for Process Control，对象链接与嵌入过程控制）的一部分，是一个工业标准，它定义了自动化设备和软件如何交换实时和历史数据。OPC DA 2.04版本是这个标准的一个重要里程碑，提供了更完善的互操作性和兼容性。在OPC DA 2.04规范中，主要包含以下几个关键知识点： 1. 接口定义：OPC DA定义了一组COM（组件对象模型）接口，使得客户端可以透明地访问来自不同来源的数据，而无需关心底层服务器的具体实现。这些接口包括了读取、写入、订阅、事件处理等功能。 2. 数据模型：OPC DA的数据模型基于服务器项（Server Items），每个项代表一个可读可写的变量。这些项可以是物理设备的测量值，也可以是计算值或其他复杂的数据结构。 3. 通信模型：OPC DA使用客户端-服务器架构，客户端通过调用服务器提供的接口来请求数据。同时，服务器可以推送数据更新到订阅了相关项的客户端，实现了双向通信。 4. 服务质量：规范中考虑了实时性和可靠性，定义了不同级别的数据更新率和错误处理机制，以满足不同的应用需求。 5. 安全性：尽管原始的OPC DA规范在安全方面可能较弱，但随着OPC UA（OPC统一架构）的出现，安全性得到了显著提升。OPC DA 2.04可能包含了对基本认证和权限管理的支持。 6. 兼容性和互操作性：OPC DA 2.04的目标是确保不同供应商的OPC DA服务器和客户端能够无缝协作，这意味着它们必须遵循同一套接口和通信协议，以实现跨平台的数据交换。 7. 错误处理和诊断：规范中包含了错误代码和诊断信息的定义，以便在出现问题时能够快速定位和解决。 8. 示例和实现指南：除了接口规范，OPC DA 2.04可能还包括了一些示例代码和实现指南，帮助开发者理解和实现OPC DA接口。 OPC DA 2.04的发布，对于自动化和制造业领域的软件开发具有重要意义，因为它降低了开发成本，提高了系统的互操作性，从而促进了整个行业的技术进步。

OPC Data Access Custom Interface Specification 2.04

It is expected that this server address space is stable and is managed within the server. The clients will

define and manage the relatively small lists of items called ‘groups’ as needed from time to time. The

interfaces described here provide the client the ability to easily define, manage, and recreate these lists

as needed through the use of ‘OPCGroups’. The clients direct the server to create, manage and delete

these groups on their behalf (persistence of the groups is the responsibility of the client application).

Although it is possible, with the usage of public groups, that the server could provide persistent storage

of these type of groups, or treat them as server defined groups.

2.6 Application Level Server and Network Node Selection

OPC Data Access supports the concept of organizing client requests into groups within a server. Such

groups can contain requests for data from only one particular OPC Server object. In order to access

data, a client application will need to specify the following:

?? The name of the OPC Data Access Server (for use by CoCreateInstance, CoCreateInstanceEx,

etc.)

?? The name of the machine hosting the OPC Data Access Server (for use by

CoCreateInstanceEx)

?? The vendor specific OPC Item Definition (the name of the specific data item in the server’s

address space)

It is beyond the scope of this specification to discuss the implications of this on the architecture and

user interface of the client program.

2.7 Synchronization and Serialization Issues

By ‘synchronization’ we mean the ability of a client to read or write values and attributes in a single

transaction. For example, most applications want to insure that the value, quality and time stamp

attributes of a particular item are in ‘sync’. Also, a reporting package might want to insure that a

group of several values read together as part of a ‘Batch Report’ are in fact part of the same batch.

Finally, a recipe download package would want to insure that all of the values in the group were sent

together and that the recipe was not started until all of the values had been received. These are just a

few examples where synchronization is important.

The short answer is that OPC itself cannot insure that all of these synchronization tasks can be

accomplished. Additional handshaking and flag passing between the client application and the device

server to signal such states as ‘ready’ and ‘complete’ will be required. There are also things that need

to be specified about the behavior of OPC servers to assure that OPC does not prevent this sort of

synchronization from being done.

It will be seen later that OPC allows explicit reads and writes of groups of items or of individual items

as well as exception based data connections (OnDataChange). Without jumping ahead too far it is

possible to make some general observations about these issues and about server behavior.

1. In general, OPC Servers should try to preserve synchronization of data items and attributes that are

read or written in a single operation. Synchronization of items read or written individually in

separate operations is not required. Clearly, data read from different physical devices is difficult

to synchronize.

2. Reads and writes of data items which can be accessed by more than one thread must be

implemented to be thread safe, to the extent that data synchronization is preserved as specified in

this specification. Examples of where this is important might include: logic within a server where

one thread services method executions while a separate thread performs the physical

communications and writes the received data into a buffer area which is shared with the first

thread. Another example might be the logic in a handler or proxy where a ‘hidden’ RPC thread

servicing an OnDataChange subscription is writing data into a shared buffer which a thread in the

client might be reading.

OPC Data Access Custom Interface Specification 2.04

3. Threading issues are always important but this is especially true on SMP systems.

By ‘Serialization’ we mean the ability of the client to control the order in which writes are performed.

1. It is STRONGLY RECOMMENDED that write requests to the same device be handled ‘in-order’

by any server implementation. For example, an application might use a ‘recipe download

complete’ flag which is set by the application after the individual recipe items are sent. In this

case, the data must be transmitted to the physical device in the same order it was output to insure

that the ‘complete’ flag is not set before all the data has actually arrived. Where the server buffers

the outgoing data and implements a separate communications manager thread to send these

outputs to the physical device (as is often the case), the server implementation must take extra care

to insure that the order of the outputs is preserved.

2. Where a client can both read values explicitly or receive updates via a callback attention must be

given to defining exactly when a callback will or will not occur. This is discussed in more detail

later.

Many of these issues will be clarified in the detailed descriptions of the methods below.

2.8 Public (aka shared) Groups

The purpose of the public group concept is to provide a way to share data configuration information

across multiple client applications. Typically, in process control systems, multiple client applications

are configured to monitor or control the same process control data using the same applications or tools.

A public group can be created, such that only one application / end-user defines the items, and other

client applications access the information in the public group by connecting to it. This facilitates

keeping the definitions of the same data in sync, since only one client has to create and configure the

attributes of the data items.

Because the information is shared across multiple clients, some restrictions may be required to make

sure that the configuration information across multiple clients remains consistent.

2.9 Persistent Storage Story

OPC Servers may implement an optional interface to facilitate OPC clients telling an OPC server to

persistent (store) the OPC server configuration information. OPC Server configuration information

may include information about the devices and data source necessary to facilitate communication

between the data source and the OPC server. Client configuration information, including the groups

and items, are not persistently stored by an opc server.

Clients are responsible for the configuration and persistent storage of the groups and items that are

required by their application..

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