OPC Data Access (OPC DA)


OPC Data Access, or just OPC DA, provides a standard way to access real-time data from process control hardware and software. Benefits include:

  • OPC DA standardizes real-time data access
  • Communication between all devices and applications is consistent
  • OPC DA is available for every major process control system
  • OPC DA secures scalability
  • With OPC DA, the communication between all devices and applications is consistent. OPC DA servers for PLCs, DCSs, or other devices provide data in exactly the same format! Similarly, HMIs, Process Historians and other applications accept OPC data in the same format. This allows all of your process control hardware and software to freely exchange data, providing enterprise-wide interoperability.

    OPC is a powerful connectivity method. As a result, OPC servers are available for almost every major device and software application on the market today. Similarly, almost every process control application supports OPC in the form of an OPC Client connection.

    Major manufacturers have adopted OPC DA as their communication standard for the transfer of real-time data to secure their system scalability. Selecting OPC DA as your communication standard allows you to easily expand your system as needed for years to come.

    OPC DA began with OPC DA 1.0, followed shortly by OPC DA 1.0a. A later specification was OPC DA 2.0, with minor clarifications appearing periodically, such as OPC DA 2.01, 2.02, 2.03, etc. The most recent specification is OPC DA 3.0. Vendors are encouraged to keep their software current, so check with your vendor about the specific version that they support.

    OPC DA Details
  • Used only to read and write REAL-TIME data
  • Provides access to single-value data items called "points"
  • Each point includes a value, a quality and a timestamp
  • Only uses latest values, not past values

  • OPC DA is used to read and write real-time data exclusively. To access previous, or historical values, you must use OPC HDA (Historical Data Access).

    Typical questions posed by an OPC client and answered by an OPC server are "What is the current flow rate of FIC101?", "Was this reading reliable?", and "At what time was the latest flow read?"

    OPC DA Timestamps
    OPC DA servers provide three attributes for every point: a value, a quality and a timestamp.

    OPC specifies that a timestamp must be provided for each point, but it does not specify where the timestamp must come from. So where does the timestamp come from?

    Sometimes a timestamp is not available from the device. For example, Modbus does not provide a timestamp from the PLC. In this situation, the OPC server provides its own timestamp. But some device vendors do provide a timestamp with each value. So when the OPC server receives a reading, it also receives the timestamp from the PLC and passes it on.

    An OPC server may be designed to ignore a timestamp, even if one is available. Since OPC does not specify where the timestamp must come from, sometimes OPC server vendors choose to ignore the device timestamp because this takes less development effort. Always ask your vendor where their timestamp comes from.

    OPC DA Summary
    So to summarize, OPC DA provides standard access to real-time data and therefore provides open connectivity. OPC systems benefit from true interoperability and scalability that are used for monitoring and supervisory control. OPC enables applications to exchange, that is read and write, the latest values and not past values. If you need to access historical data, you must use OPC Historical Data Access (HDA).

    OPC specifies that a timestamp must be provided for each point, but it does not specify where the timestamp must come from. So where does the timestamp come from?

    OPC Data Access Specification 3.0 [PDF 7,138 kb].

    Click here to visit the MatrikonOPC DA Server section.