Connecting OPC Classic to OPC UA with OPC UA Gateway

UPDATED 21.06.2022 to use the term OPC Classic for the DCOM based protocols.

We get frequent questions about how to use OPC Unified Architecture in conjunction with OPC Classic servers. Regardless of the new technical details of OPC UA, we cannot get around the fact that the majority of the communication systems worldwide are still running OPC Classic servers.

OPC UA is not directly backwards compatible with OPC Classic. Therefore if one wants to use OPC Classic and OPC UA together, interfaces for both would have to be implemented. Still, it is not necessary to avoid using OPC UA because of this. OPC UA Gateway will solve this issue for you!

Basic Usage

OPC UA Gateway can be connected to servers using multiple protocols: OPC UA, OPC DA, OPC AE and OPC HDA. All these connections are published to other clients as OPC UA, OPC DA OPC AE and OPC HDA servers. The software comes with a graphical user interface and with it connections to servers and server endpoints can be configured. Next, I will present two common use cases with OPC UA Gateway.

Note that OPC DA, AE and HDA are the so called OPC Classic protocols and they are all using Microsoft DCOM.

Use case #1

In the first use case, an OPC UA client is connected to OPC DA (Data Access) or AE (Alarms and Events) Classic servers and an OPC UA server through a firewall. This is an everyday use case when trend data and process event messages need to be transferred from the process to higher-level information systems, such as Manufacturing Execution Systems (MES). OPC Classic uses DCOM for communication between computers, and this makes firewall configuration complex, since DCOM assigns port numbers dynamically for connections. Therefore it is best to use OPC UA to communicate through a firewall because OPC UA uses only one TCP port for communication. In this use case, only one OPC UA Gateway is needed to aggregate data from three servers. OPC UA Gateway eliminates the need for other products that would combine data from multiple OPC servers.

OPC UA Gateway Firewall diagram
OPC UA Gateway used to connect an OPC UA client to multiple servers through a firewall.

Use case #2

In the second use case, an OPC DA client is connected to an OPC DA server over the Internet. OPC DA uses DCOM to communicate, and communication with DCOM over Internet cannot be done efficiently. Fortunately, OPC UA uses single port TCP communication, which can be easily used over the Internet. OPC DA nodes can be connected to separate OPC UA Gateways, which communicate with OPC UA to each other. In the OPC UA Security Model, encryption and authentication methods are introduced, which assure safe and confidential communication over the Internet.

OPC UA Gateway Internet diagram
OPC UA Gateway is used to connect an OPC DA client to an OPC DA server over Internet.


OPC UA Gateway is a simple solution to start upgrading your systems to using OPC Unified Architecture. With it, the pitfalls of OPC Classic can be avoided. It allows you to use your old systems in harmony with newer ones. There is no question of either/or, with OPC UA Gateway, both OPC UA and OPC Classic can co-exist in an industrial communication system.

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