What is OPC UA?OPC Unified Architecture
) is a industrial M2M
) communication standard.
This specification is defined and updated by the organization OPC Foundation (see https://www.opcfoundation.org).
Contrary to the original OPC specification that is based on the COM/DCOM technology by the Microsoft company (and therefore only functional in OS Windows) the OPC UA technology is based on commonly used communication standards like TCP/IP, HTTP and SOAP. It means that the OPC UA can work also on other platforms than Windows. OPC UA communication can also be built into PLCs and other devices.
The main requirements for the new standard were:
- platform independence
- multi-threaded, as well as single-threaded/single-task operation
- security based on new standards
- configurable time-outs for each service
- chunking of big datagrams
Contrary to OPC Classic
that defines process data access (OPC DA
), alarm data access (OPC AE
) and historical data access (OPC HDA
) separately, the new OPC UA
does not define these specific approaches, but only the format of messages that are being transmitted. It means that the OPC UA
standard allows transmission of all process data, alarms and historical data.
The OPC UA
communication supports two protocols. For application designers, the difference is only in URL:
- binary protocol URL specification: opc.tcp://Server
- Web service (SOAP) protocol: http://Server
Contrary to the network OPC Classic
, the OPC UA
communication does not require DCOM interface setup
. OPC UA
is a network communication by its basic principle. It means that it must employ mechanisms that provide network communication security. OPC UA
communication uses electronic signatures (certificates) in order to provide authentication
, encryption and data integrity.
OPC UA security
Each application - participant of OPC UA
communication (OPC UA
server, client or gateway) must have its own installation of application certificate, that unambiguously identifies the application and the device (computer) it is running on. OPC UA
defines 4 basic levels of security:
Level 1 – no authentication
In this case, both the client and server allow all communication. It means that all valid certificates are considered to be trusted. Application certificates provide only unverifiable information regarding the opposite side. The receiver has no means to verify the legitimity of the provider certificate. On this level, both sides automatically accept valid certificates even if these are not listed among trusted certificates. This security level does not require any setting on client side or on server side.
Level 2 – server authentication
In this case, the server allows connection of any client. Client verification (if required) is done by entering login name and password and sending these to server after the communication channel is secured. All clients must trust the server certificate. This setting is done by Administrator on the client side (the server public key must be explicitly listed in the trusted certificate list, or the server certificate must be issued by trusted certification authority). If the server certificate is not explicitly listed in trusted certificate list, then the client has to compare the DNS name in the server certificate with the DNS name of the computer it is connecting to. This procedure cannot ensure that the client connects to the correct server (OPC UA), but it can ensure it connects to the correct computer. This procedure provides reasonable level of security (similar approach is used e.g. for personal access to internet banking Web sites), but the server cannot restrict access of client applications based on their authentication.
Level 3 – client authentication
In this case, the client can connect to any server, but the server allows connection only of trusted clients. This approach is used in situations where the access must be granted only to trusted clients while there is no requirement of server legitimity. The server provides data only if the client certificate is trusted. This setup is done by system Administrator on the server (the client certificate must be explicitly listed in the trusted certificate list, or the client certificate must be signed by trusted certification authority).
Level 4 – authentication on both sides (client and server)
In this case, both the client and the server allow connection only of trusted partners. This procedure provides highest level of security, but require setup on both sides (client and server). If the server certificate is not explicitly trusted, then the client follows the same way as on level 2. This approach provides highest security and therefore is recommended by the OPC Foundation to be used as default for all clients and servers.
Communication OPC UA in the PROMOTIC system using the converter to OPC DA
The OPC UA
communication can be managed by means of the software converters OPC UA to OPC DA
From the PROMOTIC software point of view, the usage of OPC UA converters is very simple - in the PROMOTIC system the PmaOpcDaClient object must be added, the OPC UA converter is then selected as server and then the desired variables are mapped in the PROMOTIC system.
OPC UA converter must be installed on the same PC where the PROMOTIC application is running (in order to avoid the network OPC communication).
1. General setup of software OPC UA converters
In order to use these converters in the PROMOTIC software it is necessary to set them up correctly.
Following setup steps must be completed:
1. Create (or import) of certificate for OPC UA converter.
2. Definition of connectivity of the converter to OPC UA server including the connection security setup.
3. Set the OPC UA server certificate as trusted in the OPC UA converter.
4. Set the OPC UA converter certificate as trusted in the OPC UA server.
5. In the PROMOTIC application use OPC DA communication with OPC server that is running in the OPC UA converter.
2. Recommended OPC UA software converters