MAESTROrecsim OPC XML-DA is an industrial
OPC XML-DA – MODBUS TCP Gateway application
designed to read process data from OPC XML-DA servers and
make this data available to higher-level systems through a
single MODBUS TCP server.
Tag values accessible through the OPC XML-DA interface are
consolidated into a data area that can be read by SCADA,
HMI, PLC, and data acquisition applications using MODBUS TCP.
The program supports the configuration of up to
192 data points. These values can be monitored
live on the main screen across eight separate pages, with
24 data points displayed on each page.
Each data point is published as a
4-byte (32-bit) value using two 16-bit
MODBUS Holding Registers.
What Is MAESTROrecsim OPC XML-DA?
In industrial automation systems, real-time data generated by PLCs, DCSs, RTUs, and process control
software can be transferred to different applications through OPC servers. However,
some SCADA, HMI, PLC, and data acquisition systems may not directly support the
OPC XML-DA interface. Such systems may need to receive data through MODBUS TCP,
a widely used protocol in industrial communications.
MAESTROrecsim OPC XML-DA is an industrial communication gateway
application that connects to OPC XML-DA servers, reads the process data defined
on the server, and makes selected tag values accessible through a
MODBUS TCP Server.
With this architecture, process data accessible through OPC XML-DA can be read
by SCADA, HMI, PLC, and data acquisition systems that support the MODBUS TCP Client
function. This makes it easier to integrate existing OPC XML-DA data infrastructures
with systems using different communication protocols.
Basic architecture:
OPC XML-DA Server / Tags →
MAESTROrecsim OPC XML-DA →
MODBUS TCP Server →
SCADA / HMI / PLC / Data Acquisition System
The gateway transfers selected data retrieved from the OPC XML-DA server
to the MODBUS TCP Holding Register area. This enables data exchange with
systems that do not support OPC XML-DA but do support MODBUS TCP communication.
OPC XML-DA (OPC XML Data Access) is an OPC specification
defined by the OPC Foundation. It enables industrial process data to be
shared between different applications through XML-based Web Services technologies.
While classic OPC DA uses Microsoft COM/DCOM technology, OPC XML-DA
communicates through XML, SOAP, and HTTP technologies.
This approach was designed to enable OPC data to be transferred between
different applications and systems without being restricted to the
Windows COM/DCOM infrastructure.
Core technologies used by OPC XML-DA:
- XML: Represents data in a structured format.
- SOAP: Used to exchange service messages between applications.
- HTTP: Enables client-server communication through Web Services.
- WSDL: Defines the methods and message structures provided by a Web Service.
- OPC Data Access Model: Provides access to process variables along with their values, quality, and timestamp information.
OPC XML-DA was developed to bring the OPC Data Access model into a Web Services
architecture. Depending on the services supported by the server, operations
such as browsing tag lists, reading data, and subscription-based data monitoring
can be performed.
Differences Between OPC XML-DA and Classic OPC DA
| Feature |
OPC DA |
OPC XML-DA |
| Core technology |
COM / DCOM |
XML / SOAP / HTTP |
| Communication approach |
COM-based client-server interface |
Web Services-based client-server interface |
| Platform dependency |
Depends on the Windows COM/DCOM infrastructure. |
Does not depend on COM/DCOM. |
| Data representation |
OPC DA data structures |
XML-based data structures |
| Remote access |
Requires DCOM network configuration. |
Provides access through HTTP-based Web Services. |
OPC XML-DA and OPC UA
OPC XML-DA and OPC UA (Unified Architecture) are not the same
technology. OPC XML-DA was developed to provide the OPC Data Access model
through XML and Web Services. OPC UA, on the other hand, is an independent
architecture that offers broader capabilities, including information modeling,
security, alarm and event management, and historical data access.
The OPC XML-DA specification is listed by the OPC Foundation as completed
and no longer actively maintained. Although OPC UA is generally a more
up-to-date option for new system designs, integration with existing OPC XML-DA
servers in operational facilities may still be required.
Important: MAESTROrecsimOPCXML is not an OPC UA client.
It is a gateway application designed to communicate with servers that
use the OPC XML-DA interface. OPC XML-DA and OPC UA connections are
not interchangeable.
Operating Principle and Main Screen
A total of 192 data points can be configured in the program.
The main monitoring screen displays 24 data points live
at a time. By clicking the pages numbered 1–8 at the
bottom of the screen, the operator can switch to the corresponding
group of 24 data points.
Page and data capacity:
- Page 1 → Data points 1–24
- Page 2 → Data points 25–48
- Page 3 → Data points 49–72
- Page 4 → Data points 73–96
- Page 5 → Data points 97–120
- Page 6 → Data points 121–144
- Page 7 → Data points 145–168
- Page 8 → Data points 169–192
Each page displays the corresponding group of 24 data points.
Values read from the OPC XML-DA server are updated live on the
main monitoring screen.
The red dot button at the bottom of the screen starts
the automatic page-switching function. When activated, the screen
cycles sequentially through all eight pages, displaying the live
list of 24 data points on each page. This feature allows the operator
to monitor all 192 data points in sequence without manually switching
between pages.
Connecting to the OPC XML-DA Server and Selecting Tags
Clicking the 
button in the lower-left corner
of the main screen opens the OPC XML-DA configuration panel on the
right side of the program. The OPC XML-DA server address can be
entered in this panel to establish a connection to the server.
After entering the server address, click the
(DISCIVER TAG LIST) button. If the connection
is successful, the OPC tags defined on the server are displayed
in a tree structure. The user can select the required tags from
this tree and add them to the program's 192-point MODBUS list.
This allows users to select only the process variables they want
to publish through MODBUS TCP from the larger tag namespace
available on the OPC XML-DA server.
- Click the button to open the OPC configuration panel on the right side of the program.
- Enter the address of the OPC XML-DA server in the relevant field.
- Click (DISCIVER TAG LIST) to discover the tag list on the server.
- Select the required OPC tags from the displayed tree structure.
- Check that the selected tags have been added to the 192-point MODBUS list in the correct order.
OPC XML-DA tag structure: OPC XML-DA servers may
expose process variables within a hierarchical address space.
Browse operations allow the tags and groups made accessible by
the server to be discovered. This structure makes it easier
for users to locate and select the required process variables.
MODBUS TCP Publishing and Data Format
MAESTROrecsimOPC XML-DA publishes selected data read from the OPC XML-DA
server through a MODBUS TCP Server. SCADA, PLC, HMI,
or data acquisition systems connecting to the program's MODBUS TCP
interface operate as MODBUS TCP Clients.
The IP address and port number
used for MODBUS TCP publishing can be configured at the top of
the gateway interface. Since the standard TCP port for the
MODBUS TCP protocol is 502, this is the default
publishing port used by the program.
Data publishing format:
- Each data point → 4 bytes (32 bits)
- Each data point → 2 x 16-bit MODBUS Holding Registers
- 192 data points → A total capacity of 384 16-bit registers
Each 32-bit value is published using two MODBUS Holding Registers.
To interpret the data correctly in the upper-level system, both
registers must be read together, and the 32-bit data interpretation
method and byte/word order used by the device or driver must be
compatible.
MODBUS Holding Registers are 16-bit data units. Therefore, a single
32-bit process value is carried across two registers. MODBUS read
definitions in the upper-level system must be configured according
to the gateway's data mapping and two-register data structure.
Addressing note: The way register addresses
are represented as zero-based or one-based in MODBUS clients
may vary depending on the software being used. Register starting
addresses and the register/word order of 32-bit values should
be verified on both the gateway and the upper-level system
during commissioning.
Clearing and Applying the Data List
Tags selected from the OPC XML-DA server are transferred to the
program's 192-point MODBUS list. The 
(RESET)
button is used when the existing list needs to be cleared completely.
This operation resets the 192-point list. The OPC XML-DA tags must
be selected again to populate the list with new data.
Caution: The (RESET) button resets the existing
192-point configuration list. Before performing this operation,
make sure that any tag selections and configuration settings
that need to be retained have been saved. After changing the
list or settings, click the 
(APPLY) button
on the main screen on the right.
When the 
(WEB Active) option in the settings panel
is selected, the integrated WEB Server can be enabled. This allows
data published by the gateway through MODBUS TCP, together with
their MODBUS addresses, to be displayed live in a table through
a web browser.
The WEB table helps users check the values read from the OPC XML-DA
server and their corresponding MODBUS addresses during commissioning
and operation. This screen is intended for monitoring and verification.
Viewing data through the WEB Server is not a data transfer protocol
that replaces MODBUS TCP communication.
Program Continuity and Periodic Connection Renewal
When the 
(Auto-Restart Mode) option is enabled, the program
is configured to automatically restart itself when it is closed.
This option can be used in installations where the gateway application
is expected to run continuously on the operating computer.
The (Periodic Connection Renewal) option re-establishes
the connection to the OPC XML-DA server at specified intervals.
The renewal interval is configured in the relevant field in the
settings panel. This feature can be used in scenarios where the
OPC XML-DA connection is intended to be re-established periodically
during long-term operation.
Operational note: Automatic program restart
and periodic OPC XML-DA connection renewal do not, by themselves,
eliminate issues such as computer shutdown, power failure,
network faults, interruptions in MODBUS TCP communication,
or the OPC XML-DA server becoming unavailable. In applications
requiring continuous operation, the computer, OPC XML-DA server,
network connection, and MODBUS TCP communication should be
considered together.
Technical Requirements for OPC XML-DA Connectivity
OPC XML-DA uses an HTTP- and SOAP-based Web Services architecture.
Therefore, communication between the gateway and the OPC XML-DA
server takes place through the Web Services address provided
by the server.
To establish a connection, the OPC XML-DA server must be running,
the server address must be entered correctly, and the gateway
computer must have network access to the server. The HTTP service,
port configuration, and firewall rules on the server side are
also important for connectivity.
Important: Unlike classic OPC DA, OPC XML-DA
does not communicate through COM/DCOM. Therefore, DCOM permissions
and the OPCEnum discovery mechanism encountered in classic OPC DA
connections are not fundamental requirements for OPC XML-DA
connectivity. Instead, the Web Services address, HTTP access,
server configuration, and any authentication requirements
should be checked.
The services provided by OPC XML-DA servers and their connection
requirements may vary depending on the vendor and server configuration.
Particularly for remote connections, network access, firewall settings,
and the server's access policies should be evaluated together.
Making process data from OPC XML-DA servers available through
MODBUS TCP allows existing Web Services-based OPC infrastructures
to be used with different automation systems that have a MODBUS TCP
Client interface. This gateway approach can be particularly useful
when integrating systems that support OPC XML-DA but need to exchange
data through MODBUS TCP.
| Application |
Potential Use |
| Integration of existing OPC XML-DA systems |
Transferring process data from OPC XML-DA servers to systems that support a MODBUS TCP Client interface. |
| Data exchange between SCADA systems |
Making process variables accessed through OPC XML-DA readable by different SCADA systems supporting MODBUS TCP. |
| Data transfer between PLCs and control systems |
Providing selected variables from an OPC XML-DA server to PLCs and control systems that support the MODBUS TCP Client function. |
| HMI and operator panels |
Displaying process values from an OPC XML-DA infrastructure on HMI panels that support MODBUS TCP communication. |
| Energy monitoring and management |
Enabling energy management systems or data acquisition applications to retrieve power, energy, and status information from OPC XML-DA tags through MODBUS TCP. |
| Industrial recording and data acquisition systems |
Allowing recorders and data logging systems with MODBUS TCP Client support to monitor selected process variables obtained through OPC XML-DA. |
| Production and process monitoring |
Transferring temperature, pressure, flow, level, and production values exposed through OPC XML-DA by PLC, DCS, or process control software to another system. |
| HVAC and building automation |
Making boiler, chiller, air handling unit, and building control data collected by an OPC XML-DA server available to MODBUS TCP-based monitoring systems. |
| Enterprise and higher-level applications |
Transferring selected process values obtained from a Web Services-based OPC XML-DA infrastructure to other applications accessible through MODBUS TCP. |
| Preserving existing automation investments |
Providing data from an existing OPC XML-DA server to new systems requiring a MODBUS TCP interface, without replacing the original data source. |
| Testing, commissioning, and system interconnection |
Mapping OPC XML-DA tags to the MODBUS Holding Register area to verify data mapping and communication. |
Architecture note: MAESTROrecsimOPCXML exposes
selected tag values from an OPC XML-DA server through the
MODBUS TCP Holding Register area. Therefore, access to the
OPC XML-DA server and access to the gateway by MODBUS TCP
Client devices are two separate communication paths.
Web Services access on the OPC XML-DA side and the IP address,
port, and network configuration on the MODBUS TCP side must
be verified separately.
Recommended Startup and Operating Sequence
- Check that the OPC XML-DA server is running and accessible from the gateway computer.
- Start the program and specify the IP address and port number to be used for MODBUS TCP publishing.
- Verify the IP address and port information that the upper-level system will use for its MODBUS TCP connection. Note that the default port is 502.
- Click the button in the lower-left corner to open the OPC XML-DA configuration panel.
- Enter the address of the OPC XML-DA server in the relevant field.
- Click (DISCIVER TAG LIST) to display the tags defined on the server.
- Select the tags to be published through MODBUS TCP from the tree structure.
- Check that the 192-point MODBUS list contains the correct tags.
- Configure the required options, including the connection renewal interval and the number of decimal places.
- Save the settings using
(SAVE), then click (APPLY) on the main screen on the right.
- Navigate through pages 1–8 on the main screen and check that the data is updated live in groups of 24.
- Configure the MODBUS TCP Client connection in the upper-level SCADA, HMI, or PLC system. Set up the read definitions taking into account that each data point uses two Holding Registers.
- Enable the (WEB Active) and (Auto-Restart Mode) options if required.
Download MAESTROrecsim OPC XML-DA Gateway
MAESTROrecsim OPC XML-DA is designed to read selected process data
from OPC XML-DA servers and transfer it to upper-level systems
through a MODBUS TCP server. By publishing OPC XML-DA tag values
in the MODBUS TCP Holding Register area, it enables SCADA, HMI,
PLC, and data acquisition systems to access this data through
MODBUS TCP. Each 32-bit data value is provided using two
16-bit MODBUS Holding Registers.
Runtime requirement: The program requires
access to an OPC XML-DA server to operate correctly. The
server's Web Services address must be configured correctly,
and the gateway computer must have network access to the
relevant HTTP/HTTPS service. Firewall rules on the server
side and any authentication requirements must also be
configured appropriately. The IP address and port number
used for MODBUS TCP publishing can be configured through
the program. The default publishing port is
502.
Related files:
-
MAESTROrecsim.exe —
MAESTROrecsim OPC XML-DA Gateway application
-
Program configuration files —
Application configuration files
