What Is MAESTROrecsim OPC 2 MODBUS RTU?

In industrial automation systems, PLCs, DCS systems, process control software, energy management applications, and automation platforms from different manufacturers can make their real-time data available to other software through OPC DA servers. However, some SCADA, HMI, recorder, or data acquisition systems may not operate directly as OPC DA clients; they may support only a different communication interface, such as serial MODBUS RTU.

MAESTROrecsim OPC 2 MODBUS RTU connects to an OPC DA server as a client, browses the tag structure defined on the server, retrieves the tag values selected by the user, and publishes them in the MODBUS RTU Holding Register area over serial communication. This allows process data accessible through OPC DA to be read by an upper-level system operating as a MODBUS RTU Master.

Basic concept: OPC DA Server / Tags β†’ MAESTROrecsim OPC 2 MODBUS RTU β†’ MODBUS RTU Serial Line β†’ SCADA / HMI / PLC / Data Acquisition System

This architecture makes it possible to use selected process data through a serial MODBUS RTU interface without modifying the existing OPC DA infrastructure or adding a new communication driver to the source application. It can be particularly useful for sharing data with existing automation systems that use serial communication instead of Ethernet-based communication.

Operating Principle and Main Screen

A total of 192 data points can be defined in the program. The main monitoring screen displays 24 data points simultaneously in real time. The required group of 24 data points can be selected by clicking the page numbers 1–8 at the bottom of the screen.

MAESTROrecsim OPC2MSBSRTU MODBUS RTU main monitoring screen

Page and Data Capacity:

  • Page 1 β†’ Data 1–24
  • Page 2 β†’ Data 25–48
  • Page 3 β†’ Data 49–72
  • Page 4 β†’ Data 73–96
  • Page 5 β†’ Data 97–120
  • Page 6 β†’ Data 121–144
  • Page 7 β†’ Data 145–168
  • Page 8 β†’ Data 169–192

Each page displays 24 data points belonging to the corresponding group. Values read from the OPC DA server are updated on the program's live monitoring screen.

The red dot button at the bottom of the screen activates automatic page switching. When enabled, 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 observe all 192 data points in sequence without manually changing pages.

Discovering OPC DA Servers and Selecting Tags

When the button in the lower-left corner of the main screen is clicked, the OPC DA configuration window opens on the right side of the program. By entering the OPC Server ID and the IP address of the computer hosting the server, OPC DA servers on that computer can be discovered.

OPC DA server discovery and tag selection window

When one of the discovered OPC DA servers is selected, the tags defined on that 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 makes it possible to select only the data that needs to be transferred over the serial MODBUS RTU line from the potentially extensive tag space available on the OPC server.

  1. Click the button to open the OPC configuration window on the right side of the program.
  2. Enter the IP address of the computer hosting the OPC DA server.
  3. Enter the relevant OPC Server ID to discover the available servers.
  4. Select the required OPC DA server from the discovered servers.
  5. Select the tags to be published through MODBUS RTU from the server's tag tree.
  6. Verify that the selected tags have been added to the 192-point MODBUS list in the correct order.

OPC DA Tag Structure: OPC DA servers generally expose process variables within a hierarchical address space. The tree view makes it easier to navigate between folders and tag groups and select the required process variable. The OPC DA standard defines real-time data access; data access may include fields such as value, quality, and timestamp.

MODBUS RTU Publishing and Data Format

MAESTROrecsim OPC 2 MODBUS RTU publishes the selected data read from the OPC DA server through a serial communication line using the MODBUS RTU protocol. In this architecture, the program operates as a MODBUS RTU Slave. The SCADA, PLC, HMI, or data acquisition system that reads the data acts as the MODBUS RTU Master.

Data Publishing Format:

  • Each data point β†’ 4 bytes (32 bits)
  • Each data point β†’ 2 Γ— 16-bit MODBUS Holding Registers
  • 192 data points β†’ Total capacity of 384 Γ— 16-bit registers

Each 32-bit value is made available using two MODBUS Holding Registers. To ensure that the value is interpreted correctly by the upper-level system, both registers must be read together, and the 32-bit data interpretation and byte/word order used by the device or driver must be compatible.

MODBUS registers are 16-bit data fields. Therefore, a single 32-bit process value is carried across two registers. MODBUS read configurations in the upper-level system must be arranged according to the gateway's data mapping and two-register data structure.

Addressing Note: The way MODBUS clients represent register addresses as zero-based or one-based 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 upper-level system during commissioning.

Serial Communication Parameters

MODBUS RTU is a MODBUS communication mode that operates over a serial communication line rather than Ethernet or TCP/IP. Communication is performed through the computer's serial port. The physical connection can be provided through serial interfaces such as RS-232 or RS-485. RS-485 is particularly common in industrial automation systems for communication between multiple devices over the same serial line.

The serial communication parameters required for MODBUS RTU publishing can be configured in the upper section of the program.

Parameter Description
PORT Specifies the computer serial port to be used for MODBUS RTU communication, such as COM1 or COM3.
STOP BITS Defines the number of stop bits used to indicate the end of a serial character.
DATA BITS Specifies the number of data bits transmitted in each serial character. The standard character format in MODBUS RTU applications uses 8 data bits.
PARITY Specifies the parity method used for error checking in serial characters. The available options may include None, Even, or Odd.
BAUDD Specifies the serial communication speed, such as 9600, 19200, or 38400 baud.
ID Specifies the MODBUS RTU Slave address. The Master device identifies the Slave device it wants to communicate with using this address.

Serial Communication Example:

  • PORT β†’ COM3
  • BAUDD β†’ 9600
  • DATA BITS β†’ 8
  • PARITY β†’ Even
  • STOP BITS β†’ 1
  • ID β†’ 1

These values represent an example serial communication configuration. During actual operation, the communication parameters on the gateway and MODBUS RTU Master must match.

Communication Note: The serial port, baud rate, data bits, parity, and stop bits parameters must be compatible between devices connected to the same serial line. In addition, the MODBUS RTU Slave ID must match the address used by the Master device in its read requests. For RS-485 connections, line polarity, termination, and cabling must also be configured correctly.

Program Settings

Options related to the OPC connection, data display, and operating behavior can be configured from the settings section on the left side of the program. Changes are saved using the (SAVE) button. To transfer the settings to the main application, the (APPLY) button on the main screen on the right must also be clicked.

Setting Function
CONT Automatically restarts the application if the program is closed.
WEB Enables the integrated WEB Server. Published data and MODBUS addresses can be monitored live in a web table.
Nokta Aktif Uses a period (.) as the decimal separator. If not selected, a comma (,) is used.
OPC Yenileme Aralığı Specifies the time interval used for periodic OPC connection renewal.
OndalΔ±k Hane Basamak SayΔ±sΔ± Specifies the number of decimal places used when displaying data.

Saving and Applying Settings:

  1. Configure the required options and numerical settings.
  2. Click the SAKLA button to save the configuration.
  3. Click the UYGULA button on the main screen on the right.
  4. Check the OPC connection and live data values.
  5. Verify serial communication through the MODBUS RTU Master.

Clearing and Applying the Data List

Tags selected from the OPC DA server are transferred to the program's 192-point MODBUS list. The (RESET) button is used when the existing list needs to be completely cleared. This operation resets the 192-point list; OPC tags must be selected again to repopulate it.

Warning: The (RESET) button resets the existing 192-point definition list. Before performing this operation, make sure that any tag selections and configuration that need to be preserved have been saved. After changing the list or settings, click the (APPLY) button on the main screen on the right.

Integrated WEB Server

When the option in the settings section is checked, the integrated WEB Server can be activated. This allows the data published by the gateway through MODBUS RTU, together with the corresponding MODBUS addresses, to be displayed live in a table through a web browser.

MAESTROrecsimOPC integrated web server monitoring table

The WEB table helps users check values read from OPC DA tags and their MODBUS addresses during commissioning and operation. This screen is intended for monitoring and verification purposes. Viewing data through the WEB Server is not a data transfer protocol that replaces MODBUS RTU serial communication.

Program Continuity and Periodic OPC Connection Renewal

When the option is enabled, the program is configured to restart itself automatically if it is closed. This option can be used in installations where the gateway application is expected to run continuously on the operating computer.

The OPC Yenileme Aralığı (OPC Update Interval) setting allows the OPC DA server connection to be renewed at specified intervals. The renewal interval is configured in the corresponding field in the settings section. This feature can be used in scenarios where the OPC connection is expected to be re-established periodically during long-term operation.

Operational Note: Automatic program restart or periodic OPC connection renewal does not, by itself, eliminate problems such as computer shutdown, power failure, network faults, physical problems on the serial communication line, or an unavailable OPC server. In applications requiring continuous operation, the computer, OPC DA server, serial port, communication line, and DCOM configuration should be evaluated together.

Network and Security Requirements for OPC DA Connections

OPC DA is part of the OPC Classic family and uses Microsoft COM/DCOM technology. If the OPC DA server and the gateway are running on the same computer, the connection can be established through local COM. If they are running on different computers, Windows network access, DCOM permissions, user accounts, and firewall rules must be configured correctly.

The OPCEnum service may be used to discover OPC servers on remote computers. Therefore, when troubleshooting server discovery and connection problems, the status of the OPCEnum service on the target computer, the OPC Server ID/ProgID information, and DCOM access permissions should be checked.

Important: Remote OPC DA connections may involve TCP port 135 and dynamically assigned RPC ports used by the system for DCOM/RPC communication. Firewall and network rules should be configured to permit access only to the required clients and servers, in accordance with the organization's information security policies. DCOM settings may vary depending on the operating system and OPC server vendor.

Typical Applications

Making OPC DA data available through MODBUS RTU allows process information in an existing OPC infrastructure to be transferred to different systems equipped with a serial MODBUS RTU Master interface. This gateway approach can be particularly useful in architectures where the upper-level system uses MODBUS RTU over RS-232 or RS-485 serial communication instead of Ethernet.

Application Potential Use
Integration with Legacy SCADA Systems Transferring process values obtained from an OPC DA server to legacy SCADA systems that support only a serial MODBUS RTU Master interface.
Data Sharing Between PLCs and Control Systems Making selected variables provided through OPC DA readable by PLCs or control systems equipped with a MODBUS RTU Master interface.
Serial Communication-Enabled HMI Systems Transferring process values from an OPC DA infrastructure to HMI panels using serial MODBUS RTU communication.
Energy Monitoring and Management Enabling energy management systems or data acquisition devices to obtain power, energy, and status information available in OPC DA tags through MODBUS RTU.
Industrial Recorders Allowing recorder and data logging devices with serial MODBUS RTU Master support to monitor selected process variables obtained through OPC DA.
Production and Process Monitoring Transferring temperature, pressure, flow, level, and production values provided through OPC DA by DCS, PLC, or process control software to another system supporting serial MODBUS RTU.
HVAC and Building Automation Making boiler, chiller, air handling unit, and building control data collected by an OPC DA server available to serial MODBUS RTU-based monitoring systems.
Preserving Existing Automation Investments Making existing data accessible through an OPC DA server available to new systems requiring a serial MODBUS RTU interface, without reconfiguring the original data sources.
Testing, Commissioning, and System Interconnection Enabling the verification of serial communication and data mapping by transferring OPC DA tags to the MODBUS RTU register area.

Architecture Note: MAESTROrecsim OPC2MSBSRTU makes selected tag values from the OPC DA server available in the MODBUS RTU Holding Register area. Therefore, access to the OPC DA server and access by the MODBUS RTU Master device to the gateway over the serial line are two separate communication paths. The network and DCOM configuration on the OPC side, and the serial port, line connection, and communication parameters on the MODBUS RTU side, must be verified separately.

Recommended Startup and Operating Sequence

  1. Check network access to the computer running the OPC DA server and verify the required DCOM permissions.
  2. Start the program and select the serial PORT to be used.
  3. Set the BAUDD, DATA BITS, PARITY, and STOP BITS values for MODBUS RTU communication.
  4. Enter the ID address to be used for the MODBUS RTU Slave.
  5. Click the button in the lower-left corner to open the OPC configuration window.
  6. Enter the IP address of the OPC DA server and the OPC Server ID to discover the available servers.
  7. Select the discovered OPC DA server and choose the tags to be transferred from its tree structure.
  8. Check that the 192-point list has been populated with the correct tags.
  9. Configure the required options and specify the OPC renewal interval and the number of decimal places.
  10. Save the settings using the (SAVE) button and click (APPLY) on the main screen on the right.
  11. Navigate through pages 1–8 on the main screen and check that the data is updated live in groups of 24.
  12. Configure the MODBUS RTU Master connection in the upper-level SCADA/HMI or PLC system. Set the serial communication parameters and Slave ID, and configure the read operations taking into account that each data point uses two Holding Registers.
  13. Enable the and options if required.

Download MAESTROrecsim OPC 2 MODBUS RTU Gateway

MAESTROrecsim OPC 2 MODBUS RTU is designed to retrieve selected process data from OPC DA servers and transfer it to upper-level systems through serial MODBUS RTU communication. By publishing OPC DA tag values in the MODBUS RTU Holding Register area, it allows SCADA, HMI, PLC, and data acquisition systems to access this data over serial communication. Each 32-bit value is made available using two 16-bit MODBUS Holding Registers.

⬇ Download MAESTROrecsim OPC 2 MODBUS RTU

Operating Requirements: The program requires access to the OPC DA server to operate correctly. For connections to OPC DA servers on remote computers, the required network access, DCOM permissions, and firewall rules must be configured. The serial port, baud rate, data bits, parity, stop bits, and Slave ID parameters used for MODBUS RTU communication can be configured within the program. The serial communication parameters of the gateway and the MODBUS RTU Master device must be compatible.

Related Files:

  • MAESTROrecsim.exe β€” MAESTROrecsim OPC 2 MODBUS RTU Gateway application
  • Program configuration files β€” Application configuration files
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