The following instructions are for both Modbus RTU (TModBusRTUDriver class) and Modbus TCP (TModBusTCPDriver class). To configure a protocol+tag connections you have to do:

  1. Insert a communication port appropriate for the chosen Modbus protocol:
  2. Insert the chosen protocol component;
  3. Connect the protocol to the port, through the “CommunicationPort” property;
  4. Insert the tags manually or through the “Tag builder” tool
  5. If you insert the tags manually, set the properties as described below;
  6. Connect the tag to the protocol driver through the ProtocolDriver property present in each tag.

Both protocol classes support the following tag classes:

  • TPLCTagNumberplctagnumber
  • TPLCBlockplcblock
  • TPLCStructplcstruct
  • TPLCStringplcstring


To configure a tag to use Modbus, you must configure the following tag properties:

  • PLCStation: Modbus equipment address. For Modbus TCP, this property has its value set to 1, it usually depends on the Modbus Server implementation.
  • MemAddress: Address of the input/output/register that you want to read/write. The addresses start from zero, always. Do not use the address notation 1xxxxx, 2xxxxx, 3xxxxx, 4xxxxx, as it is not supported.
  • MemReadFuntion: Function that will be used to read the tag. See table below.
  • MemWriteFuntion: Function that will be used to write tag values. See table below.

For the MemReadFunction and MemWriteFunction properties the following values are accepted according to the desired memory area:

DESIRED AREA MemReadFunction MemWriteFunction
Digital inputs 2 0
Coils (digital outputs) 1

5 (TPLCTagNumberplctagnumber)

15 (TPLCTagNumberplctagnumber TPLCBlockplcblock TPLCStructplcstruct TPLCStringplcstring)

Registers 3

6 (TPLCTagNumberplctagnumber)

16 (TPLCTagNumberplctagnumber TPLCBlockplcblock TPLCStructplcstruct TPLCStringplcstring)

Analog registers (analog inputs) 4 0
Device status 7 0

You need to know the ModBus functions your device supports.

16 thoughts on “Modbus RTU and TCP

  1. Popescu Dragos Reply

    I need to read digital inputs from an ICPDAS modbus module, M-7054, using standard 02 function “(0x02) Read discrete inputs”.
    The string to send into module is “adr_byte 0x02 start_input (2bytes) count (2bytes) CRC (2bytes)”. The response will be according to the standard.

    Please tell me how do I do this, using PascalScada Tags?
    I’m using 0.7.3 version.

    • Fabio Luis Girardi Post authorReply


      Few steps:

      1) Insert a TSerialPortDriver into your app and set up it with the same serial settings of your device and activate it.
      2) Insert a TModbusRTUDriver and set CommunicationPort Property to point to Serial Port created on the previous step.
      3) Insert a TPLCTagNumber and set:
      3.1) MemAddress to point to your Digital Input (address starts from zero – 0)
      3.2) MemReadFunction to 2 “(0x02) Read discrete inputs”
      3.3) PLCStation to point to your Modbus slave address;
      3.4) Set property ProtocolDriver to point to ModbusDriver created in step 2.
      3.5) Assure that TagType is pttDefault or pttBit;

      4) Put a THMILabel into your app/form and link it with the tag created at step 3.

      5) If everything is set accordingly, you will see the label showing values 0 or 1, depending on the state of your digital input. The digital input can vary depending on your Modbus device.

  2. jayakrishnan Reply

    Hi fabio

    i have installed the software and done some basic works like configuring modbus device etc, after that i was trying to display the datas in the chart but i was unable to bind the tags to the chart. Could you pls instruct me how to do this and following features like alarming and reporting or pls provide me a deatiled documentation of those above mentioned .

  3. jayakrishnan Reply

    Hi fabio,

    i have installed pascal scada and done some stuff like creating tags and configured a modbus tcp device. when i try to get data of the device to be displayed in the charts provided in the package i was not able to bind the tags to chart. Could you pls tell me how to bind the tags and also an explanation about some features like alarms, reporting etc. Pls provide me a deatiled documentation .

  4. Fabio Luis Girardi Post authorReply


    There are missing features on PascalSCADA, due to I have tried to keep it working with Lazarus and Delphi:

    ** Alarm historic
    ** Historic
    ** Recipes
    ** User management stored in file/database;
    ** Tag chart connection.

    Since I cutted off the Delphi support, some things have better progress, since I don’t need anymore to think multiple IDE support. I have two things started, but not finished:

    ** Lazarus TAChart series datasource linked with PLC Tags (started, near to be finished)
    ** Database abstraction (I’m researching the better way to achieve this)

    So, for now, I’ll have to do your chartings using the components of TAChart with few lines of code, to add new values and delete old values.

  5. Konrad Reply

    Hi, Does PascalSCADA supports MODBUS ASCII protocol?
    I see WEST ASCII there, but I think it is quite different from MODBUS ASCII.

    • Fabio Luis Girardi Post authorReply

      Hi Konrad!

      Modbus ASCII isn’t supported yet. At least, I don’t reserved time to do that and anybody else…

      Also, you are the first that had requested this.

  6. Boban Spasic Reply

    Hi Fabio,

    I have a Project with 1xModBusRTU and 4xModBusTCP drivers (I want to read from 4 IP addresses).
    The problems start when I want to close Lazarus and the Project is open in Lazarus – Lazarus locks my Windows10 with 100% CPU usage.
    The compiled project does the same – 100% CPU usage.
    OS: Windows 10 64bit, Lazarus versions tested 2.0.10 and 2.2.0RC1
    I would send you my code if you want to take a look at it.

    With just one ModBusRTU and one ModBusTCP drivers it works fine.

    • Fabio Luis Girardi Post authorReply

      Can you test the same application on Windows 7 environment? I don’t have any Windows 10 environment configured.

    • Fabio Luis Girardi Post authorReply

      Or if you have time and can share your environment with me, I can investigate using anydesk/teamviewer… If you agree, please send e-mail to fabio at pascalscada dot com

      • Boban Spasic Reply

        I apologize for late answer. I think I solved the problem.
        I have 4x TTCP_UDPPort and if I set the events for all the error handling (OnCommErrorReading … OnCommPortOpenError) for every port – here is the problem with 100% CPU usage.

        Here is my code for the events (all 4 ports are using the same procedures, no extra procedures for Port2-Port4):

        procedure TfrmMain.TCPPort1CommPortClosed(Sender: TObject);
        portName: String;
        portName := TTCP_UDPPort(Sender).Name;
        mmLog.Lines.Add(portName + ‘ Port closed’);

        procedure TfrmMain.TCPPort1CommPortCloseError(Sender: TObject);
        portName: String;
        portName := TTCP_UDPPort(Sender).Name;
        mmLog.Lines.Add(portName + ‘ Port close error’);

        procedure TfrmMain.TCPPort1CommPortDisconnected(Sender: TObject);
        portName: String;
        portName := TTCP_UDPPort(Sender).Name;
        mmLog.Lines.Add(portName + ‘ Port disconnected’);

        procedure TfrmMain.TCPPort1CommPortOpened(Sender: TObject);
        portName: String;
        portName := TTCP_UDPPort(Sender).Name;
        mmLog.Lines.Add(portName + ‘ Port opened’);

        procedure TfrmMain.TCPPort1CommPortOpenError(Sender: TObject);
        portName: String;
        portName := TTCP_UDPPort(Sender).Name;
        if TTCP_UDPPort(Sender).LastOSErrorNumber 0 then
        mmLog.Lines.Add(portName + ‘ Port open error: ‘ + IntToStr(TTCP_UDPPort(Sender).LastOSErrorNumber) +
        ‘: ‘ + TTCP_UDPPort(Sender).LastOSErrorMessage);
        tbRun.Down := False;
        isRunning := False;

        After removing these events, it looks it runs fine, although I didn’t do some extensive testing.

  7. Laurent Belanger Reply

    Hi Fabio,

    Is it possible to implement Modbus TCP master / slave communication between two freepascal app using PascalSCADA?


    • Fabio Luis Girardi Post authorReply

      Hi Mr. Belanger

      PascalSCADA has only master/client implementation of Modbus RTU/TCP protocol stack. PascalSCADA can’t communicate with other PascalSCADA implementation because it doesn’t have slave/server implementation of these protocols. I don’t have free time to do that, but it’s possible to do. I finished the move from SVN to Github. So, now is more easy receive these cool features

  8. Reid Simonsen Reply

    I am kind of new on Modbus. I have installed pascalscada on Lazarus. Now I would like to use it with Arduino. I need some sample code on pascalscada and Arduino. Where can I find some sample code for it?

    • Fabio Luis Girardi Post authorReply

      Hi Mr. Simonsen!

      Well, I’m not a arduino enthusiast, so I can’t help you on the arduino side. The unique thing that I know is that if arduino supports the standart modbus funcitons (0x01, 0x02, 0x03, 0x04, 0x05, 0x06, 0x0f and 0x10) PascalSCADA will communicate with it. The unique thing that maybe causes some difficulties, is a setting to avoid pascalscada send packets when it’s in idle state that will make your arduino cpu usage high. But this settings is just a property that should be set to false.

      I helped some people in the past, with the promise of when the prototype is done, they will send-me a full arduino example. For now, I received nothing.

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