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.

5 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.

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