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.

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