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parse-modbus-tcp-frame.md

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Parse a Modbus TCP Frame

Use BinaryParsec.Protocols.Modbus.ModbusTcp when application code needs an MBAP-aware Modbus TCP parser with a shared PDU model.

Result-based parsing

open BinaryParsec.Protocols.Modbus

match ModbusTcp.TryParseFrame frameBytes with
| Ok frame ->
    printfn "transaction=%d unit=%d function=%d payload-bytes=%d"
        frame.TransactionId
        frame.UnitId
        frame.Pdu.FunctionCode
        frame.Pdu.Payload.Length
| Error error ->
    printfn "parse failed at byte %d: %s" error.Position.ByteOffset error.Message

Throwing convenience parsing

open BinaryParsec.Protocols.Modbus

let frame = ModbusTcp.ParseFrame frameBytes

ParseFrame raises InvalidDataException when the MBAP framing fields or shared PDU bytes are invalid.

Notes

  • TransactionId and UnitId stay on the transport frame model.
  • Pdu exposes the shared Modbus payload after MBAP framing is removed.
  • Pdu.FunctionCode reports the logical function code.
  • Pdu.RawFunctionCode preserves the transmitted function byte.