type
 TForm1 = 
 private
   procedure Connect;
   procedure Disconnect;
   procedure OnAppIdle(Sender: TObject; var Done: Boolean); 
   procedure SendBuf(var Buf; const Size: Cardinal);

   FPort: THandle;
   FDCB: TDCB;
   FStat: TComStat;
 end;  

procedure TForm1.FormCreate(Sender: TObject);
begin
  FPort := INVALID_HANDLE_VALUE;
  Application.OnIdle := OnAppIdle;
  ...
  ...
end;

procedure TForm1.Connect;
begin
  Disconnect;
    
  FPort := CreateFile('COM1', GENERIC_READ or GENERIC_WRITE, 0, nil ,OPEN_EXISTING, FILE_ATTRIBUTE_NORMAL, 0);
  if FPort <> INVALID_HANDLE_VALUE then begin
    SetCommMask(FPort, EV_RXFLAG);
    GetCommState(FPort, FDCB);
    FDCB.BaudRate := CBR_115200;
    FDCB.Parity := NOPARITY;
    FDCB.ByteSize := 8;
    FDCB.StopBits := OneStopBit;
    if not SetCommState(FPort, FDCB) then begin
      MessageDlg('Невозможно задать парамерты порта', mtError, [mbOk], 0);
      CloseHandle(FPort);
      FPort := INVALID_HANDLE_VALUE;
    end else begin
      PurgeComm(FPort, PURGE_TXABORT or PURGE_RXABORT or PURGE_TXCLEAR or PURGE_RXCLEAR);
    end;
  end;
end;

Procedure TForm1.Disconnect;
begin
  if FPort <> INVALID_HANDLE_VALUE then begin
    CloseHandle(FPort);
    FPort := INVALID_HANDLE_VALUE;
  end;
end;

procedure TForm1.OnAppIdle(Sender: TObject; var Done: Boolean);
var
  P: Pointer;
  BR: Cardinal;
begin
  if FPort <> INVALID_HANDLE_VALUE then begin
    ClearCommError(FPort, FErrs, @FStat);
    GetMem(P, FStat.cbInQue);
    ReadFile(FPort, P^, FStat.cbInQue, BR, nil);
    //
    //Тут обрабатываем данные в P
    //
    FreeMem(P);
    Done:=False;
  end;
end;

procedure TForm1.SendBuf(var Buf; const Size: Cardinal);
var
  BW: Cardinal;
begin
  if FPort <> INVALID_HANDLE_VALUE then begin
    WriteFile(FPort, Buf, Size, BW, nil);
  end;
end;