Detailed explanation of the 0 character receiving routine using the serial port control MSComm in Delphi

//The following is the MSCOMM parameter setting process when creating a form

//MSComm1.InputMode := comInputModeBinary;

// and MSComm1.InputMode := comInputModeText;

//The experimental results basically have little effect on Delghi

procedure TForm1.FormCreate(Sender: TObject);
var
  str: string;
begin
//MSCOMM parameter settingsMSComm1.CommPort
  := 1;//Use COM1
  MSComm1.Settings := ''9600,N,8,1'';//Set communication port parametersMSComm1.InBufferSize
  := 32;//Set MSComm1 receive buffer to 32 bytesMSComm1.OutBufferSize
  := 2;//Set MSComm1 send buffer to 2
  bytesMSComm1.InputMode := comInputModeBinary;//Set the receive data mode to binary formMSComm1.InputLen
  := 1;//Set the number of bytes read from the receive buffer at one time for Input to be 1
  MSComm1.SThreshold := 1;//Set the number of bytes read from the send buffer at one time for Output to be 1
  MSComm1.InBufferCount := 0;//Clear receive bufferMSComm1.OutBufferCount
  := 0;//Clear send bufferMSComm1.RThreshold
  := 1;//Set to generate OnComm event when receiving a
  byteMSComm1.PortOpen := true;//Open serial port 1
/////////////////////////////////////////////////////////////////////////////
  Buffers := '''';
  CheckSum := 0;
//Send serial port command
  Command := 34;
  str := ''$'' + #2 + #$22 + #1;//Read total MP3 tracksstr
  := str + Char(GetCheckSum(str));//Calculate
  checksumMSComm1.Output := str;//Send serial port
commandend;

//The following is the receiving event processing process, which is equivalent to serial port interrupt in MCU

//Note that there are two statements

//while MSComm1.InBufferCount > 0 do //Input FiFO is not empty
//if str = '''' then str := #0; //0 character processing
//Example received data is #24#02#00#01#03

//At this time, InBufferCount=5. If Input is set, the number of bytes read from the receive buffer at one time is unlimited

//That is: MSComm1.InputLen := 0; then str := MSComm1.Input; str seems to be #24#02#00#01#03

//But the actual value is ''??''#24#02. No result is obtained, at least the #01#03 after the 0 character cannot be read.

//Use MSComm1.InputLen := 1; and cooperate with while MSComm1.InBufferCount > 0 do

//When reading 0 characters, since str = '''' (null), accessing str[1] will cause an exception and cause the program to terminate.

//So use if str = '''' then str := #0; to fill the character #0 into str[1] and the length of str is also 1.

//Therefore, the key point is to use the if str = '''' then str := #0; statement to overcome the difficulty of reading 0 characters~~~

procedure TForm1.MSComm1Comm(Sender: TObject);
var
  str: string;
  i: integer;
begin
  case MSComm1.CommEvent of
    comEvReceive://receive event processing
      begin
        while MSComm1.InBufferCount > 0 do//Input FiFO is not empty
        begin
          str := MSComm1.Input;//Only take 1 character from FIFO, because MSComm1.InputLen := 1
          if str = '''' then str := #0; //0 character processing
          if (Buffers = '''') and (str = ''$'') then//Synchronizer test
          begin
            Buffers := str;//Store synchronization symbol ''$''
            CheckSum := 0;//Initialize checksum
          end
          else if (Buffers <> '''') and (Buffers[1] = ''$'') then begin//Must start with synchronization symbol
            Buffers := Buffers + str;//Add data string
            CheckSum := CheckSum xor Byte(str[1]);//Calculate the checksum (except the synchronization character ''$'')
            if Length(Buffers) = Byte(Buffers[2]) + 3 then//Terminator test
            begin
              if CheckSum = 0 then//At this time, the checksum must be 0 to indicate that the checksum is correct
              begin
                case Command of
                  $22: begin//Get the total number of songs
                        ComboBox1.Items.Clear;
                        for i := 1 to Byte(Buffers[4]) do
                        begin
                          str := ''第'' + inttostr(i) + ''首歌曲'';
                          ComboBox1.Items.Add(str);//
                        end;
                        Command := 0;
                      end;
                  1: ;
                else ;
                end;
              end;
              Buffers := '''';//Clear the buffer after receiving
              CheckSum := 0;//Initialize the checksum
            end;
          end
          else
          begin
            Buffers := ''''; //Receive error, clear the buffer and discard all data
            CheckSum := 0; //Initialize checksum
          end;
        end;
      end;
  end;
end;