串口通讯的一段代码不明白,高分求教，急请高手帮忙，马上揭贴给分

function hex(c:char):Integer ;
var
      x:integer;
begin
     if c=' ' then
        x:=0
     else if (Ord(c)>=ord('0')) and (Ord(c)<=ord('9')) then
        x:=Ord(c)-Ord('0')
     else if (Ord(c)>=ord('a')) and (Ord(c)<=ord('f')) then
        x:=Ord(c)-Ord('a')+10
     else if (Ord(c)>=ord('A')) and (Ord(c)<=ord('F')) then
        x:=Ord(c)-Ord('A')+10
     else
        //输入错误
        x:=-1;        
      Result:=x;
end;//该函数接收1个至2个字符
//转换成功.输出对应16进制数的值
//转换失败.输出-1。
function HexToInt(S:String): Integer;
var
    tmpInt1,tmpInt2:Integer ;
begin
   if Length(S)=1 then
   begin
      Result:=hex(S[1]);
   end
   else if Length(S)=2 then
   begin
      tmpInt1:=hex(S[1]);
      tmpInt2:=hex(S[2]);
      if (tmpInt1=-1) or (tmpInt2=-1) then
          Result:=-1
      else
          Result:= tmpInt1*16+tmpInt2;
      end
    else
        //输入错误，转换失败    
        Result:=-1;
end;//发送数据
procedure TMainForm.btnSendClick(Sender: TObject);
var
    Len:Integer;
    i,count,tmpInt:Integer;
    tmpVar:Variant;
    tmpStr,Output:String;
    
begin
    if not MSComm.PortOpen then
    begin
        showmessage('没有打开串口！');
        Exit;
    end
    else begin
        //发送二进制数，需要使用Variant变量矩阵，矩阵大小自动调节
        if HexSend then
        begin
            Output:=mmSend.Text;
            Len:=Length(Output);
            if Len>0 then
            begin
               i:=1;
               count:=1;
               //创建一个Variant数组
               tmpVar:=VarArrayCreate([1,1],varByte);
               while(i<Len) do
               begin
                  //每3个字符串中截取2个字符，转换为16进制
                  tmpStr:=Copy(Output,i,2);
                  tmpStr:=LowerCase(tmpStr);
                  tmpInt:=HexToInt(tmpStr);
                  if tmpInt=-1 then
                  begin
                      showmessage('发送的数据格式有问题！');
                      exit;
                  end
                  else begin
                      tmpVar[Count]:=tmpInt;
                      Inc(count);
                     //增大Variant数组
                      VarArrayRedim(tmpVar,count);
                  end;
                  i:=i+3;
               end;
               MSComm.Output :=tmpVar;               
            end;
        end
        else begin
            MSComm.Output :=mmSend.Text;
        end;
    end;
end;
这是一段串口调试精灵的源代码，我不明白为什么要当选择用二进制方式传输时要每每3个字符串中截取2个字符，转换为16进制，谁能给我讲讲