求助高手解答串口程序

大侠们，能不梦帮我解答一下这个程序是什么意思啊，最好是一步一步的说明他的作用和目的，小弟刚开始学习，许多地方不懂，请多多指教
function read_dm_com1(startAddress:string):string;
  var
  outstring,keyorder,fcd,instring,z,zz,resu:string;
  i,s:integer;
  j,jj,y,time_out:integer;

  //error_com:boolean;

  begin
     j:=ord('@');
     mainform.MSComm1.InBufferCount :=0;
     keyorder:=startaddress;
     outstring:='@'+'00'+'RD'+keyorder+'0001';

    for i:=2 to length(outstring) do
      begin
      zz:=copy(outstring,i,1);
      jj:=ord(zz[1]);
      y:=j xor jj;
      j:=y;
      end;
    z:=inttohex(y,1);
    fcd:=z;
    outstring:=outstring+fcd+'*'+chr(13);
    MainForm.MSComm1.Output :=outstring;
    time_out:=1;
    repeat
      time_out:=time_out+1;
    //  delay(1);
    sleep(1)
    until ((time_out=50)or(mainform.MSComm1.InBufferCount>= 15));
    if time_out=50 then
       result:='0'
    else
       begin
        instring:=MainForm.MSComm1.Input;
        result:=copy(instring,8,4);
       end;


  end;