function NumToHex(num1,num2) {
// Convert number from decimal to hexadecimal
// num1 refers to the source element
// num 2 refers to the destination element
// first retrieve the source element value
strNum=document.forms[0].elements[num1].value;
for(i = 0; i < strNum.length; i++) {
// loop through each digit
chr=strNum.substring(i, i + 1);
// isNan - check for not a number
// see if not numeric or space 
if((isNaN(chr))||(chr == ' ')) {
// show alert message
alert('You must enter a digit between 0 and 9!');
// set cursor on source element
document.forms[0].elements[num1].select();
// clear destination element
document.forms[0].elements[num2].value='';
// return
return false;
   }
}
// valid digit so check value
if(strNum > 255) {
// invalid number
alert('You must enter a number between 0 and 255!');
// set cursor on source element
document.forms[0].elements[num1].select();
// clear destination element
document.forms[0].elements[num2].value='';
return false;
}
else {
// now convert to hexadecimal
base = strNum / 16;
rem = strNum % 16;
base = base - (rem / 16);
// convert base to hexadecimal
baseS = MakeHex(base);
// convert remainder to hexadecimal
remS = MakeHex(rem);
// write destination value
document.forms[0].elements[num2].value=baseS + '' + remS;
// Change background to values in elements 3,4,5
ChangeBackground(3, 4, 5);
return true;
   }
}


function MakeHex(x) {
// convert passed decimal into hex
if((x >= 0) && (x <= 9))
// between 0 and 9 so return as is
return x;
else {
switch(x) {
// return remaining numbers
case 10: return "A"; 
case 11: return "B";  
case 12: return "C";  
case 13: return "D";  
case 14: return "E";  
case 15: return "F";  
      }
   }
}


function HexToNum(num1,num2) {
// Convert number from hexadecimal to decimal
// num1 refers to the source element
// num 2 refers to the destination element
// first retrieve the source element value
numberS = document.forms[0].elements[num1].value;
// check first digit
tens = MakeNum(numberS.substring(0,1));
if(tens == 'X') {
// if invalid number
document.forms[0].elements[num1].select();
document.forms[0].elements[num2].value='';
return false;
}
ones = 0;
if(numberS.length > 1) 
// means two characters entered
// get the numbers and check them
ones=MakeNum(numberS.substring(1,2));
if(ones == 'X') {
// invalid digit entered
document.forms[0].elements[num1].select();
document.forms[0].elements[num2].value='';
return false;
}
// write decimal number
document.forms[0].elements[num2].value = (tens * 16) + (ones * 1);
// convert hex to upper case
document.forms[0].elements[num1].value = 
		document.forms[0].elements[num1].value.toUpperCase();
// change background colour
ChangeBackground(3, 4, 5);
return true;
}



function MakeNum(str) {
// validate the passed number
if((str >= 0) && (str <= 9))
return str;
switch(str.toUpperCase()) {
// return decimal equivalent
case "A": return 10;
case "B": return 11;
case "C": return 12;
case "D": return 13;
case "E": return 14;
case "F": return 15;
// invalid number entered
default:  alert('Choose a number between 0 and 9 or a letter between A and F!');
return 'X';
   }
}



function ChangeBackground(num1, num2, num3) {
// change the background colour
// use the passed elements to build the colour
document.bgColor = '#'+document.forms[0].elements[num1].value + 
                       document.forms[0].elements[num2].value + 
                       document.forms[0].elements[num3].value;
}