var kLetAlfaNum = ['0','11','2','3','4','5','6','7','8','9'];
var kBuf = ['','','','','','','','','',''];
var index = 0;
var nowTime = new Date().valueOf();
var keyStroke = '';
var timeOutMsecs = 1000;

function focusField(nForm, nField)
{
    if (document.forms[nForm].elements[nField])
    {
        document.forms[nForm].elements[nField].focus();
    }
	return false;
}

function tecla()
{
	if (event.keyCode == 0) return;

	var nField = document.getElementById(event.srcElement.id).name;
	var nForm = document.getElementById(event.srcElement.id).form.name;

	kLet = kLetAlfaNum;
					
	var field = document.forms[nForm].elements[nField];
	var texto = field.value;
	var code = parseInt(event.keyCode);

    var iKeyStroke = -1;								
	if (code >= 48 && code <= 57)
	{
		iKeyStroke = code-48;
	}
				
	switch (iKeyStroke)
	{
		case 0:
		case 1:
		case 2:
		case 3:
		case 4:
		case 5:
		case 6:
		case 7:
		case 8:
		case 9:
			var now = new Date().valueOf();
			if ((now - nowTime) > timeOutMsecs && keyStroke != '')
			{
				kBuf = ['', '', '', '', '', '', '', '', '', ''];
				index++;
			}
			else
			{
				if (code != keyStroke && keyStroke != '')
				{
					kBuf = ['', '', '', '', '', '', '', '', '', ''];
					index++;
				}
			}
            
            var borrar = false;
			if (kBuf[iKeyStroke] == '' || kBuf[iKeyStroke].length == kLet[iKeyStroke].length)
			{
				kBuf[iKeyStroke] = kLet[iKeyStroke].charAt(0);
			}
			else
			{
			    if (iKeyStroke == 1)
			    {
			        borrar = true;
			    }

			    kBuf[iKeyStroke] += kLet[iKeyStroke].charAt(kBuf[iKeyStroke].length);
			}
			
			if (borrar)
			{
			    if (index > 0)
			    {
			        index--;
			    }
			    
			    field.value = texto.substr(0, index);
			    
			    keyStroke = '';
			    nowTime = new Date().valueOf();
			}
			else
			{
			    letter = kBuf[iKeyStroke].charAt(kBuf[iKeyStroke].length-1).toUpperCase();
			    field.value = texto.substr(0, index) + letter;

       			keyStroke = code;
		        nowTime = now;
			}

			break;
		default:
			switch(code)
			{
				case 42: // asterisco
				case 67:
				case 99:
                    return false;
					break;
				case 35: // almohadilla
				case 45: 
				case 65: 
				case 97: 
					return false;
					break;
				default:
					break;
			}
			break;
	}
	field.focus();
	event.keyCode = null;
}