Joliat,
<!DOCTYPE html>
<html>
<head>
<title>Untitled</title>
<meta charset="utf-8">
<style type="text/css">
</style>
<script>
window.addEventListener("DOMContentLoaded", function() {
var all_block = document.querySelector("#all_block"),
inp = document.querySelectorAll("#is105, #is132, #is123"),
f = function() {
all_block.value = [].map.call(inp, function(el) {
//el.value = +el.value || 1;
return el.id + "x" + el.value
})
};
[].forEach.call(inp, function(item) {
item.addEventListener("input", f)
})
});
</script>
</head>
<body>
<input id="all_block" type="text" value="is105x1,is132x5,is123x1" size="45">
<input id="is105" type="text" value="1" >
<input id="is132" type="text" value="1" >
<input id="is123" type="text" value="1" >
</body>
</html>