[OpenLayers-Users] OpenTopoMap layer in Openlayer 2

[Alberto]

Hi

I'm trying to add  Opentopomap layer on my  htm page with oter imagery 
(mapnik, openmapsurfer imagery, lonvia etc).  All is right but 
opentopomap don't work...

I use this:

var otm = new OpenLayers.Layer.OSM("OpenTopoMap", 
"http://a.tile.opentopomap.org/{z}/{x}/{y}.png",
             {type: 'png', getURL: osm_getTileURL, numZoomLevels:20, 
displayOutsideMaxExtent: true, visibility: true, isBaseLayer: 
true,attribution: 'Kartendaten: © OpenStreetMap-Mitwirkende, SRTM | 
Kartendarstellung: © OpenTopoMap (CC-BY-SA)'} );

and thist to get z, x, y:

function osm_getTileURL(bounds) {
         var res = this.map.getResolution();
         var x = Math.round((bounds.left - this.maxExtent.left) / (res * 
this.tileSize.w));
         var y = Math.round((this.maxExtent.top - bounds.top) / (res * 
this.tileSize.h));
         var z = this.map.getZoom();
         var limit = Math.pow(2, z);

         if (y < 0 || y >= limit) {
           return OpenLayers.Util.getImagesLocation() + "404.png";
         } else {
           x = ((x % limit) + limit) % limit;
           return this.url + z + "/" + x + "/" + y + "." + this.type;
         }
       }

Any help?

Many thanks

Alberto