Select the datatable instance in a function under fnServerData

Hell again!

I don't completely understand things in javascript so I was hoping someone could answer this question.

The following code works great! And it is repeated on all 4 tables. I wanted to create a function for it and send the variables I need but that won't work for some reason. Here is the code I am talking about:

Works:

                  "fnServerData": function ( sSource, aoData, fnCallback ) {
				/* ... additional variables ... */
				$.getJSON( sSource, aoData, function (json) {
					if(json.error == 'expired'){
						timeoutLogoff();
					}else{
						/* Create the select elements on the first run */
						if ( json.sEcho == 1 )
						{
							build_dropdowns('Assignments',json,oTable);
						}
						/* DataTables callback */
						fnCallback(json)
					}
				} );
			}

note that build_dropdowns is a function that will create teh drop downs based off the json data. Here is it's code:

          function build_dropdowns(tableID, json, refName){
			$("#"+tableID+" tfoot th.dropdown").each( function () {
				index = refName.fnVisibleToColumnIndex($(this).index());
				$(this).html(fnCreateSelect(json.select[index], index));
				$('select', $(this)).change( function () {
					refName.fnFilter( $(this).val(), refName.fnVisibleToColumnIndex($(this).parent().index() ));
				} );
			} );
		}

Notice how this function needs the refName (like oTable or wTable) to call fnFilter etc... I couldn't figure out a way to get the instance of datatable based off the ID of the table.

So, I changed fnServerData to this:

//**in the global scope area at the top of the page**//
                function build_fnServerData(table,refName,sSource, aoData,fnCallback){
			$.getJSON( sSource, aoData, function (json) {
				if(json.error == 'expired'){
					timeoutLogoff();
				}else{
					if ( json.sEcho == 1 ) {
						build_dropdowns(table,json,refName); // Create the select elements on the first run
					}
					return fnCallback(json)
				}
			} );
		}
 /* ... */

//**in the wTable datatable setup area**//
                                       "fnServerData": function ( sSource, aoData, fnCallback ) {
						build_fnServerData('WO',wTable,sSource, aoData,fnCallback);
					}

I don't know the best solution. Is there a way to reference wTable dynamically?
When I pass wTable from fnServerData it says "wTable not defined" but if I hard code wTable into the build_dropdowns function within build_fnServerData it will work... Is this because getJSON is asyncronous so when getJSON is called the datatables object is created? Is there a way to dynamically get the datatable instance like $('#WO').thisInstanceOfDatatables? Should I pass a string (or have a global has value to convert) and use eval to use the correct variable name when it is needed?

Thanks for the help!