Perhaps your Mango is quite old? The sleep function was added sometime ago as a wrapper for Thread.sleep. You may still be able to do java.lang.Thread.sleep(XXXX); if you are using Java 8.

RuntimeManager.sleep(200)

@phildunlap I tried to use the sleep code for something else but I get this:

TypeError: { dataSourceEnabled(xid): boolean, enableDataSource(xid): -1 0 1, disableDataSource(xid): -1 0 1, dataPointEnabled(xid): boolean, enableDataPoint(xid): -1 0 1, disableDataPoint(xid): -1 0 1, } has no such function "Sleep" in at line number 3
at line:

is RuntimeManager.sleep(XXXX) a valid function? I've seen it posted elsewhere but its not working for me..

I would consider putting a RuntimeManager.sleep(200); into that while loop, though. Also, the calls to runtime manager should be synchronous, so you shouldn't need to pause in the middle at all. Furthermore, should the disable data source fail (maybe you don't have permission to edit that data source?) you'll get stuck in the while loop.

But otherwise, Dan's solution will work in a great number of Mango versions in a script polling periodically.

What I'd do, if you're using the latest Mango, is have a scripting data source update on change of the slave monitor point (currently the best way to disable scripting data source polling is to use a year in your cron, like 0 0 0 1 0 ? 2050 but we'll probably add the ability to outright disable polling), or have an event detector with a script set point event handler on it, and in either case a script body like

if(inCondition(slaveMonitor)) { //Could maybe move this logic into the event detector if not using a scripting data source
    RuntimeManager.disableDataSource('DS_Modbus');
    RuntimeManager.enableDataSource('DS_Modbus');
}

if(RuntimeManager.isDataSourceEnabled('DS_Modbus') === true)
{
    if (--Condition that occurs that means you need to reset, i.e slave monitor shows datasource is offline--)
    {
         RuntimeManager.disableDataSource('DS_Modbus');  // disable DataSource 
         while (RuntimeManager.isDataSourceEnabled('DS_Modbus') === true) {}    //loops until DataSource is disabled since it might take a few seconds to disable depending on your timeouts, etc..
         RuntimeManager.enableDataSource('DS_Modbus');    // re-enable DataSource  
        }
}