Added ADS reserved index groups and implemented access with index gro…

[PLCHome]

Added ADS reserved index groups and implemented access with index group and index offset.

One reason was the Event-Driven Detection of Changes to the Symbol Table. If the symbol table changes because, for instance, a new PLC program is written into the controller, the handles must be ascertained once again. The example below illustrates how changes to the symbol table can be detected.

var start = true;

var myHandle = {
    indexGroup: ads.ADSIGRP.SYM_VERSION,
    indexOffset: 0,
    bytelength: ads.BYTE,  
}

var client = ads.connect(options, function() {
    start = true;
    this.notify(myHandle);
})

client.on('notification', function(handle){
    if (start) {
      console.log('symbol table version '+handle.value)
    } else {
      console.log('symbol table changed '+handle.value)
    }

    start = false;
})

process.on('SIGINT', function() {
    client.end(function() {
        process.exit()
    })
})

client.on('error', function(error) {
    console.log(error)
})