111.js

/*
so the server.js starts and runs 
the init function in device.js 
That function sets up the serial communications and calls a read on the SN provided from the device. 
Using that SN for the port number in this example. 

The issue at hand is this: 
there is a unknown delay period where the serialport it open but it is sending back 
garbage data that doesn't meet the checksum or just times out (expected) 
so there are a handful of errors in the read function before I get the SN back. 

for ever error I see, the group.process method is called (idk why? shouldn't it only call for the passing read?) 
which leads to me listening on the same port for as many errors as I see (dozen +)

Why is it doing this?
*/

device.js

function init(cb) {
	// setting up the serial connection.
	getSN(cb);
}
function getSN(cb) {
	read({
		start: 149,
		length: 1,
		process: function(res) {
			var serialPort = res.values[0] * 256 + res.values[1];
			if( serialPort > 1024 ) { // valid value and not garbage
				exec('some ssh command here', puts);
				cb(serialPort);
			} else {
				getSN(cb);
			}
		}
	});
}
function read(group, cb) {
	var req = master.readHoldingRegisters(group.start, group.length, {timeout: 1000,unit: 1});

    req.on('complete', function(err, res) {
    	if( err ) {
    		console.log("READ ERROR", err);
    		errors.increment(); // ignore me
    		read(group, cb);
    		return;
    	}
    	errors.reset(); // ignore me
    	group.process(res, group);
		if( cb ) { // i'm used for another part of the program, you can ignore me as well. 
			cb();
		}
    }).on('error', function(err){
    	// do nothing here. errors are handled above but the library insists I put this
    });
}

server.js

device.init(start);

function start(devicePort) {
	port = devicePort;
	app.listen(port);
	console.log( "app listening on port " + port );
	// other shit 
}