Hi Grog. Thanks for the great detailed explanation. I am "generalizing" past the arduino, thus allowing you to to virtually manage i2c device data and i2c device connections to any running  i2c Controller services (like Arduino, Raspi, etc.) The main goal Ray was looking for was to be able to have a basic i2c device service that can be used for new i2c devices that currently dont have a service, like the pcf does, in mrl. Instead of writing a class for each and every i2c device, I can generalize the devices into an "I2cDevice" class that stores the  device address, controller, and device bus it is attached to. I put that I2cDevice into a concurrent hashmap in my I2cDeviceManager.   The manager has methods that allow the user to get/set the device address and device bus, as well as methods that, given that information and the controller service the device should be connected to, will call on that device's controller to attach it.  This should both dramatically expand the possible i2c devices that can be used, but also simplifies the coding for writing a script to read/write to an i2c device.  Now obviously the user still has to start the controller services, but once those are running, the user either chooses an existing but detached i2cDevice in the manager, or they add a new i2cDevice object to the manager for which they can attach to an existing controller service.   

My only question about attachable is with the following lines of code found in the pcf8574 class (which is an i2cControl). You'll notice that the second method only checks if the "attachable" service is an I2CController before it calls the attachI2CController class.  Why have the second method at all?  Why not just do a check in the first method to see if the service is an I2CController  and if so call attachI2CController.   As I said before, my only guess is that something else, like some kind of backend mrl code or another service, passes the supposed I2CController service as an attachable. I cant see any other reason why its necessary to have a separate method to do that check, unless perhaps its just split up for cleaner/more understandable code?   If im modeling my generic "I2cDevice" class after the pcf class (which is an i2cControl), i want to be certain that all the methods I incorporate are actually necessary. Thanks for the help so far :)

public void attach(String service) throws Exception {
    attach((Attachable) Runtime.getService(service));
}

@Override
public void attach(Attachable service) throws Exception {

    if (I2CController.class.isAssignableFrom(service.getClass())) {
        attachI2CController((I2CController) service);
        return;
    }
}

public void attachI2CController(I2CController controller) {

    if (isAttached(controller))
        return;

    if (this.controllerName != controller.getName()) {
        log.error("Trying to attached to {}, but already attached to ({})", controller.getName(), this.controllerName);
        return;
    }

    this.controller = controller;
    isAttached = true;
    controller.attachI2CControl(this);
    log.info("Attached {} device on bus: {} address {}", controllerName, deviceBus, deviceAddress);
    broadcastState();
}