Hi pros,
we would like to add a simple light tower with green, yellow and red on it. What is the simplest way to hook this up to the UR robot to monitor robot run, pause and stopped states of the robot with limited programming? Can we setup IOs to do this?

I know @Enric use a lot of them.

The hardest part is knowing the difference between pause and stop as setting an output to be high when the program is not running goes high when the program is stopped or paused. You will also need to reset the output low by having a background thread running that just keeps that output low when the program is running. The running is straightforward as it goes high when the program is running and low when its not.  You could wire this to a relay and then use the relay to cycle between running and not running on the light tower, this keeps the programming to an absolute minimum as nothing is required to switch between these two states using the relay. 

What we have done is we wire yellow to be NOT running (stopped or paused) and Green to be running.  We then use the red light internally in the program when we want to notify an operator that there is something the robot needs help with, for instance out of raw material or time to take away the finished goods.  We then can control this light and make it blink if we want.  Then if we have different notifications that we want to visually indicate to the operator we can do that through controlling how it blinks.  For instance, I need help, flash the SOS pattern, take material away, long off (2 seconds), short on (1/2 second), give me more material, short off (1/2 second), long on (2 seconds), some other notification, 1 second on, 1 second off.  Obviously you just play around with the timing until you get what you and the operator can decipher with a quick look at the light.

So that I understand correctly, you want the green light to flash when the program is not playing? Or is the program playing and you are just waiting to resume operation for the operator to push the button?

You can simply have a thread that is running in the background with some conditions that start the thread so that when you want the light to flash you start a loop running where the light turns on, waits, turns off, waits.  When the opertor pushes the button you would just need to set whatever flag you are using to flash the light to false and then in the thread you would simply have an else statement where the light then goes solid.  

Here is an example thread that you could use in the program

thread_1():
  if (waitingOnOperator == True):
    set GreenLight High
    wait (0.5)
    set GreenLight Low
    wait (0.5)
  else:
    set GreenLight High
    wait (waitingOnOperator == True)
  end
  sync()
end

waitingOnOperator = True
wait (operatorButton) == High
waitingOnOperator = False