Machine On button.

Hi,
I've been using LinuxCNC for a couple of years now, but apart from setting up using the stepconf wizard, I have not attempted any other interaction with Linux.
So please assume no prior knowledge!
I would like to be able to switch on my machine, using a relay interface board controlled by a bob pin.
Unlike Estop, Machine On does not appear as a choice in Stepconf.
I thought this might be as simple as adding a couple of lines to the .hal file, similar to those used for the coolant button. However I just get a message reporting that Machine On isn't recognised.
Any help in getting the machine on button to actually do what it is supposed to do would be much appreciated. 
Thanks, Dougal.

Many thanks for your reply!
I will go and try that right now and report back.

Gmoccapy started now without error, but the button does not change the state of pin 16 unfortunately.  I've attached pic of line 55 added.
Any more ideas?
Thanks, Dougal.

Just to be clear, what I want to do is to switch on a singe power supply providing power to the computer and machine (but not switching either on). I will then start the computer and Linuxcnc. When the Gmoccapy screen opens I toggle the software estop (which I do not use) and this enables the machine on button. I wish to be able to press this button to ground a relay on pin 16 on the BoB (which is already receiving power from the usb). This relay will then switch main power on to the machine.
Is this what the second version of code will accomplish?
Thanks,
Dougal.

Yup, that works just as I want  .
Many thanks for the help.
Dougal.

Despite the fact that, following the above, switching on the machine from gmoccapy now, and happily, does exactly that, I am trying to understand a little of how and why.

I think I can now see that the first response, 'machine.on', appears to be correct.
However, when that didn't work and I attached the snippet of code, the respondent was alerted to the code being that code run before the gui started and changed the response to 'machine.is-on'.
I put the line of code in the configuration  .hal file simply because it was the only file I have ever altered.
It now appears to me that, since the action I wanted occurs after the gui is open, the code should have appeared in a postgui hal file.
So my question now please is, to achieve the action stated above, what should the line of code look like to attach the gmoccapy 'machine on' button to parallel port pin 16 and switch on the machine, where should the line of code be written and where would I find that place?

It would also be interesting to understand why 'machine.is-on' works to toggle the button on/off when it is described as an input, presumably to display the current state of the machine.

Thanks, Dougal. 

Thank you for that very clear explanation.
Reading that document, now from a different perspective, makes sense.
Dougal.