Touch off having a hard time with HAL side

I have figured out making a touchoff button on my Pyvcp panel. What I'm not sure about (despite hours of reading examples and looking at the 2.8 documentation) is how to make it actually do anything, which I think is a HAL question (let me know if I'm in the wrong place).

I'm using LinuxCNC/AXIS version 2.8.4-76 etc.
In Stepconf, there is a page for custom HAL commands, where I have entered (as the first and only MDI command):
   MDI_COMMAND = o<z_touch>call

1. How does HAL know where to find the subroutine, i.e. the path to the gcode z_touch?

Here is the xml corresponding to the touch off button:
        <button>
          <font>("Helvetica",14)</font>
          <width>3</width>
          <halpin>"z-zero"</halpin>
          <text>"Z"</text>
        </button>
I need to connect the 'z-zero' button to my touch off subroutine. I think I need a line like this:
net do-z-touchoff halui.mdi-command-00 <= pyvcp.z-zero

2. Does this go in custom_postgui.hal ?
3. Is that all I need?

I also want to use a spinbox to input the touch plate thickness, but I'll leave that question for the next round.

Thank you,
Steve
 

I get an error from Axis:
Unknown word where unary operation could be. Position: Relative Actual

Searching took me to a page that said this certainly came from gcode interpretation. I have reduced my sub to just start and end statements:
o<z_touch> sub
o<z_touch> endsub
and still get the same error.

Nowhere in the various config files is the subroutine file name used, only the directory. Is this correct?

Thanks

I made your change, saved and restarted Axis. Same error message.

Here are my various mods:

.ini file
[RS274NGC]
PARAMETER_FILE = linuxcnc.var
SUBROUTINE_PATH = ncsubroutines:/home/steve/linuxcnc/subroutines

HAL
HALUI = halui

[HALUI]
# add halui MDI commands here (max 64)
MDI_COMMAND = MDI_COMMAND = (debug,MDI button pressed)

postgui_call_list.hal
# touch off Z
net ztouchoff halui.mdi-command-00 <= pyvcp.z-zero

custompanel.xml
<button>
<font>("Helvetica",14)</font>
<width>3</width>
<halpin>"z-zero"</halpin>
<text>"Z"</text>
</button>

There are other mods, the jog button pyvcp example. They work fine until I try my added touchoff Z button, when I get the error and the jog buttons no longer work. So I think it's the Z button section in error, rather than some other part.

Well that was a silly mistake. Thank you very much!