Z probe triggers on homing but not during probing

I am setting up a Duet with a Piezo z probe.
After considerable messing about, the probe triggers fine if I do a 'homez'. When I try a G30 probing run, it would appear that the Duet is disabling the probe input. I can not trigger the probe even with a sharp manual tap, the red light is off all the time.
I get an error message saying the z probe was not triggered during the probing run.

When I revert to I0 in the M558 command and do a G30, the probing run aborts immediately with a message saying the probe was already triggered when the run started.
I am pretty sure there is a problem in my config.g file but I can't for the life of me figure out what it is. It is probably staring me in my face but I can't see it.
Here are the relevant config.g lines:

; Endstops
M574 X1 Y1 S1 ; Set active high (S1) endstops for x and y
M574 Z1 S2 ; Z endstop is the z probe both Z0 and Z1 seem to work to activate probe and not use regular endstop

; set up for piezo z probe
M558 P8 I1 R0.9 F400 ;digital mode (P8), active high (I1), delay (R0.9), speed (F400)

G31 X0 Y0 Z-0.2 P500 ; probe X, Y, Z offsets , Trigger point of probe
; G31 must come after M558

Is there any reason why the Z probe trigers fine during a homing run but does not trigger during a probing run ?

Thanks

Edited 1 time(s). Last edit at 06/25/2019 05:36PM by jens53.

Can you confirm that the piezo unit is connected to the 4-pin Z Probe connector?

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Quote
dc42
Can you confirm that the piezo unit is connected to the 4-pin Z Probe connector?

Thanks for replying. Yes, it is. I also have a manual switch connected to the z endstop but it is not in the picture at this time.
I just made some edits in the original question because I had made an error and I also tried to clarify things a bit.
Yes, the probe is definitely connected and working correctly. When running the homeall/homez routines, the probe is acting like it should and the nozzle stops just below the bed as the probe triggers the stop. The mechanical limit stop is not being activated at this point.
For the longest time I didn't grasp the concept that homing and probing are two entirely different things but now that I understand it, It would seem that homing is working fine and probing is not working at all. The fact that homing works would seem to indicate the sensor works.

It's driving me squirrely

MAJOR UPDATE:

At the risk of getting a reputation for an 'old fart' that makes 'old fart mistakes', I have fixed my problem. No wonder I was unable to find a solution on the internet because nobody in their right mind would admit to reversing the z-probe-mod line with the gnd line which is what I managed to do.
I have never considered the wiring because the homing worked ... go figure ..... key thing is G30 now TRIGGERS when I poke at the nozzle.

Edited 1 time(s). Last edit at 06/25/2019 11:33PM by jens53.