Hmm... So then it is pretty weird.
I'm following the same process to map the flippers (Left and Right Shift keys) and Start (1 key):
Open the Config Tool...
Go to Settings...
Scroll down to Button Inputs...
(I have not set a Shift button from the default of 0 (zero))
For each button I set:
the Port: Click on port and choose one of the pins from the list in the Main Board section (it shows a picture of a standard Pinscape Main Board... although I'm using the AIO I'm under the impression 1 is 1, 2 is 2, etc...
the PC Input: Click on the key cap icon and then press the intended button.
I do that for my four buttons (two flippers, Start, and Enter) then hit Program KL25Z... wait for it to refresh... and then go to the Button Tester.
In my list of buttons I have:
1 Button 1 (digital in) key=1 Start Game
2 through 5 = not connected
6 Button 6 (digital in) key=Enter Plunger/Launch Ball
7-8 = not connected
9 Button 9 (digital in) key = Shift Left Left Flipper
10 Button 10 (digital in) key = Shift Right Right Flipper
Each button has its positive wire going to the corresponding input port on the AIO unit.
But I get the same thing every time... Flippers and Start work flawlessly. They test fine with the button test and work well in VPX. But the Enter button is just "dead"... no action in Button Test or VPX.
Things I've tried:
1. Checked the physical button: I swapped it with a flipper button and there is no effect - the flipper still works but Enter does not. I've also checked the button with a multimeter and it behaves as expected.
2. Changed the Plunger/Launch Ball function in VPX to another key (I chose "P"). Then I re-mapped the button to the P key. Oddly, this did not work either. I ended up with a "dead" P button.
I'm sure I'm doing something dumb... but I will take a break and look at it with fresh eyes in the morning.
Thanks all!