It's all home brew, but the hardware isn't the problem.
Mach3 isn't toggling pin 1 of the D25 connector, even though I believe the setup
is correct to make spindle operations toggle pin 1.
I believe the screenshots attached to the initial post show a configuration for no
PWM, just M3 G code operating pin 1 and that energizing my spindle relay.
I know the hardware path from the PC to the spindle relay works, as I can cause
the spindle to start and stop by checking/unchecking the "Ports and Pins" page
"Spidle" "Step Low Active" box. So Mach 3 has control of the spindle, but I can't
make the spindle start under program control, or with the "Spindle Toggle" button
on the "Diagnostics" page.
Thanks, Jim