parport pwmgen pulse too weak

I'm trying to use the onboard parallel port of my computer to command spindle speed with PWM pulse. In halscope things look ok, I think:

But this is the signal on an actual oscilloscope (directly connected to parport plug pins on rear of computer):

The timing is right, and it matches what I'm doing in the GUI when I change the spindle speed, eg. at a lower speed duty cycle we get:

But obviously this ain't gonna work with the VFD. It's almost like there's a large capacitor in the circuit...  has anybody come across this problem?

HAL file: www.iforce2d.net/tmp/my-mill2.hal.txt

btw what's up with the embedded images here, am I doing something wrong? They never show up correctly.

Thanks Tommy, it works nicely on pin 1,2,3,4.... and now also works on the original pin (16) that wasn't working earlier for many hours. Such fun.

My next problem is that I realize the cable I just bought is a null-modem one, useless for this purpose as far as I can tell. Such fun. No, I was not using the cable earlier, that must have been a bad connection.

The AC coupling may have been wire to wire capacitance from the
PWM driven wire in the null modem cable to an undriven wire.