EDIT: I have solved this issue. Solution is below, but first I’ll leave the description of the problem from my original post…
I have a new Altmill that I’ve put together with a 2.2KW 220V 4-pole G-Penny Spindle that requires 800Hz input to get 24000rpm out. I’m using the SLB-EXT B5.0.11 firmware. My VFD is an H100 (not the one from Sienci, but a suitable variant for 2.2KW 220V single phase). I am using the H100 profile in gSender which selects the H100 plugin in the firmware. My problem is that the spindle RPM (as reported by the VFD front panel) is half the speed I am requesting. In other words, I need it to be that M3 S12000 will yeild 400Hz on the VFD display, but M3 S12000 yeilds 200 on the VFD which is as expected for a 400Hz spindle. To work with my 800Hz spindle I modified $468, $469, $470, and $471 values. However my changes to these values seem to have no effect whatsoever.
Solution:
It turns out the H100 plugin in the SLB B5.0.11 firmware completely ignores $468, $469, $470, and $471 values. It always behaves as if those values where 10, 60, 60, 10. The default values in those grblHal settings are 50,60,60,100, which are confusing (but completely ignored). My first attempt to work around this was to use the MODVFD plugin instead of H100. That plugin does honor $468-$471, but it also uses a different way of sending start commands so it didn’t work with my H100. My final solution was to actually modify the source code of the H100 plugin firmware. The new plugin code is working great for me. I wrote it to be backwards compatible with 400Hz spindles, and I will reach out to Sienci if they want to provide this update for everyone.