[adva]

Hi,
is there a way to script the S g-code?

To set the spindle speed, normally the speed is converted to a frequency or PWM, which is then converted to 0-10V, which is then converted back to the speed by the VFD.
But many VFDs also have a digital inteface, e.g. UART or RS-485. So: Why not skip all the conversion, and send the speed directly to the VFD? (via UART, or e.g. an I2C-to-RS485-converter)

So: Is there any way to catch PlanetCNC's spindle-speed-changes / S-words -- e.g. with a script like for M-codes or via a callback in the API?

[theRat]

This is very possible. I know this because I have built and I2C to RS485 converter and written the scripts (for a Nowforever vfd). The scripts get rather complicated as the serial comms requires the use of crc's to validate commands.
You don't need to intercept the S commands, you do it all in the M3/M4 scripts (ie. set the speed for the vfd only when you are starting the VFD). The rest of the time it doesn't matter since the actual speed is zero!