is there any limitation other than voltage range, which isn't a problem for SPI ?
Is there also a hardware requirement for the serial decode? The EDUX doesn't have the same number of protocols supported that the DSOX does.
My guess is that SPI decoding is missing on the EDUX because it requires the use of EXT TRIG input as 3rd channel, and EDUX has limited capabilities on this input compared to the DSOX. But keysight charges the same $$$ for serial decode for EDUX and DSOX even when SPI is missing on EDUX.
In EDUX mode (Product ID 22) Bus function does not allow user to select EXT as a signal for the BUS. You can only select CH1 and CH2. In DSOX mode (Product ID 24) you can add all 3 inputs to a BUS. Even when the manual says that EXT TRIG input has only 1 voltage range, you can change the probe attenuation and it automatically changes the voltage range. i.e. with probe 1:1 voltage range goes up to 8V. If you change probe to 10:1, voltage range goes up to 80V. I have not tested other probe attenuation values.
Unless triggering from EXT, the button is disabled and if you press it, the scope says: The external trigger view (the signal's digital waveform) is not available in EDUX1000-Series oscilloscopes when the trigger source is not 'EXT' or when the serial bus decode is enabled.
Unfortunately I don't have the decode option to test if SPI gets enabled in EDUX scopes with Product ID 24.
I am about to populate the missing components on the EXT Trig input section to see if the self test error goes away.