The main bypass external is indeed broken in V3.5. We apologize for this and will issue a fix shortly.
Otherwise, this and the FX A/B externals work as Italo describes, with one strangeness that may be the cause of the confusion.
When the control is in the ON/OFF mode, it toggles the bypass state when the MIDI input goes from below 64 to 64 or above, and toggles again when the MIDI input goes from 64 or above to below 64. This means that the control operation depends on the previous bypass state rather than being absolute.
This is not quite as one would expect – it would be more normal for it to be ON at 64 and above, and OFF at 63 and below. In most cases the described implementation will give the same result, but with fast operation it could get out of step.
If you truly need a defined ON/OFF state you can use a SYSEX message as described below – this will work with the current V3.5 release.
F0 1C 70 01 3C 31 30 30 33 30 35 30 37 20 31 F7 – for bypass on
F0 1C 70 01 3C 31 30 30 33 30 35 30 37 20 30 F7 – for bypass off
Note that the F0 and F7 are standard MIDI sysex start and end codes and may be supplied automatically by your pedal.
