Midi based keyboards with extra output pulses

[HOW]

While testing some of our handheld Midi Data Loggers I noticed that when using a Yamaha PSR-420 keyboard the logger would display messages starting with FF or FE and only intermittently display the correct 3 byte code. So I tested the same keyboard with a PC based midi data logger and only "note on" messages were displayed usually and sometimes no messages were displayed when keying. However GrandOrgue interpreted the messages without a glitch. As a matter of fact, you could take a piece of trim and push all the notes at once in rapid succession with all the stops active of a 50 stop organ and the processor (single core 2.6Ghz) would only be at 45% usage!
So, I looked at the midi output from the Yamaha with a digital scope and discovered that there are continuous low pulses even when no notes are played. Does anyone know the reason for these pulses (they show up as either FF or FE as the first byte)?
Hlabs notes midi converters send continuous "all notes off" messages when no notes are being played in order to prevent ciphers due to faulty midi cables, but the Yamaha does not seem to be sending any midi message with this pulse train.
Best regards,
Stan

[el_supremo]

Hi Stan,
My Yamaha SY77 synthesizer has a feature called "Active Sensing". It sends FE at intervals of approximately 170ms. If it receives FE and no MIDI data are received within the next 300ms (approx.) then it clears the MIDI receive buffer and if any keys are on they are turned off.
It is a sort of failsafe mechanism.
I can always tell if my SY77 is on (it has another keyboard on top of it) by looking at the MidiMan interface because the transmit light flicks on and off continuously if the SY77 is powered.
Perhaps the logger doesn't interpret them correctly.

Pete

[HOW]

Hello Pete,
Thank you for your reply and answer. That makes sense. After checking further, the first byte is always interpreted as FE. Perhaps we should have our logger either ignore this command or clear after a few milliseconds. I will see if the software engineer can easily modify the program.
Best regards,
Stan