On the basis of what you say, you cannot PLAY anything including percussion on your digital piano, but you can RECORD such, for playback using another device that does support percussion playback, or via setting channels, playback to your piano for the other sounds, with the percussion being handled by a supporting device using channel 10.

In detail, you could record track by track, using channels, to your sequencer (maybe GarageBand can do this), then you could record alongside this to Channel 10 using the relevant notes/keys for the percussion you want, using multiple tracks if need be, to build up a percussion track. When done, you replay the complete recording incl the usual voices, and the percussion on Ch 10. If you're using your piano and something else, then you make sure that your piano is set to receive channels OTHER than 10, and the something else receives on Ch 10 ONLY.

One problem may be that the EP will NOT send/receive on Ch 10, as that Ch is usually used for Perc. This may not be a big deal, you record on another Ch, the data is stored and THEN you change that Ch on the recording to Ch 10. Ch 10 is the default for percussion, you CAN alter settings on devices to use another channel, but this may be a fiddle.

If your EP has midi IN/OUT, then it should be able to do what you want. Check though what channels it can use for either IN or OUT.. Maybe you've been trying to use it on channels that do not operate. I've got a Casio Kbd that will receive on various ch, but send on very few, other devices may work on 1 ch only?

Geoff

1. Getting a MIDI connection between your Yamaha P-155 and computer

You'll need a USB MIDI interface to connect the DIN MIDI ports on your Yamaha P-155 keyboard to a USB port on your computer.

More information: You can't use a USB A port to connect to a "computer host".

In general, USB A ports (flat rectangle shape) are on the end of a connection that acts as a "computer host", and USB B ports (square shape) are on the end of a connection that acts as a "peripheral device".

Yamaha keyboards typically have one or both of these kinds of USB ports on them:
a USB A port labeled "TO DEVICE" to connect USB thumb drives or other storage devices.
a USB B port labeled "TO HOST" to connect to a computer for USB MIDI communications.

Since your keyboard only has a USB A "TO DEVICE" port, you'll have to use your keyboard's DIN MIDI ports for MIDI communications. You'll need a USB MIDI interface to connect your keyboard to your computer, as shown in page 50 of the manual. (Actually, page 50 of the manual is kind of confusing, it seems to say there are "two ways" to connect to a computer, but both ways described seem to be exactly the same thing -- use a USB MIDI interface to connect the DIN MIDI ports on the keyboard to a USB port on the computer.)

2. Record MIDI percussion sounds in GarageBand.

I'm a little unclear on how to tell GarageBand you want to record MIDI notes and have them sound like a drum kit, but I think you should try to create a "software instrument track" and select a "drum kit patch":

Intro to tracks in GarageBand on Mac
Intro to patches in GarageBand on Mac

You don't want to hear any sounds from your Yamaha P-155 keyboard as you play so you can either turn its volume all the way down, or set its Local Control mode off. (In the manual, see "Basic Procedure in Function" on page 39 for how to select functions, then read the description of function "F7.3 Local Control ON/OFF" on page 44.)

Hey thanks for all this context George and Bavi, makes sense the "To Device" is used for a flash storage device and wouldn't do MIDI when there is a clearly labelled MIDI output right next to it. Idk why I thought midi would work via USB-to-Device.

- Good call on the manual, thanks. I downloaded and bookmarked that!
- I was muting the keyboard's audio by plugging in a 1/8" or 1/4" jack for headphones or whatever that thingy is, as a hack. But I will check the Local Control ON/OFF you reference, thanks.

Solved:
I bought a MIDI to USB-A cable and it works great! The store didn't have a MIDI to USB-C, so I plug the USB-A into one of those white Mac adapters that connects to the mac's USB-C port.

I opened Garageband and all worked immediately. Now playing drum kits and synths on my keyboard. Running the audio output from Mac->ScarlettInterface->Headphones.

For me, fully-weighted hammer action isn't ideal for drums and some synths, but works for now. Beat pads on a MIDI Controller (I was looking at the Arturia and Akai MIDI controllers at the store) are more ideal for drums, but I'll hold off on that for now.

I'll see if Garageband will let me assign the drumkits to higher octaves on the keyboard, because those keys feel a little faster in their response/recovery. Easier/faster to press. Depends on what instrument voice I'm trying to play really. So far I'm fine with them on the lower octaves for just getting started.

Crazy thing happened, it stopped working, so I did a factory reset (pg. 46 in the Manual) of all setting on the Keyboard, and it started working again. Whew.

I must have messed up one of the Midi Functions. I went through each one and made sure Defaults were still 'ON' but was still stuck, so that's what I looked up Factory reset.

I'm glad you got it working!

[quotePost id=17281]I'll see if Garageband will let me assign the drumkits to higher octaves on the keyboard[/quotePost] Alternatively for a shift of one octave, you can press the TRANSPOSE and -/NO buttons on your keyboard to change its transpose setting to -12 (down one octave). Then when you play the keys one octave higher than before, the keyboard will send MIDI notes that are one octave lower than the keys you are playing.

[quotePost id=17282]Crazy thing happened, it stopped working[/quotePost]If you are having problems, be aware of a particular kind of poor quality USB MIDI interfaces. Take a look at this thread and my reply.
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Thanks I'm trying the TRANSPOSE and -/NO buttons for this.

Good call, I'll test the normal piano sounds (notes, chords) via MIDI. Thanks for the links.