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I've just learned that with digital piano and MIDI-to-USB connection the cable or converter is an active element and it is charged not from digital piano (MIDI side) but from tablet/computer (USB side).

Thus the question (to be 200% sure) -- if I connect Roland FP-10 via USB (pure USB, because this model does not have MIDI, just regular USB) with tablet:

  1. the tablet will be charged by FP-10
  2. FP-10 will be "charged" by tablet (more precisely tablet will discharge itself attempting to charge FP-10)
  3. there will be no charging any way

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To address your first paragraph:

The MIDI-to-USB converter being an ‘active element’ means that the converter itself needs power.

MIDI doesn't carry power (only a ground, and a balanced pair for the signal).  But USB must provide power (at least 5V 100mA, usually 0.5A or more).  So the converter must take its power from the USB connection.  (However, it's unlikely to have an internal battery, so there's nothing to ‘charge’.)

In fact, the MIDI signal is electrically separated with an opto-isolator, so even if it did carry power, there would be no way to transfer power between the ends of a MIDI cable, regardless of whether it connected to a USB converter or directly into another device.

However, your second paragraph is about a direct USB connection between devices…

USB can carry power.  However, it's directional.  (Before USB ‘C’, this was enforced physically: ‘hosts’ such as computers and hubs had A-type sockets, while devices such as printers had incompatible B-type sockets, and cables would always have an A-type plug at one end and a B-type plug at the other.)  In general, I think the host must provide power for the device to use, never vice versa — and since the computer or tablet must be the host, then it can't receive power from the electric piano.

More recent USB versions have added more flexibility for charging, but the Roland FP-10 has only a traditional USB B-type socket, so it won't be able to provide power through it.

All of which points to your option 3 (no charging), either way!

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  • Thank you for such in-depth answer.
    – greenoldman
    Commented Jan 28 at 4:26
  • I don't believe the part about the host is right. With USB C you can have host being charged by the peripheral device. For example, a laptop can be charged by a docking station. That being said, I haven't heard of musical instruments making use of that... but perhaps that's coming? The OP's example with a piano and a tablet actually would seem quite practical.
    – user1079505
    Commented Jan 28 at 16:45
  • @user1079505 As I said, more recent versions of USB have a lot more flexibility about the direction, voltage, and current you can use to charge things.  (USB Power Delivery is the main standard, though there are others including several proprietary ones.)  I can't find any information on when the FP-10 was introduced or which USB version(s) it supports — but the fact that it uses an old-style B-type port, and doesn't mention USB2 or USB3 suggests it's only original USB (1), which didn't have any of that.
    – gidds
    Commented Jan 28 at 19:07
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Such functionality would be mentioned in the manual. As I couldn't easily find anything regarding charging via USB, I would assume there is no charging in either direction.

From my experience, this is a regular behavior of larger pianos with speakers and dedicated power adapter, like FP-10. In turn, MIDI keyboards, which produce no sound or audio signal on their own, often draw the power from the USB cable.

I haven't heard of an instrument that could provide power to the tablet. I think the name of such functionality would be power delivery.

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  • Many thanks. Just clarification, I didn't mean solely MIDI controller in the opening sentence, just digital piano which does not have USB, so usually you end up buying some MIDI-to-USB kit to connect it computer/tablet/etc.
    – greenoldman
    Commented Jan 27 at 13:12

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