According to its manual, the MPK225 can do what you want, but you need a separate power supply.
The same applies to the Novation Impulse 25, Alesis VI25, iRig Keys 2, and iKeyboard 3X. (Some of them need a USB power supply, some need a special power supply.)
All of them can be used standalone, but some of them can be programmed only from a computer; check the manuals.

Have you checked out the USB <--> DIN connectors? These allow you to connect a device with say USB only to a device that is DIN only.

Beware that there are at least one very cheap version available that is sold widely that is prone not to work properly, and this variant should be avoided. By 'cheap' here I mean below $12. There are however various others, for example a device from Roland, which works VERY much better although it costs more (but I think still less than $50).

There has been previous discussion here with people who have had problems with the cheap device, and that then found the Roland to work perfectly.

Such a connector may well allow you to use the Yamaha device as the 'controller' for the Roland module?

Geoff

Thank you both for your help. I'll have to check out your suggestions. Geoff, the Roland UM-ONE Mk 2 you suggested has the wrong connector at the USB end. I'll have to check with Sweetwater, Roland or someplace to see what its primary use is.

Clemens, I'll check out those other brands at Guitar Center, though the kids there aren't too good with technical questions.

Gerry

There are a number of different USB plugs/sockets, they each have a specific designation but I'm not sure what they are. There are certainly little adaptors to link certain different types. I have adaptors which I use to link the normal large flat type with the square type, as I have a printer that uses that. I have other adaptors which connect with both of the smaller types as used withhand held devices.

Which connector does your keyboard use. I'd guess the larger square one (the plug is about 1 cm square)? I assume that the Roland device has the standard flat plug (about 1/2" x 1/8".

Update - now found the PDF of the manual, and I note that the P-125 has a Type B USB connector, which is the 'square' one I referred to. I suspect that the Roland device uses an Type A connector, which is the standard flatish one. You can certainly get adaptors for this, as I have one. You can also get a cable with an A plug on one end and a B plug on the other, as I have one of those as well (used by a couple of external hard disk modules).

Geoff

Geoff, my synth has the square socket. The only place I've seen the rectangular receptacle is on computers which leads me to guess that the purpose of the Roland adapter cable is to extract MIDI information from a computer and send it to a synth or controller.

I'm beginning to doubt if there's any way to convert the USB information to the DIN connecter without the use of a computer. Further, last night I dug out my Juno 2 and connected it via MIDI to the VR-09. It seems that though the VR-09 responded to the Juno, it ignored any parameters I changed in the signal. My guess the MIDI master in the sky is suggesting I give up on this project!

Gerry

Yes, you've got a problem.

I've now found the Midi Implementation Chart for the Yamaha device (P-125) and this makes it clear that while the keyboard is capable of receiving and responding to a fair range of midi instructions, it is VERY limited regarding SENDING midi data. Mostly this is Played Notes, Program Changes and some Controls generated by the foot pedals. I suspect that you would wish to do a lot more than that. The P-125 is just not designed to be any sort of 'controller', that may allow various keys to to programmed regarding midi instructuins, esp relating to various controllers, so that things could be implemented during playing.

Maybe you need an additional programmable controller - the devices mentioned by Clemens above may provide some of the facilities you need, and could be used to supplement the P-125 (not replace).

Geoff

It sounds like you could use any MIDI keyboard that has a DIN MIDI OUT port to send messages to the Roland VR-09, such as the Akai MPK225 or Roland Juno 2 that you have mentioned.

If you want to send messages from the Yamaha P-125 to the Roland VR-09, it sounds like a "USB MIDI Host" could be a solution. The purpose of a "USB MIDI Host" is to convert a USB type B port (the square shape) which sends MIDI messages into DIN MIDI ports. Then you could use a MIDI cable to connect to the Roland VR-09.

A recent thread discussed using a Yamaha P-125 with a USB MIDI Host. In that thread, I said I believe you would need to find a USB MIDI Host that will work with USB hubs, because the Yamaha P-125's multiple USB functions will probably appear as a USB hub with multiple devices connected to it.


About USB

In a very generalized simplification, products with a USB port can be a computer "host" with a type A port (flat rectangle shape) or a peripheral "device" with a type B port (square shape). Simple USB connections will use a USB A to B cable to connect a host to a device, for example, connecting a computer (host) to a printer (device).

When converting USB MIDI to DIN MIDI, it matters which side the USB connection is on, host or device:

To connect a computer's USB A port to a DIN MIDI device, you need a USB MIDI interface, like the Roland UM-ONE mk2.

To connect a MIDI device's USB B port to another MIDI device's DIN MIDI ports (without using a computer as a go-between), you need a "USB MIDI Host".

This is a considerable help.........

https://www.youtube.com/watch?v=neXswJJcatc&t=3s

Gerry

USB MIDI to DIN MIDI with no computer? You need a MIDI Host unit, e.g. the Kenton MIDI Host

https://kentonuk.com/product/midi-usb-host-mk3/