length USB cable to pc

I did this research a while ago so I'll try to clarify this. The specs of the USB interface imply that the maximum cable length should be 5 meters. That means one single cable or two, say, 2 and 3 meters common USB cables connected to each other. If the length is more than 5 meters there 'may' be some errors in the data. Also you are able to avoid this 5 meters limitation by using so called 'active' USB cables, you should look for them in a dedicated computer supermarkets. There are quite many ways for doing such task, from USB cables that, again, may or not require even an additional power supply or the ones that use the LAN technology to deliver the USB connection through tens or even hundreds of meters. The price is somewhat higher than a conventional USB cable though not so extremely unrealistically high.

Your setup will perform perfectly right. There is nothing about the delay of the signal in all cases, there will be just zero loss in timing realistically speaking. My advice is just to go for more expensive 5 meter conventional USB cables, the more thicker ones, you know. I have quite experience with them and the cheaper ones may lead to disconnects from time to time or even introduce some buggy behaviour.

Or, as stamkorg says, you may consider using the actual MIDI cable from your DP to the audio interface if you have the MIDI sockets on your keyboard. I know nothing about the MIDI cable length limitations. Also I can't predict what will be more superior in terms of latency, direct MIDI connection or the MIDI over USB to your computer.

Also, sorry to say that and hope being wrong, the Focusrite USB audio interfaces have quite bad reputation around in terms of latency. Though no complains about other parameters reported about them.

MIDI max length is 15 meters in theory. Much more than USB. In fact it is even much more. So yes, using a MIDI cable from the kb to the interface is a good solution. This being said, I'm using a 8 m USB cable here without any problem... Latency will not change with cable length: it works fine or not.

1) No, you are not pushing any limits, you are absolutely in the safe area, just better try not to buy the cheapest one 5-m USB cable in the store.

2) By the picture from your first post I assume that you may use a very short (or any length) MIDI cable to connect the piano to the audio interface and then to connect the audio interface to the computer with a 5-m USB cable with no problems. That will keep all your stuff perfectly organized as far as I can see it.

Speaking about the audio interfaces the RME stuff are very popular and considered pretty Hi-End class if you can afford it, the Prism Audio interfaces seems to be even more top level though (Lynx Audio or Apogee devices sure can be called 'good' also). I honestly don't think that you 'need' to go that far, try your setup connected and working, it should do the job right for you (and for most people), just if you desperately want to upgrade there are sure options to choose from.

I cant say that super confidently that MIDI cabling is that much 'better' than USB - it is not such kind of a scalable project to grade it you know :) It will flawlessly work in both cases, just go and connect it and start playing :) Both ways of connection will do the same. ...Ok, go for MIDI cable, you will have the one more free USB port on your computer then :)

I can't tell the source, just some time ago I was looking for a new audio interface to buy and was thinking about entry level Focusrite USB interfaces, but from some research I could see quite many reports about issues with latency. Glad to hear that it is not a problem in your case. I tried to google the  same topic now but almost could not find anything. Are you interested to check your Scarlett 2i2 RTL (Round Trip Latency) with a test utility? If you don't mind here is a link: http://www.oblique-audio.com/free/rtlutility Note that you need to connect the output of the interface directly to its input with a cable to perform this test. The reason to use this method is because the interface latency shown in, say, a DAW audio preferences is a so called "reported by a driver" and sometimes manufactures fake this data.

I'm using antique M-Audio Audiophile 2496 internal card with Windows 8.1 with quite impressively low latency timings. Here are my results for some most useable settings:

buffer size (samples), sample rate, measured RTL
64 44100 4.467
64 48000 4.124
64 88200 2.248
64 96000 2.045

Interesting. From your tests one could see that Scarlett and Behringer is 5 ms. behind the internal audio. But what is the delay for the internal soundcard?

If you are still interested you may still test RTL manually. You should have some sound sample, some kind of a click sound. Then put it to some audio channel for the playback, interconnect the output with the input on yours Scarlett directly with a cables (or one for mono) then set another audio channel for the recording from the hardware input. Hit rec/play. Check how far behind this new recorded audio is. This is Round Trip Latency time for your card.

Thank you very much, I was very interested to check it. 9 ms. roundtrip is absolutely within the usable limits for live monitoring/playing. Lets not forget that the "play" latency will be in this case 5 ms. or lower.