How to have access to a USB port from inside the case?

Question

I'm working on making a case for the project SamplerBox.

I would like 2 or 4 USB ports of the Raspberry Pi to be reachable from outside the case, like the photo below.

But then, how to access USB from inside the case? I need to access USB inside the case to connect a 2nd SD card reader.

Why a 2nd SD card reader?

interal microSD card : for the OS + software
2nd SD card reader : for user sample sets (it's an electronic music instrument / sampler, and the user should be able to load sample sets; you can imagine this SD card reader as the SD card reader of a digital camera: useful for user data)

Why not use SPI instead of USB ? The transfer rates will be 10 times slower, and this is not an option.

Problem : USB is not available from GPIO (this would solve the problem!).

Question

Do you have an idea / solution for making a case such that:

the 2 or 4 USB ports are available outside the case for the user (for example to connect a USB MIDI keyboard, etc.)
inside the case, one USB port should be connected to a PCB SD card reader under the hood

?

Is there a reason you're using SD cards other than compactness? It might save a lot of headaches to switch to USB storage. — goobering, Nov 16 '16 at 01:02
@goobering Yes mostly for compactness reason. I don't find it good in terms of user experience to have a USB stick that goes out of the box. It would be a bit like this on a digital camera :) But you're right, lots of headaches indeed to find a solution to have a SD reader. — Basj, Nov 16 '16 at 21:13

score 1 · Answer 1 · edited Apr 13 '17 at 12:56

Take a look at the bottom side of the Pi's PCB (see here) there is a number of test pads - labelled PPxx. Those located near the USB jack are actually connected to the USB data lines. You need to figure out which pads belong to which jack but that should be eays enough. People may say that wiring for USB is critical with respect to timing of high frequency signals but I say: give it a try. Keep the cable short and of equal length and it might work out. This is an example to have an USB hub work this way on a Pi Zero.

Do not forget to block the corresponding external jack to prevent the user plug something in there since there cannot be two devices at the same data line with one host.

PP27 USB VCC
PP28
PP35 VCC +5V
PP36 USB D-
PP39
PP40
PP41 USB D+
PP42 USB D-
PP43 USB D+
PP44 USB D-
PP45 USB D+
PP46 USB D-
PP47 USB D+
PP48 GND

Source

An alternative unrelated to USB would be the use of GPIO pins to connect an SD card (after all that is how the on-board SD card is allready connected). See here.

Thanks! I'll probably try this once! (But still unsolved for me, because I was looking for a easily reprocible solution that doesn't require to hack the hardware of the Pi, because very few users of http://www.samplerbox.org would risk to destroy their Pi by soldering it) — Basj, Nov 16 '16 at 21:21

score 1 · Answer 2 · answered Nov 16 '16 at 01:00

1

Use two USB A male to female adapters, or build small USB male-female PCBs, plugged into two USB sockets to extend them out from the Pi. Build the case around those, leaving two internal sockets to connect to.

answered Nov 16 '16 at 01:00

goobering

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This might be a reasonable idea to stick on this method (that I currently us for the project). Thanks. – Basj Nov 16 '16 at 21:21

How to have access to a USB port from inside the case?

Question

2 Answers2