Hi I’m having an issue with what I thought was going to be super simple.
I have a Teensy 4.1 connected via its micro usb b connector to a USB B and mini USB B breakout pcb’s.


I need to be able to connect 2x mini pc’s to the teensy but only ever one at a time so one host to one device which I thought would be ok.
This works perfectly if I just connect mini pc 1 to mini usb or mini pc 2 to usb b.
the issue is that I can connect pc1 ok but if I have an open ended usb cable plugged but not connected to pc2 then pc1 will error device not detected, and vice versa if I have pc2 connected and floating wire not connected to pc1 then device not detected.
this is making no sense how I’m trying to describe the problem so I’m at work am tried to draw some diagrams.
The usb cable from mini usb b is built inside my project and can only be disconnected from pc1 at the pc end.
Does any of this make sense?
I need to understand why a floating usb cable stops the device being recognised?
Can I make it work with how I have it setup?
Thanks and sorry for my terrible description!

ceefna

 

You need to read up on how the USB works. The host can not handshake with two devices connected in parallel.

 

You need to read up on how the USB works. The host can not handshake with two devices connected in parallel.

If you look at the diagrams there are not two devices connected in parallel. Only one is connected at one time.

 

I think your best bet is going to be to get some sort of adapter to be able to use the one cord with both PCs. It's probably a matter of the extra cord adding enough capacitance to throw off timings. USB is very particular.

The diagrams are worth 1000 words as they say. Makes it easy to follow what you are trying to say.

 

USB is a matched impedance differential protocol. Your usb isn’t being recognized because you have stub lengths that are not terminated properly and creating reflections on the line. The shorter stub lengths aren’t adding as many reflections.

 

Thanks for all your replies, I have come up with a solution that is not very pretty but does exactly what I need.
I used 2x motherboard splitters, connected one of the 4 outputs on each board together so the board that has its input disconnected is ignored when the other input is used. Doesn’t matter if there is a floating cable and connects every time via which cable I choose.
Can you see any issues with this? Apart from accidental connection of both pc’s but that will not happen.
many thanks
Ceefner

 

Thanks for all your replies, I have come up with a solution that is not very pretty but does exactly what I need.
I used 2x motherboard splitters, connected one of the 4 outputs on each board together so the board that has its input disconnected is ignored when the other input is used. Doesn’t matter if there is a floating cable and connects every time via which cable I choose.
Can you see any issues with this? Apart from accidental connection of both pc’s but that will not happen.
many thanks
Ceefner

This seems like an extremely complicated replacement for a couple of resistors in series with the data lines. Try something in the range of 20-30Ω on each side. Your problem, as mentioned above is an impedance mismatch causing reflections from the unterminated data lines.