Multiport USB hubs/switches on Raspberry Pi

[Rodney W. Grimes]

> Does anyone have experience using multiport USB hubs on freebsd-arm,
> specifically in the context of Raspberry Pi 2 or 3?
> 
> I'm interested in trying to use a hub/switch (not sure there's a
> difference in the case of USB) to construct a terminal server for
> serial console access on a small cluster of Pi's. I'd want about
> seven or eight ports, each holding a USB-serial adapter that would
> then connect to the GPIO UART pins on one Pi in the cluster. I've
> used both USB 3.0 and USB 1.1 hubs, and in both cases odd things 
> seem to happen from time to time. One potential example is 
> 
> 
> https://www.amazon.com/AmazonBasics-Port-USB-2-0-Hub/dp/B00GGYU46O/ref=sxin_2?keywords=powered+usb+hub&pd_rd_i=B00GGYU46O&pd_rd_r=83f1e3f0-a1e6-40aa-bdd8-aa86a7c0b360&pd_rd_w=foNmt&pd_rd_wg=ncKZR&pf_rd_p=0395a9fd-d124-46c0-a48f-d8582ed1a45c&pf_rd_r=2P9WWPAZGMAA29NMVVV2&qid=1552234319&s=gateway
>  
> 
> If anyone has experience along these lines or a suggestion for/against
> particular USB devices or versions I'd be curious to know.

I am going to say some EE general issues that people run into
when trying to do this.

a)  Good power is a must, you must provide adaquate power to
    all parts of any system, and that power must be delivered
    without IR losses or drops.  (Much of the PC consumer stuff
    horribly overloads wires, connectors, and under utilizies
    proper filter capacitors (aka, the "bad caps" situation has
    more to do with not using proper caps than it has to do
    with any defects in the caps them selves, run near any caps
    current rating and it is dead in about 2000 hours.)
    I would suggest adding 5x-10x what ever the current input filter
    cap on each item in your cluster is to clean up the local
    noise.

    Do not try to deliver power over 24 (2 amps) or even 22  (3 amps)
    awg wire!  Most wall warts use 24 awg wire, at an ampacity of
    2 amps your just asking for lots of noise and sag.  

b)  Good ground is as important as a), one must design a proper
    star connected ground layout when doing this type of stuff,
    any loops in the layout must be broken.  Noise must be
    minimized on this ground system.  Ground noise is a direct
    loss of signal noise margin, and at 3.3V ttl levels it
    takes very little noise to destroy the signal.

c)  Central power.   Trying to do complicated large blobs of
    things like this that are powered from a bunch of wall
    warts is just a royal pita to get a and b right, so it
    is usually much better to have 1 power source for all of
    it.   (One person resopnded to the thread about there pick
    and place machine, I am sure that this is what he has done in
    it.)

> Thanks for reading,
> bob prohaska
-- 
Rod Grimes                                                 rgrimes at freebsd.org