Disapearing pl2303 usb serial adapter on rpi2

[bob prohaska]

On Sat, Dec 23, 2017 at 04:55:56PM +0200, Bob Bishop wrote:
> 
> Note that "The USB ports on a Raspberry Pi have a design loading of 100mA each...???[1] which is the absolute minimum allowed by the USB spec.
                                ^^^^^^^ maximum?
 
> 
> [1]https://www.raspberrypi.org/documentation/hardware/raspberrypi/usb/README.md
>

The PL2303TA data sheet at

https://cdn-shop.adafruit.com/datasheets/DS_PL2303TA_d20120504.pdf

claims the operating current is 10 mA maximum, plus output. The output 
drive rating is 4 mA, which puts the total far below the Pi's limit.

The other  USB loads on the RPI2 are a mouse, keyboard and Sandisk
Extreme 64 GB USB flash drive, which holds /var, /usr /tmp and swap.

The mouse is marked 5 V, 100 mA, the keyboard is marked 5 V 1.5 A,
but contains a hub of its own, so I don't know how much power the
keyboard itself uses. Both are old Dell units, it's fairly clear 
the keyboard rating includes outboard load and it's unlikely the
mouse ever draws full nameplate current. A Logitech hubless keyboard
is marked 100 mA, which seems a plausible upper limit for keyboard alone. 

The big mystery is the USB flash drive; I can't find any specs on
power consumption, the only measure is a "touch test", which reveals
the drive is lukewarm to the touch with a make session running in
/usr/ports. I'd guess it's around half a watt, but that's only a guess.
However, there has so far been no sign of misbehavior from the USB
flash drive.

Are there any software tools that can display USB power consumption?

At the discovery of the USB/serial problems around a year ago there
was a faint positional dependence; two of the hosts in the group seemed
more prone to trouble than the other two when using the PL2303 adapters.
That by itself suggested some sort of physical cause, but until Rod Grimes
mentioned connector shroud grounding it never occurred to me to check. 

Having learned since that the shrouds weren't securely grounded I've 
resoldered one and installed it, not expecting it to make much difference
since pin 4 _is_ securely grounded and the cables are unshielded anyway.
That adapter has been up for 18 hours, a recent record, though by no
means an all-time record. For the moment the best course seems to be 
waiting and watching.

 
Thanks to everyone for reading!

bob prohaska