Disapearing pl2303 usb serial adapter on rpi2

[bob prohaska]

On Thu, Dec 21, 2017 at 08:19:43AM -0800, Rodney W. Grimes wrote:
> 
> What do you mean by unpowering both ends?
> If this is one of the serial adapters that has a power pin
> to plug in with the Gnd/Rx/Tx pins I would highly recommend
> you leave that power pin disconnected as connecting it can
> lead to power supplies in parallel and "that is a bad thing".
> -- 
> Rod Grimes                                                 rgrimes at freebsd.org

I should have written "disconnecting". The red wire wasn't in use. 
In the past unplugging and replugging the pl2303's usb connector 
by itself wouldn't allow FreeBSD to recognize  the adapter, it was 
necessary to also unplug the data pigtails. 

The same appears true now; the adapter wasn't recognized by FreeBSD,
despite repeated disconnect/reconnect cycles of the usb connector, 
but removed to an RPI3 (running Raspbian) it was recognized immediately.

When moved back to the RPI2 running -current, it was recognized and
at the moment seems to behave normally. For now I'll just leave it
alone to see how long it stays up.

I've got four of these adapters, and they all seem to exhibit the 
same behavior, but not to the same extent, among the four RPI2's on
my local network. Two FTDI usb/serial adapters were substituted for
the two most troublesome pl2303 devices, and all was well until the 
update to r326951. 

Is there a way to determine where/what is getting stuck? There seem
to be at least three possibilities: usb, ucom and uplcom. Other usb
devices seem unaffected.

Thanks for reading, and any suggestions.

bob prohaska