In an effort to provide a kind of "half-fail-safe" access to the 
RPI2s in the event of power outages and similar catastrophes a
quartet of pl2303TA-based USB-to-serial adapter cables were bought
and connected in a ring, with eash RPi2 functioning as a terminal
server for the next RPi2 in the ring. All four RPI2s were connected
to a LAN by wired Ethernet for normal access. The logic was that if 
any one Pi came up multi-user it would provide access to the console
of the next via ssh for troubleshooting, and so on around the ring. 

In practice, two of the pl2303s have been quite stable, the other
two seem to lock up hard after variable amounts of time. Here's
an example of the console output on lockup under R301569:

login: 

FreeBSD/arm (ns1.zefox.net) (ttyu0)

login: uplcom0: at uhub1, port 4, addr 5 (disconnected)
usbd_req_re_enumerate: addr=5, set address failed! (USB_ERR_IOERROR, ignored)
usbd_setup_device_desc: getting device descriptor at addr 5 failed, USB_ERR_IOERROR
usbd_req_re_enumerate: addr=5, set address failed! (USB_ERR_IOERROR, ignored)
usbd_setup_device_desc: getting device descriptor at addr 5 failed, USB_ERR_IOERROR


Since two adapters seemed to work consistently while two others failed it
made sense to swap the adapters around, to see if the problems followed
the adapters or stayed with the hosts. Initially the problems followed
the adapters, then one formerly-reliable adapter failed too. 

After more testing, there still seems to be more trouble with the two
adapters plugged into the pair of RPI2s ethernetted to the DSL modem. 
The pair of RPI2s ethernetted to a separate switch (which is also cabled
to the DSL modem) are not entirely free of trouble but experience it 

On 6/22/16 all four RPI2s had their Ethernet connected to the 5-port
switch, which is cabled to the DSL modem/router by a single cable.
All of the RPI2s have isolated power supplies so this layout should
eliminate any potention for ground loops beyond those furnished by
the serial cables themselves.

The USB-serial adapters do have continuity from serial ground to USB
ground, but it's fairly resistive, around 0.4 ohm. 

6/24/16
Over the past day both usb-serial adapters on www.zefox.net and ns1.zefox.net
have hung. Those are the two machines most inclined to trouble, and the two
which were formerly connected directly to the DSL modem. Clearly moving the
network cables didn't help. ns2.zefox.net and www.zefox.com, as before, have
exhibited no hangups. It also appears that unplugging the adapter in www.zefox.net
triggers a reboot, which does not happen on ns1.zefox.net.