Disapearing pl2303 usb serial adapter on rpi2

[bob prohaska]

On Fri, Dec 22, 2017 at 12:02:46AM -0800, Rodney W. Grimes wrote:
> 
> 
> Cmos latchup could exibit that behavior, a next test would be
> to just remove the gnd pin from the data end next time the
> connection stops working and see if that clears the fault.
> 
Tried it, no effect.

> You could also try removing and replacing the data end one pin
> at a time and see if the fault clears after any one of these,

Tried that too, likewise no effect. It seems mandatory to lift
all three serial-end pins _and_ the usb connector.

> I am suspecting a latch condition in the FTDI TX pin output
                                           ^^^^ did you mean Prolific?
The two FTDI usb-serial adapters I have work perfectly.


> buffer caused by out of range voltage caused most likely
> by excess cable/capacitance induced ringing.
> 
The cables are about three feet long, as furnished by the manufacturer.
I do wonder if slowing the serial speed down might help; a console
certainly needn't run at 115200 bps.

It might help if I indicate the layout of the system, since I gather it's
perahps a little unusual. There are four hosts, call them com, net,
ns1 and ns2. They're all networked to a hub, with the usb/serial cables
arranged so that each RPI2 provides terminal service to the next host
in the ring:

com-usb/serial-ns1-usb/serial/-ns2-usb/serial-net-usb/serial-com

There was a faint tendency for hosts net and ns1 to have more usb/serial
adapter lockups, so those hosts got FTDI adapters and all was well until
the host called com (the test box) was upgraded to r326951. Serial link
uptimes went from weeks or months to hours, for that host only.

The serial cables obviously ground all four RPI2s together, I think the
ethernet likewise distributes ground. Wall warts are connected to a single
outlet strip, are isolated and do not distribute ground. All the cables are 
confined to a small shelf about one by two feet in size.

It wouldn't be surprising if this turned out to be a wiring issue, but
proof has so far proved elusive. Just for completeness, there's a photo
of the setup at 
http://www.zefox.net/~fbsd/com_net_ns1_ns2
Left to right, the hosts are com, net, ns1 and ns2. I admit it isn't 
pretty, but is far from the biggest cable nightmare seen elsewhere.

There is one device not shown, a networked printer sitting on the
table above the shelf, plugged into the same outlet strip that powers 
everything in the photo. It has a three-wire cord and probably provides
a single point ground for the whole network. In principle that should
be good. Multiple paths to ground are understood to be bad in general 
and have been mostly avoided.

Apologies for the length, thanks for reading!

bob prohaska