Date: Fri, 22 Dec 2017 12:01:54 -0800 (PST) From: "Rodney W. Grimes" <freebsd-rwg@pdx.rh.CN85.dnsmgr.net> To: bob prohaska <fbsd@www.zefox.net> Cc: "Jukka A. Ukkonen" <jau789@gmail.com>, freebsd-arm@freebsd.org Subject: Re: Disapearing pl2303 usb serial adapter on rpi2 Message-ID: <201712222001.vBMK1sqR038282@pdx.rh.CN85.dnsmgr.net> In-Reply-To: <20171222183516.GB23984@www.zefox.net>
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> 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. I would need a circuit layout diagram to be certain, but I suspect that if this device does latch up it is going to stay latched until all sources of voltage are removed. > > 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. Try lifting everything but the data end gnd, that would be the USB connector, and RX and TX. > > 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. Ok, me bad, I should try to find a datasheet on the Prolific and see what its ESD/latchup protection rathings are. The FTDI is pretty well protected. > > > 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. Maybe, but probably not going to have an effect, but always worth a try. > 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 Ok, that is good additional data. > 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. I think you are correct in that the serial cables are tying all 4 systems grounds togeather, I am unclear on what the USB ground isolation rules are, I would actually suspect they are isolated. I put an ohm meter on my ftdi USB serial adapter and its gnd pin is infact tied to the shell of the USB connector, so that is a ground path. Ethernet does NOT have any ground wires, they are differential pairs and isolated via the transceiver. My pedantic side does not like this stringy "using a signal cable" as a system ground. I would rather see all 4 RPI's tied with a good ground to each other, and disconnect the serial cable gnd wire. > 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. Looks okay, but this is still a poor system ground in my mind. This is not an earth ground, but a signal ground, and those are usually best done in a star topology, not a string. > 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. There is no ground signal path in your ethernet cables, so you do not haved a "ground loop" that I can see. > Apologies for the length, thanks for reading! > > bob prohaska > > -- Rod Grimes rgrimes@freebsd.orghome | help
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