Date: Tue, 21 Jun 2016 17:19:53 -0600 From: Ian Lepore <ian@freebsd.org> To: bob prohaska <fbsd@www.zefox.net>, James Elstone <james.c.elstone@gmail.com> Cc: freebsd-arm@freebsd.org Subject: Re: pl2303 lockups on rpi2 Message-ID: <1466551193.34556.76.camel@freebsd.org> In-Reply-To: <20160621223531.GC2449@www.zefox.net> References: <fbsd@www.zefox.net> <20160619020311.GC38492@www.zefox.net> <20160619034248.1D097406057@ip-64-139-1-69.sjc.megapath.net> <20160621041439.GA2449@www.zefox.net> <CANocirkCFXr8zDQZQv_Cm6j4UYDFHEuvQ2Fwn7wmwPuqkvTWaw@mail.gmail.com> <20160621223531.GC2449@www.zefox.net>
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On Tue, 2016-06-21 at 15:35 -0700, bob prohaska wrote: > On Tue, Jun 21, 2016 at 10:34:41PM +0100, James Elstone wrote: > > Hi Bob, > > > > What sort of flow control (rts/cts, xon/xoff, or none)? > > > > I suspect you cleared the control signals rather than unfreeze?? > > There are none to clear: The serial end is on an RPI2, with only > TX, RX and ground. The USB end is on a second RPI2. > > In that particular case, I tried unplugging and replugging the > USB end, to no avail. Then I dragged over an iMac with Prolific's > drivers installed and plugged the USB end into the iMac. The > PL2303 still wasn't recognized. While it was still plugged into > the iMac, I lifted the TX, RX and ground connections on the PL2303 > cable. The PL2303 recognition message immediately showed up on the > iMac's console. > > As it happens, the two RPI2s in question share a ground through > the wired Ethernet. They're connected to two different switches, > which are in turn connected by a length of cable. > > The ground cable on the PL2303 forms a second, parallel ground, > amounting to a loop. This is universally considered bad practice > but is often gotten away with. Perhaps I'm not so lucky. > > The Prolific driver installation instructions specify installing the > software, connecting the USB end next and connecting the serial end > last. I thought they were just pacifying the pedantic among us, > but maybe there's a physical reason behind the advice. > > Next time a PL2303 locks up I'll begin by lifting the serial cable > connections, ground first, to see what the uplcom driver does. If > it subsequently recognizes the PL2303 I _think_ that supports the > notion of a wiring problem. > > Thanks for reading and special thanks to Hal Murray for introducing > the > idea of serial devices being "held" by signaling voltages, > > bob prohaska > If the serial side has active signals and the usb side is unconnected then you are essentially applying power to some of the IO pins of an otherwise unpowered chip. Sometimes you end up accidentally powering the chip that way and it kinda-sorta works. Other times it just fries the chip. And sometimes it's harmless (usually only because the active signal line(s) can't source enough current to cause any harm). -- Ian
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