On Wednesday, October 06, 2010 02:55:53 pm Alan Stern did opine: > On Wed, 6 Oct 2010, gene heskett wrote: > > I just spent an hour drilling more holes in the wall/floor here, so I > > could get those two long extension cables to a direct from the mobo > > port. The ports I used are on a breakout back panel slot, and add > > about 15" to the cable length, plus of course the big lump from the > > connector junctions at the slot filler. Neither one works, error > > -71. So, move those two to a powered hub which adds about 4 feet of > > cable from the hub to the port, and both are now working. So if the > > mobo ports have as much signal driver power as the 4 port hub, then > > its the extra 15" of cable from the mobo to the slot filler and the > > (low) impedance lumps at the slot filler sockets that are killing it. > > Damn if I do, and damned if I don't. > > Maybe... Hardware faults can be very difficult to pin down. For > example: I've got a USB-IDE disk drive that doesn't work when attached > to my home computer. When plugged in to my office computer (using the > same cable), it works fine. When I attach a USB flash drive to that > cable in place of the disk drive, it works on the home computer. And > when I replace that cable with a different one, the disk drive works on > the home computer. Since replacing any of the three components fixes > the problem, how can you say which is at fault? > > > I guess the upshot of this is that as an rf broadcast engineer, I have > > learned that the VSWR present on USB cables is in no way, properly > > terminated. It it was, then it should be somewhat like the scsi buss, > > whose original specs 35-40 years ago claimed to have a max cable > > length limit of 39 meters, if it was properly terminated on both ends > > to absorb the echos from 5ns signal edges. > > There _are_ standards specifying what the USB physical and electrical > characteristics are supposed to be. If you've got the right equipment, > you can check the USB terminations. > > > But between the engineers and the production floor was a bean counter > > who bought the cheapest +-20% parts he could buy, so we have had 35+ > > years of scsi having a reputation that not even sacrificing virgins > > would make it work reliably. Maybe it still has that rep, I haven't > > used it for a tape drive in yonks, vtapes on a hard drive are easily > > 50x more dependable and 100x faster to recover from. > > > > I have fixed a lot of scsi systems right, and without using up any > > hard to find virgins, made them truly bulletproof. Bean counters > > will be the death of anything they touch, could we start a bounty > > system on them? ;-) > > There's no question that a _lot_ of USB devices are designed to be as > low-cost as possible, which means cutting a bunch of corners and giving > up compliance to the spec. For many companies, if the device works > okay when plugged in to a Windows machine using a standard cable, > that's all they care about. > > Alan Stern I'm still snooping, but haven't resorted to my 100mhz dual trace scope yet. I just set that up as a powered 4 port connected to the mobo, with both extension cable/hubs and an FDTI usb-ser adapter plugged into this same 4 port hub giving me /dev/ttyUSB0 for heyu to use. Everything seems happy, and it frees up a 7 port I'll take downstairs and see if it will work in place of the 4 port CyberPower I moved up here and connected as above. Yes, I just took the 7 port Alps down and hooked it up, and now everything is again running on one extension cable. Traffic on that particular circuit isn't normally going both ways, what I am doing is shipping the printer port of an old CoCo3 up here, through cups and back to a laser printer down there so I effectively have a 22 ppm laser printer on a 30 year old computer that about 300 of us refuse to let die. So this branch now has a 4 port powered hub plugged into a motherboard connector, 3 loads plugged into it (one is the scanner) and two of those are the extension cables I paid $40 & change each for. A 7 port Alps is plugged into one of the extensions for a local hub in the basement, and the two devices there then plugged into the 7 port, and again, both are working. Interchange the 4 port and the 7 port, and its DOA. I think I need a beer. ;-) Thanks Alan. Till next time, Cheers. -- Cheers, Gene "There are four boxes to be used in defense of liberty: soap, ballot, jury, and ammo. Please use in that order." -Ed Howdershelt (Author) Bershere's Formula for Failure: There are only two kinds of people who fail: those who listen to nobody... and those who listen to everybody. -- To unsubscribe from this list: send the line "unsubscribe linux-usb" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
