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by Julian Stacey

Corrections etc Welcome


Too many (self included) have had or still see USB problems, I guess too often it gets blamed, on mystical `non USB-2 compliant cables' , which might occasionally even be true, if perhaps some cables aren't screened too well, or are too thin, but I think for many things there's a much simpler explanation: Inadequate power & bad connections.

FreeBSD allows power ratings on devices to be seen with `usbdevs -v`

USB spec. allows devices to draw no more than 0.5 Amps = 500 mA.

But devices such as external disks Lie & say "self powered", even when no power supply is connected. What this must mean is "Device is capable of or should be self powered" - but does not have intelligence to realise currently Not self powered. ... & sometimes such disks may work, & sometimes not ... (what's the betting more likely if the hub power is plugged in ;-)

Power Consumptions

Power consumption varies. If your computer or hub power supply is strained, (or cable kinked & some wires fractured to make a high resistance) not all devices may be affected the same way.

Example: Some USB memory sticks power ratings from `usbdevs -v`

  • 256 M - Celldisk 100 mA
  • 512 M - Kingston 100 mA
  • 512 M - MyFlash 80 mA (stick now dead & gone)
  • 1 Gig - Integral 200 mA (this has a 1.4M pretend floppy lock/ file system, maybe using more high power logic ?)
  • 2 Gig Sony - (Many were Defunct) 100 mA
  • 2 Gig SanDisk 100 mA
Other devices
  • Havit USB Keyboard is rated on back as 0.25 Amps
  • USB to PS/2 converter "Semi Tech PS/2 Keyboard - PS/2 Mouse" reports with `usbdevs -v` 100 mA, regardless if mouse & keyboard plugged into it.
  • Larger Havit PS/2 keyboard (that I use through a USB to PS/2 converter is not labelled for power, so no idea what it draws
  • A small blue Kensington mouse isn't power labelled, but `usbdevs -v` reports 100 mA
  • A mouse given away as a pharmaceutical promotion reports: "low speed, power 98 mA, config 1, USB Optical Mouse(0x00c3)" (nice to see a non round number :-)
  • [ I'll solder up a socket to monitor power consumption later. ]
  • ( Internal mouse pad inside laptop reports 100mA. )
  • Disk: Toshiba MK1403MAV 12.7mm IDE 1.44GB Paper label say 0.7 A.
  • MK6021GAS HDD2183 MQ60kfGG 9.5mm IDE 16 16,383 2 1 60GB 5V 4.7W on startup, 2.6 Watts seeking, 2.3 R/W

Really thin cables in spring loaded rewind drums

I've just confirmed (as expected): That two ultra thin usb cables, on rewinding drums, wont pass enough power to an external disk (small laptop disk in small box, not a big chunky 3.5" disk) to start disk spinning (but will supply enough power to keep disk spinning if already started), So external power needs to be supplied direct to disk from transformer (even though thin cable carries sufficient power to turn on LED that doubtless works at lower voltage, & too high a cable resistance to supply enough power than disk motor & drive & adapter electronics. Confirmed as when wobbling it, I could not feel disk spinning.

With external power direct to disk, then it works (& one can feel it spinning too). Reason of course must be cable is simply too thin for disk power. Same disk works without direct power, if using a bit thicker USB extension cable to a powered hub.

The disk was not recognised (no /dev/da* on FreeBSD) unless the hub was powered up, or external power for the disk

This all at USB-1 speeds, so no mystical speculation about magic of USB 1 v. USB 2 cables applies ;-)

Power Supplies

Of 3 USB power supplies:
  • 2 came with 2 port USB 2 card from Belkin.
  • 1 Manufacturer DVE is rated 5V 2.4 Amp. Plug is too short to fit reliably in the Belkin Cardbus (credit card size) USB adaptor. Plug is far too short (7.5 mm) to do much more than wobble quite unreliably in drive box garyj@ gave me. Plug was long enough to fit in a Belkin 4 port USB-1 hub though. I cut off 2 mm of plastic on end of plug.
  • Other PSU looks identical shape but labelled Achme Corp, rated 3.0 Amp & Longer plug, 9.5 mm, which sits well in Belkin USB cardbus adapter & in garyj@ drive box. Belkin card has 3 A written on it.
  • The transformer of Zediworks 4 port USB2 hub is labelled 2.1 Amps Zediworks (subd reports an NEC chip) Hub also falsely reports "self powered" even when no power is plugged into hub or disc
  • All are 5V, positive centre pin, though different size round plugs.

Plug Sizes

Small plugs: One should also remember with these smaller than normal size USB plugs, that they're not as rugged as the full size ones, if they wobble the power may get interrupted to the disk, the disk may then reset confusing the Op Systems.

One small USB plug I had was too short: 6mm instead of 7 mm It didn't fit properly into external disk chassis (chassis from garyj@, with Grey plastic fascia) Of course thus a potential double problem of failing power & failing USB signal. Cutting off 1 mm of plastic made it work.

Hubs crash

Sometimes the USB hub locks up (crashes). Same solution as anything else, removing external power & USB cable to laptop (& drive if powered I guess) seems to reset (reboot) it.

MP3 players can crash FreeBSD

A power related problem. Likely it could do similar to other OS's eg Linux & XP.

Cabling - Pin Outs

On both full size flat (A) plug & square (B) plug, 2 of the pins (outer 2 on A) are longer to stabilise power before devices can signal on other 2 wires.

USB port on main boards

Pin Name Description Cable Colour
1 Vcc +5V DC Red
2 D- Data - White
3 D+ Data + Green
4 Gnd Ground Black

Plug at peripherals.
(B) Plug on cable, from single upstream side of a hub to a PC.
Socket (jack) at controller.
(A) As on a PC chassis & 4 outputs of a Hub.
(B) Single Socket on chassis of a 4 port hub

Wire Colour Code Other
Red, Orange Power, VCC, SVCC, +5V
White, Yellow P-, Data-, SBD-
Green, Grey P+, DATA+, SBD+
Black, Blue, Brown Ground, Grnd, Gnd


With a cable (from garyj@), transparent, with: 1 male flat plug forking to 2 cables:

  • 1 long thick cable, plaited screen, to a mini usb male plug
  • short thin cable, with simpler spiral (non plaited) screen going to a combination male + female:

Ohm Meter shows:
  • The thick long cable implements all 4 wires end to end.
  • The Male + Female plug implements all 4 wires.
  • The short thin cable only implements 2 wires: power.
Some uses:
  • Normal Single Use Configuration: A plug on thick cable connecting straight to camera or external disk.
  • Use just 1 socket to also Charge a 2nd device connected to the combi. male + female via thin cable, while simultaneously the standard size cable A plug connects PC to miniature device eg disk drive/ camera to be controlled from PC.
  • Extra power for non self powered External Disk (laptop sized) : If no external power supply is plugged in (or available), & disk perhaps wants 1.0 Amps; 2 USB sockets on your hub/ PC/ laptop could supply it, even if each limit supply to just the USB specified 0.5 Amps. (Note if silicon is not identical one may deliver more than the other [& possibly overheat or limit at less than 1.0 ?]
  • Use just 1 socket to also Charge miniature plug connected device (eg a camera or phone), while simultaneously connecting through male + female adapter to another cable with an A plug.

With a black cable with 1 Full size male plug with 2 black cables from it
  • 1 cable going to mini male plug
  • 1 cable going to another full size male plug

It's same electrically as previous cable set: The thinner of the 2 cables only has 2 power wires, no signal wires. As is has no female socket, the last of the cases listed above is not viable.

Grey cable with Male to male type A plugs each end, &
Male type A plug adaptor

I'm not clear what the purpose of these is, but ...
With a double socket I've cut off a dead main board I'll be able to make a male to female current measuring breakout lead.

My etc/devd/berklix.conf (ex usbd.conf)

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