10

Not all cables seem to be created equal but I've never got to the bottom of why so many (cheap) cables I try only charge - no data. Anyone know why? Certain pins not connecting?

I do know that longer cables or car to usb converters fail to transmit enough power for the phone to register and not charging but I'm getting the opposite. I don't think I am alone (same problem for Nexus One users here)

10

Broadly speaking, if you have a "USB" cable that only connects pins 1 & 4 then you will get power and no data (Best Buy was selling cables like that as Universal USB Charging Cables a while back, not sure if they still do). Likewise, if you have a cable with only 2 & 3, you will get data but no power. If you're curious, here's a Wikipedia page with the standard USB Pinout.

Note: Micro uses the same pinout as USB A.

  • Thanks. I checked those pins and they were good. Seems like my phone had dirty 2 & 3 connections and after a lot of wiggling about the cheaper cables seem to work most of time where previously they never worked. – KCD Oct 17 '11 at 0:53
5

As devnul3 says, a power-only cable is a USB cable where the data pins are not connected. The main reason they exist is because they can be made ever so slightly cheaper by virtue of only needing 2 connections instead of 4 (and also because you don't need to worry about preserving the quality of the very high frequency signals on the data lines).

This said, there are cases where a power-only cable is preferable to a "fully featured" USB cable: when you want to charge your device from some random USB connector somewhere. As is becoming more widely known, a USB connector can be used for many things besides the ones the user expects, so if you use a "normal" USB cable, you may end up unwittingly providing priviledged access to your device.

IOW when you're charging from a USB connector that you have no reason to trust, a power-only cable is not only ever-so-slightly cheaper, but it's also a bit safer.

3

Sorry to disagree with the other answers, but those assumptions are simply wrong:

  1. To be compliant with Battery Charging Spec. or any other comparable charging mechanism, the data lines require certain voltages or a shortcut (e.g. BC with <200 Ohms) to identify the charger type, i.e. the charger provides that signals - hence the data lines are required for the attached device to identify the charger.
  2. Even if only data shall be transmitted, at least data lines and GND are required (see USB 2.0 Spec. Figure 7-19). Hence, is no good reason to keep the VCC line disconnected.
  3. If the cable-connectors have the USB logo on them those cables HAVE to be compliant with USB standard, which in turn means that all pins have to be connected.
  • Interesting. Charging has changed since this question was asked so it seems faulty/fraudulent cables will fail to negotiate a higher current and take much longer to charge or not charge at all depending on the devices – KCD Dec 9 '17 at 16:25

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