Electronic – Why must USB low-speed devices use a captive cable

cablescapacitanceusb

The USB 2.0 spec says: "A low-speed device is required to have a captive cable." However, I can't find a good explanation for why this is the case. I have seen hints towards two possible explanations:

  1. To prevent users from accidentally using a (non-captive) low-speed cable with a full-speed or high-speed device. However, there is an error in this logic: To achieve that, the spec would only have to demand that low-speed cables must be captive. It would not have to demand that low-speed devices use a captive cable, the difference being that a low-speed device could use a non-captive full/high-speed cable.

  2. Differences in cable capacitance. Section 7.1.2.1 says: "A low-speed device (including cable)
    may have a capacitance of as little as 200 pF and as much as 450 pF". Unfortunately I found it hard to find numbers for high-speed cables. Things get more complex because different cable lengths exist, but then, I don't understand why the spec disallows using these cables altogether instead of saying, if a low-speed devices has a B-receptacle then it must handle all standard cables, and only if it can't do that a captive cable is required. Also, in case standard cables have a capacitance that already exceeds 450pF (as noted, I don't know the actual numbers), then hub drivers are strong enough to handle that because they must (for fullspeed), so they could do the same for lowspeed — and for the device, again, a captive cable could be required only if its driver is too weak for the capacitance range of standard cables.

Best Answer

The reason is, as in your first explanation, to prevent the accidental use of a low-speed cable with a full- or high-speed device.

Yes, you're technically correct that they could simply mandate that low-speed cables be made captive - but having removable cables being used on a low-speed device would create a market for cheap uncertified cables being sold with (or not sold with) an "only for use with low-speed devices" warning that people could ignore and use on other devices.

Making low-speed devices use captive cables simply removes that market and any chance of ever using the wrong cable.

Related Topic