On Sat, Feb 23, 2019 at 11:21:39AM -0500, rhkramer@gmail.com wrote: [...] > > Why would you want this ? From an electrical or logical point of view, > > it makes no sense. > > Well, two reasons: > > * in most cases (other than hot plug) you want to make connections first, > before you turn on the power (I mean, for example, you don't assemble your > computer with the power on) > > * and somewhat similarly (but in a very different context) in a 3 pin power > plug for an ordinary household outlet (in the US), the ground pin is longer > than the power pins so that the ground is connected before power is applied. Good point, so the hotplug case seems counterintuitive. But consider that in the first case, components are designed to handle the case of power coming and going. If you connect them with power running, you can run into exactly that situation that one data pin is connected before the circuit is powered up: the best thing happening is that the input's overvoltage protection (imagine a diode going up from the input to the positive rail) conducts more current than it's designed for. In the second case, the first contact is *protective ground*. It's there to protect you from a defective device shorting the hot end to its chassis. The power carrying pins are connected "at the same time" (i.e. in some random order, and possibly a couple of times, if you chose your time resolution high enough). Cheers -- tomás
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