On Sun, 2020-02-02 at 18:49 -0800, John Jason Jordan wrote: > On Sun, 02 Feb 2020 20:47:18 -0500 > tomas.kuchta.li...@gmail.com dijo: > > > > It was advertised as being USB 2.0. The interesting part is what > > > happens when I plug it into a USB 3.0 port. (None of my computers > > > have USB 2.0 ports any more.) Not only does the drive not need both > > > forks inserted, it also doesn't seem to matter which one I use. > > On the contrary - all of your ports are usb3 and also usb2 - it is > > part of the compatibility spec for the root device to have both 2 and > > 3 controllers hanging on the USB-A connectors. > > I am only concerned with power availability. My understanding is that a > USB 3.0 port can supply close to twice the current of a USB 2.0 port > (1.8x). So if the port is capable of being USB 3.0, with the power > availability of USB 3.0, when I plug in a USB 2.0 device it will drop > the available current to the USB 2.0 level? If so, is this because USB > 2.0 cables have fewer wires, or why? > > I'm still wondering about my USB 2.0 optical drive; why it functions > fine with either one of its connectors plugged in and not the other > connector. I don't have any USB 2.0 ports to plug it into to see if it > then needs both forks of the cable plugged in. I just assumed that it > would, else why would the manufacturer have included a cable that forks > into two connectors at the computer end?
In nut shell - on any USB 2 and 3.0 compatible port * USB 2 device cannot ask for more than 500mA in 100mA increments - and USB host (your PC) should give it at least 100mA until negotiated up to 500mA. * USB 3 device cannot ask for more than 900mA in 150mA increments - and USB host (your PC) should give it at least 150mA until negotiated up to 900mA. There are also multi-lane USB 3.2 gen 2 devices which can ask for up-to 1.5A in 250mA increments - and PC ports to match that (they are currently rare) Note: The wording "at least" because the port can be made by manufacturer to supply more as part of: * Battery Charging (BC) 1.1 - which can supply up to 1.5A at 5V outside normal USB device enumeration * Battery Charging (BC) 1.2 - which can supply up to 5A at 5V outside normal USB device enumeration 5A is maximum the USB connectors and cables can be made to handle. I personally have not seen any USB A type connector or cable for more than 2.4A. This is probably because standard PC power supply should give at least 2A on 5V rail when connected to AC. So , if a PC wants to guarantee more ON or OFF - it has to make sure that the power supply can supply it too. Typically only laptops and pre-assembled PCs can make any guarantees about its power supply. This is outside of the scope of this case/question - there is also USB power delivery (PD) standard using mostly USB-C connectors (though USB A/micro could, but mostly do not implement - PD 1.0). This requires active data communication to negotiate supply of up to 5A at 5V-20V for PD 2.0 and PD 3.0 USB 2 optical drive is not PD capable device. more at: https://en.wikipedia.org/wiki/USB So - you optical drive most likely negotiates max USB 2 current 500mA and then it takes whatever it needs and the USB port can supply - in terms of PC port ~ 1A or more if BC 1.1/2. If you connect to hub - things are limited to total 900mA for all ports unless - it is powered hub which can then behave like a PC port within its design/power supply. By design - USB 2 device with the Y cable should be designed to fit max 1A if both Y cable leads are connected to PC port. Single USB 3 PC port should be able to give 900mA - most likely enough for that drive. I hope that this is conclusive. -Tomas _______________________________________________ PLUG mailing list PLUG@pdxlinux.org http://lists.pdxlinux.org/mailman/listinfo/plug
