Help with USB power (Technical)

d4mo · Guru Joined: 27 Jun 2005 Posts: 430

Ok, I am considering a little project here but I need some info first, and I can't seem to find what I'm looking for on the net.

Ok USB has 4 wires. 5v..Data +...Data -....Ground. My question is this. With the 5v wire...its there power flowing through that all the time, or could a program shut the power off completly? Also with the Data + and data - is that from the Device to the Computer or can it go in reverse?

EDIT: Basically I wanna build one of these http://www.gizmodo.com/gadgets/gadgets/magnetic-usb-power-strip-176470.php
I just need to know how I can turn on/off the power in the USB cable.

Cuardin · Posted: Thu Sep 21, 2006 7:50 am Post subject:

I see what you are trying to do, but I think you missunderstand the purpose of these power-lines. If you turn them off, you are completely turning off the USB port, which some USB controllers do support, but I don't think that is part of any standard. Besides, I think the most you can get out of the USB power is 500mA and at 5V that is 2.5W, which will hardly power anything at all.

If you do want something to be controlled by USB, you should look at http://www.ftdichip.com/. They have real good chips that have a USB device connector on one side, and a serial port on the other end. If you connect the USB-end to a computer it will claim to be a COM-port and behave just like one. Then you can controll the Serial end of the chip just like a serial port. Hook one of the controll lines to a power-relay and you are pretty much done.
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d4mo · Guru Joined: 27 Jun 2005 Posts: 430

Well I already actually have a USB to Serial converter.

But as for the usb, I would use the usb to trip a relay of some sort that would in turn, make the power strip turn on. Wouldn't that be possible>

Cuardin · Posted: Thu Sep 21, 2006 8:40 pm Post subject:

Yeah, that would work. But USB is a packet-based protocol, much like IP, so you need some circuit that can read USB packets and then set a control-line accordingly. USB-RS232 would definatley work. AS I mentioned, use the control lines in the RS232.
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d4mo · Guru Joined: 27 Jun 2005 Posts: 430

Why couldn't I just use a serial port thats already on my computer if thats the case?

Cuardin · Posted: Fri Sep 22, 2006 11:17 am Post subject:

No reason. But serial ports are a seriously endangered species these days.
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