Interesting John, but some of what you say is contrary to what I'm finding on the web. For example, I'm reading that USB development didn't start until 1994. I'm also finding multiple sources where they say that USB can provide up to 5 V and 500 MA. I'll include some links and paste some text below. Note that one of the sources talks about how devices are connected in low power consumption mode and then negotiate for a higher power consumption. this may explain why you found that your USB port was only providing 1.5 V and 135 MA when you were testing it with your probe.
>From http://en.wikipedia.org/wiki/Universal_Serial_Bus, it says, among other things, that USB development wasn't started until 1994. Here's what it says in the history section of that web site: The Universal Serial Bus (USB) is a standard for peripheral devices. It began development in 1994 by a group of seven companies: Compaq, DEC, IBM, Intel, Microsoft, NEC and Nortel. USB was intended to make it fundamentally easier to connect external devices to PCs by replacing the multitude of connectors at the back of PCs, addressing the usability issues of existing interfaces, and simplifying software configuration of all devices connected to USB, as well as permitting greater bandwidths for external devices. The first silicon for USB was made available by Intel in 1995. [5] The USB 1.0 specification was introduced in January 1996. The original USB 1.0 specification had a data transfer rate of 12 Mbit /s. [5] The first widely used version of USB was 1.1, which was released in September 1998. It allowed for a 12 Mbps data rate for higher-speed devices such as disk drives, and a lower 1.5 Mbps rate for low bandwidth devices such as joysticks. [6] The USB 2.0 specification was released in April 2000 and was standardized by the USB-IF at the end of 2001. Hewlett-Packard, Intel, Lucent Technologies (now Alcatel-Lucent following its merger with Alcatel in 2006), NEC and Philips jointly led the initiative to develop a higher data transfer rate, with the resulting specification achieving 480 Mbit/s, a fortyfold increase over 12 Mbit/s for the original USB 1.0. data. I found another web site, http://www.wordiq.com/definition/USB, where it does a good job of explaining how the power supplied by the USB port works. Power supply The USB connector provides a single 5 volt wire from which connected USB devices may power themselves. A given segment of the bus is specified to deliver up to 500 mA. This is often enough to power several devices, although this budget must be shared among all devices downstream of an unpowered hub. A bus-powered device may use as much of that power as allowed by the port it is plugged into. Bus-powered hubs can continue to distribute the bus provided power to connected devices but the USB specification only allows for a single level of bus-powered devices from a bus-powered hub. This disallows connection of a bus-powered hub to another bus-powered hub. Many hubs include external power supplies which will power devices connected through them without taking power from the bus. Devices that need more than 500 mA or higher than 5 volts must provide their own power. When USB devices (including hubs) are first connected they are interrogated by the host controller, which enquires of each their maximum power requirements. The host operating system typically keeps track of the power requirements of the USB network and may warn the computer's operator when a given segment requires more power than is available (and will generally shut down devices or hubs in order to keep power consumption within the available resource). A number of devices use this power supply without participating in a proper USB network. The typical example is a USB-powered reading light, but fans, battery chargers (particularly for mobile telephones) and even miniature vacuum cleaners are available. In most cases, these items contain no electronic circuitry, and thus are not proper USB devices at all. This can cause problems with some computers - the USB specification requires that devices connect in a low-power mode (100 mA maximum) and state how much current they need, before switching, with the host's permission, into high-power mode. USB-powered devices attempting to draw large currents without requesting the power will not work with certain USB controllers, and will either disrupt other devices on the bus or fail to work themselves (or both). -- Christopher [email protected] -----Original Message----- From: [email protected] [mailto:[email protected]] On Behalf Of Jörgen Hansson Sent: Sunday, June 06, 2010 8:53 PM To: PC Audio Discussion List Subject: Re: HOW USB PORTS WORK Hi there John! wow! that was the best info I have ever heard of, I didn't know that the USB port was in use in 1986. I thought that the USB port came out when the windows systems came out in the 90's, cause the first PC I saw that on was from 1998 Regards, Jörgen Hansson! Tel +46 703-601296 www.jorgenhansson.com skype: djtropical4532 ----- Original Message ----- From: "DJ DOCTOR P" <[email protected]> To: "PC audio discussion list. " <[email protected]> Sent: Monday, June 07, 2010 3:33 AM Subject: HOW USB PORTS WORK > Hello all list members, > I'm gonna try to explain how your USB ports work. > And in the process of this, I'm also try to put away allot of > misconceptions and myths about it. > In 1986, the USB port was developed to transmit and receive digital > information or if you will, data from the computer to other digital > devices such as, printers scanners and fax machines. > All of these devices usually have their own power supplies. > Even though, 1.5 volts is needed to open up the data ports that are built > in to the devices, the devices themselves, aren't powered by the USB > ports. > These days, the USB ports are used to open up the data ports on memory > chips that are built in to storage devices such as, thumb drives external > hard drives and MP3 players and IPods. > As I said in an earlier post, your lab top and desk top computers have 12 > volt power supplies powering them. > In some electronic equipment, there are power/currant limiting resisters > to reduce the voltage to what the engineer desires. > But inside your computers, there is a chip that does a triple job. > This chip cuts the high voltage down to what the storage devices can > handle which is, 1.5 volts in to 135 MA. > This chip also handles the transmitting and the receiving of data. > This is why we're able to transfer audio/video files to our MP3 players > and IPods. > The same thing is also true about transferring pictures from digital > cameras to our computers. > The USB port was never mint to power devices alone. > But in order to make the USB port a viable power supply, a couple of > changes will have to be made. > First, the voltage will have to be razed from 1.5 volts to pretty close to > what the computers power supply is delivering. > Next, the currant will have to be razed from 135 MA to about 1500 MA. > But if we do this, the storage devices will overload and burn out. > unless, you increase their capacity too as well. > I hope I clarified the way your USB ports work. > My best regards. > John. 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