Why Powered USB Is Needed, Part 2: The Future of USB

This article describes a version of USB that is not related to the new USB 3 spec that Intel has released for 2010 products

The Universal Serial Bus, or USB, is right now the most common serial peripheral bus in existence. Allowing all the most common devices to connect to your computer, to each other, through hubs, and now even wireless USB has become the dominant method of low bandwidth communications between devices and their peripherals.

However, USB is not without flaws, in fact, it has tons of issues that other less accepted standards have already solved, and USB has either not solved them or solved them only recently. One of those problems is that, although USB does provide electrical power to peripherals, it is rarely enough to run devices that suck juice like no tomorrow. Powered USB exists to solve this problem.

I will tell you why Powered USB will never be widely accepted, and also why we need it. However, to do so, this article is split into two three parts: the first part discusses the history of USB and previous peripheral ports, and why it it became widely accepted, the second part contains the meat of my argument on why Powered USB is both needed, yet failing to be accepted, and the third part describes a possible future USB 3 specification in detail.

This is part 2. Part 1 is available here, and part 3 is available here.

Short Introduction to Powered USB

At the end of Part 1, I said USB does not provide enough power for certain devices, a total of 2.5 watts at 5 volts. This is enough for any device that uses little power: keyboards, mice, USB flash keys, etc, etc. Compared to Firewire 400, which can provide similar data transfer performance to USB 2.0, devices can use up to 45 watts at 30 volts.

Powered USB can output 144 watts at 24 volts, 72 watts at 12 volts, and 30 watts at 5 volts (all at 6 amps current), while Firewire does up to 45 watts at up to 30 volts (1.5 amps current), and USB 1.1 and 2.0 does 2.5 watts at 5 volts (500mA of current).

According to recent comments of this article, my original math was wrong: at 24 volts Powered USB only provides three times more power than Firewire, and at 5 volts Powered USB provides five times more power than normal USB.

In Firewire 400 against USB 2.0, Firewire comes out as the better bus for many devices due to the fact that it can supply enough power to, for example, run a two or three drive enclosure or an external DVD burner; not only that, it does perform better than USB 2.0 for data transfers due to the fact you can never get 480mbps transfers in the real world, only around 240 to 360mbps, whereas on Firewire 400 you really can get to 400mbps.

Firewire, for devices that require power and bandwidth, gives USB 2.0 a severe beat down but has trouble taking on Powered USB.

Don’t understand how much power 144 watts at 24 volts is? You can drive printers, scanners, large RAID enclosures, multiple DVD burners for parallel/mass burning, even medium sized LCD monitors. You could drive a pair of large speakers with this much power in addition to sending them digital sound to play.

As you can see in the image, the top part of the plug is the power plug combined with the bottom part of the plug that is a standard USB data plug. This power plug’s power output not being standardized is where everything goes wrong.

The plug did it with the crowbar in the library

The Powered USB specification manages the second half of this (very ugly) plug, where all the extra power comes from. According to the specification, you may have different layouts of power pins to supply 5, 12, or 24 volts, and each plug can only do one of the three.

Can you imagine how confusing this would be to end users? Powered USB has gone back to the days of having incompatible but similar functioning ports on the same computer. Had problems telling your grandmother about PS/2 and SCSI ports, and why she can’t plug her printer into either? Now tell her why she can’t plug her PUSB-5v device into a PUSB-12v plug.

USB does need a powered extension to compete with and possibly eliminate Firewire. I have a dozen devices that have separate power cords and power bricks and it makes for cable spaghetti behind my computer. If all my devices supported some sane future form of Powered USB, I’d lose at least four or five of these power cords.

So how can this be fixed?

For New Powered USB to move forward to the home desktop, I envision that both the USB Working Group and the Powered USB Working Group needs to release new versions of their specifications. First, Powered USB needs a new version (lets call it New Powered USB): they have to standardize on one voltage. I suggest using 12 volts, or use a floating voltage design like Firewire does (12 to 30 volts instead of 30 volts fixed), as this would be most beneficial to devices that require high voltages.

Second, I suggest the USB Working Group should release USB 3.0 already. As I mentioned before in this article, Firewire 400 is marginally faster than USB 2.0, however what I did not mention is that Firewire 800 is about three times faster than USB 2.0 and is already available in a couple devices. I expect to be able to do at least 800mbps or 1600mbps of real performance (akin to USB 2.0’s 240-360mbps real performance) or even more.

Third, I suggest that power strips (the kind you plug your computer into) add USB to New Powered USB bridges that simply pass the USB data over, but add the power pins and power the devices directly from the power strip. Adding these plugs would allow people to power new devices with older computers or with smaller devices (ultra-small laptops, PDAs, etc) that can’t power devices on their own.

With these three suggestions, I can bet you that New Powered USB would become a common home standard, and at least part of the cable spaghetti problem would go away, and I can also bet you that Firewire might also disappear as well.

Continue to part 3.

Written by
Open Source software architect and technologist. He's just this guy, you know? Follow him him on Google+.
Published in
Transmissions from the Little Blue Marble

Published March 30th, 2007


70 Responses

M Lapin: This isn’t a technical writing issue. Apparently, my math on how to calculate watts, amps, and voltage from each other.

Also, as long as I’ve spoke english, three times more is 300% not 400%.

You’re still having power problems here.

You say : “Powered USB can output 144 watts at 24 volts … Firewire does … 45 watts at … 30 volts (1.5 amps current), and USB 1.1 and 2.0 does 2.5 watts at 5 volts (500mA of current).

“According to recent comments of this article, my original math was wrong: at 24 volts Powered USB only provides three times more power than Firewire, and at 5 volts Powered USB provides five times more power than normal USB.”

This still isn’t correct. Now the problem is the wording. You should have said “at 24 volts Powered USB provides three times AS MUCH power AS Firewire.”

“Three times more” means the original amount plus 3 times the original amount – or four times AS MUCH. Three times more than 100 would be 400.

English is a challenging language for every purpose, and it’s especially so for technical writing. A local college’s or university’s course in technical writing might help to eliminate similar errors in future articles.

peteredworthy: You’ve double posted this comment on both part 2 and 3. I’ll answer the question there.

Patrick, does the usb specification include a minimum voltage the insulation has to suitable for?

Raising the voltage allows much higher wattages, but it does increase the component count on the powered device, as it will need to step the voltage down, and the current up, to meet the requirements of the devices. Although the device would probably already contain a voltage controller to handle any voltage drop due to the resistance of the cable.

Using USB2’s 1.5 amp cables and 100v would give 150w theoretical and not require anymore connectors. The raise in voltage would be negotiated just like everything else, to ensure nothing was electrocuted.

Up to 120VDC is classed as ‘extra low voltage’ and so would not require any significant differences than using 12v, so long as the insulation was sufficient.

“The environmental impact from using all that electricity will be devastating.

LOL…you ARE kidding, right?

Right? Lord I hope so.

OK, L’il Al…Some real science 101. The driving force behind the earth warming comes down to carbon dioxide, correct? Any first year collegs student will know this. I understand your concern and I am here to help with a sure fire way for you and anyone else worshiping at this alter to contribute, and You don’t have to sacrifice your computing power.

Stop exhaling.


>>Powered USB is the last thing we need. Computers are already using too much power. The environmental impact from using all that electricity will be devastating.

Devastating? A little alarmist, don’t you think? I’m guessing you don’t live alone, and thus don’t pay close attention to your power consumption, because my computer and its peripherals consume the least amount of power in my apartment, other than my 30w halogen lamps. It’s TVs, refrigerators and other appliances that consume the most power. In comparison, computers are just table stakes. I even lived in an apartment once where we had four computers running non-stop, and our power consumption was still really low, paying an average of $75/month for power between two people.

That said, I would just like to not be dependent on power bricks for my peripherals anymore. It’s beyond ridiculous how much cable spaghetti it creates.

Environmentalist: actually P-USB will be good for the environment. At present I have speakers, external backup drive and monitor all driven from individual transformers, which are all variously inefficient and take power even when the device they are driving is turned off. With P-USB you get smart power from your computer’s power supply, which is a lot more efficient.

Different ports for different power levels is just silly, why don’t they just supply one type of power, and have the device modulate the power to what it requires simillar to how firewire 800 does. Speaking of firewire, I don’t see what the problem with both existing. Firewire will always be faster than USB simply because it doesn’t use the north/south bridge. Furthermore with planned specs like firewire 1600. USB has little chance in keeping up.

well I know that power supplies comes for a determined power, imagine If I add a power hungry device and I cross the power barrier. I guess that won’t be nice if plugging your camera shuts down your PC. ON the other side you make computers more expensive if you put them huge power supplies that wont be used (hey is not even environmental friendly since they always waste energy even if not used at full capacity). With an USB device with a power supply there is no problem. You only plug the power source if you use it and won’t break the barrier.

FireWire (aka ieee1394 and friends) vs USB has more to it than bandwidth or power… personally I see a place for both.

The peer-to-peer part of FireWire (ie: all devices on the bus are “equals”) allows lots of useful stuff (like TCPIP networking) – OHCI (etc) is only a small part of this.

The USB 2.0 stuff is great, but trying to endlessly improve on something that’s only got “problems” from a blinkered point of view is crazy : everyone pays for that – you, me, your grandmother and my idiot father :)

My scanner (CanoScan LiDE 600F) already does run entirely on USB power.

There is one major thing that the article failed to mention. Prior to being told this I was all completly pro USB. Don’t get me wrong I do like USB, But if i am going to buy something and it is offered in both USB or Firewire. Even thought it is a little bit more expensive I am going Firewire. Simply because Firewire can be daisy chained, with out the loss of bandwidth. No hub needed, fewer devices, fewer cords, etc…. I think everyone knows were i was going with that. If not I guess its not that important to you. So if the USB, and PUSB people are reading. Take note, among the suggestions in the article Add the ability to daisy chain the devices, so that there is no need to have a Hub!!!! LESS IS SO MUCH MORE!!!

You sound like you really don’t like firewire, but, in my opinion, it is the better standard.

Not only does Firewire have a nicer plug, which causes much less damage to the port if you try and shove it in the wrong way, firewire 800 is much faster then USB (at least at this stage) and Firewire supports P2P networking, where every device can be assigned an IP address, and have equal status on the bus.

The only problem I see with firewire is its lack of ubiquity and presence on consumer devices.

I do like USB, and I appreciate what it has done for consumer computing, but Firewire seems far more forward thinking.

What I got out of this article is that firewire 800 is massively better than USB is, yet you think we should just upgrade USB instead.
That’s a little odd. (although making the switch would be very difficult.)

>> …and I can also bet you that Firewire might also disappear as well.

WTF? You say (rightly) that Firewire400 beats USB2 (and that’s because of all the crappy client/server-type communication going on), that Firewire800 is THREE times faster than THAT, And THEN you can turn around and say that you think USB (even higher-powered – something that can ALSO be done on Firewire) could STILL beat it? Please: explain.

(btw: FW 4-pin cables do not have power – only the 6-pin ones do. 4-pins are a lot smaller and are usually found on battery-powered cameras and such; but some suppliers DO make “firewire power supplies” so that (6-pin) cable-powered devices can be used in 4-pin inputs.)

You can’t standardize on one voltage. Different devices will require different ones. I’m surprised they were able to standardize on three. And just “floating it”, well, you’re talking about it like that solves everything…

Honestly, I don’t think Power USB is really a very good idea. The reasons in this article for why we need it seem to boil down to “Firewire has it!” I’ve never seen a Firewire device actually depend on the Firewire interface for power, and certainly not substantial amounts of power. It’s just not a feature that’s really relevant to any practical or common use.

The solution isn’t to develop a new plug that can supply multiple voltages, but just to up the maximum 5V amperage by a moderate amount. Even 1 Amp is sufficient to power 2.5″ Laptop HDD enclosures, and 2-3A should be plenty for desktop HDD enclosures. The current USB2 cable spec supports up to 1.5A, so bumping up the current capacity of the socket and controller electronics should be relatively easy, and it would also be possible to get even higher current levels by specifying slightly thicker power wires in a newer standard. While this may not give us the capacity to power really high-drain devices like scanners and printers from USB, I really can’t see requiring an electrical outlet as being a substantial hinderance in the usefulness of high-drain devices.

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