Existing USB ports may face trouble powering new breed of peripherals

Contrary to popular belief, at least from a power perspective, not all USB ports are created equal. That's what I learned yesterday during an e-mail exchange with Buffalo Technology's Brian Verenkoff.

Contrary to popular belief, at least from a power perspective, not all USB ports are created equal. That's what I learned yesterday during an e-mail exchange with Buffalo Technology's Brian Verenkoff. Earlier this week, as a part of ZDNet's Deputy Tester of the Week program, I announced that the next person to be named a ZDNet deputy reviewer would get a free 80GB USB-based hard drive from Buffalo Technology.

There are tons of USB drives on the market. Buffalo's unique selling proposition for this particular drive, according to the box it comes in (the drive actually ships at the end of the month) is that, in its turbo mode that requires special software drivers, it can run up to 64 percent faster. It's a claim that whoever we name to be the ZDNet deputy tester will hopefully be able to confirm or deny. With everyone on vacation this week though, I had a tough time figuring out what one of the extra cables that came with the drive is for (you can see me mention this in the video attached to the aforementioned blog post). Included in the box was a regular USB cable for connecting the drive's USB port to a PC's USB port. But also included in the box was another cable with a USB jack on one end, and a power jack that fits the drive's power port on the other. The first obvious question to me was, isn't the first cable all that's needed for both data and power?

As it turns out, depending on your PC, maybe not. And that's because, as I said earlier, not all USB ports are created equal. Yes, they all offer 5 volts. But where they often differ, according to Verenkoff, is on amperage which could be equally important to the peripheral people are looking to power -- particular power hungry devices like hard drives.

Verenkoff did such a great job explaining this to me via e-mail, that I'm just going to cut and paste his explanation here into this blog post. It's a must read.

[The first issue] is Amps vs Volts. Amps is volume, volts is pressure. The best example is a garden hose. Volts is the pressure of the water coming out of a hose. You can raise the pressure by putting your finger over the end of the hose (increasing the pressure and making the water go further). However, the actual amount of water coming out of the hose doesn't change. The amount of water being supplied through the hose is amps. Amps or volume doesn't change as the pressure increases or decreases. Watts is actually the measure of amps and voltage combined, but it is not applicable to the discussion we're having.

Where as all USB ports support the 5 volt requirement in the USB specification, amperage varies from one system to the next. 500 milliamps (ma) appears to be the standard with many notebooks supporting more. You have to remember that originally, USB was designed for mice, flash memory, and other low power devices. Any device you plug into a USB port on a computer and do not plug into an AC adapter is running at 5v and generally low amperage.

Devices like USB keychain flash drives and mice run at 5v but only require limited amounts of power (or amperage). However, larger devices like 3.5" USB Drives, external speakers, etc. require significant power and often need to be plugged into the wall. The USB cable on those types of devices just uses the 5v USB as a communications protocol to send data, not to try and power the device.

A USB hard disk drive literally has a hard disk drive (HDD) inside of it. A HDD itself has a motor which spins the platters as well as another motor that moves the heads around the platters. This requires a lot of power since things are moving. A 2.5" USB HDD (portable HDD) is one of the few devices that has moving parts that can still be powered by the limited amperage of many USB ports.

Every computer has different amperage at their USB ports. For example, even in some of the new Macbooks, we've observed a low amperage of 500ma. USB HDDs happen to be the main device that really becomes a problem on low amperage USB ports. Today, most computer manufacturers realize that USB HDDs will be used and are taking measures to make sure there's enough amperage in their USB ports. [But, there are still some systems out there (new and old) whose USB amperage is too low to power USB HDDs.] Many older laptops for example were designed before USB HDDs were widely used.

All USB HDDs require different amounts of power. The variable here is the actual HDD inside. Generally, larger drives require more power because there are more platters inside. When there are more platters it takes more power for the motor to spin them. Also, more platters = more heads for the platters. Those heads are on their own motors as well. Finally, faster RPM drives require more power to spin their motors faster (just like running a car engine faster uses more gas). There are also slight variations in the amount of power between similar drives. For instance, a Western Digital 80GB 2.5" HDD may use more or less power than an almost identical Seagate 80GB 2.5" HDD.

As far as Buffalo, our experience shows that 80 and 120GB drives work fine without any additional power. We are just now producing the 160s and 250s and haven't tested them yet, but we will soon. However, there have been reported problems with other manufacturer's 80s and 120s NOT working without another form of power.It's also worth noting that many manufacturers do not provide ANY alternative way of increasing amperage.

So, the reason we provide the power assist cable is to make sure that our customers have a backup if they need it. We've had this cable since our original drives, mainly so people with older notebooks could use them, however, now it's nice to have because of the 500ma Macbook limitation. This may come more into play as we see 250 and 300GB (and higher) USB Portable HDDs in the near future. The cable is completely optional and only required if you need more than 500ma.

Unfortunately [for those checking notebook data sheets to see if their USB amperage is enough to power all drives], we cannot say an exact mA number for what our drives use because it's ever changing based on the HDDs. Even HDDs with the same part number can have small internal changes that fluctuate that number. We're not a HDD manufacturer, but we've been closely testing the drives we select for our products to give them the best chance of working with only 500ma. In the event they don't, the power assist cable is available.

Using the power assist cable provides another 500ma over from the second port (for a total of 1000ma of power from both ports. It uses the first USB port for 500ma power and USB communication (data) and uses the second port ONLY for 500ma power (no data travels over that USB cable).

I recommend Googling the following string: "western digital passport not enough power" You can read forever about the problems they're having [DB's note: example here] and here's a thread from Apple's support forum that illustrates the 500ma problem.

The HDD manufacturers have been trying to reduce their power draw. Most people tend to be blaming the USB HDD manufacturers even though their not the ones who make the notebook or the USB port. It appears this problem is most significant on Apple's and most other notebooks haven't had this problem for years. In Buffalo's case, we don't make the notebook or the actual HDD inside so we look to find drives that draw the least power.

Wow. Interesting explanation and it certainly opens my eyes to some important issues having to do with USB ports. I wonder if this also explains why some AC adapters with a USB jack on one end won't charge just any USB-based rechargeable device. For example, I have a bunch of chargers that output their charge through a mini-USB jack, but some simply won't charge my Motorola Q or my wife's Motorola Razr.

In Buffalo's case, the net net is that you won't need second cable in all situations. But just in case you do, it's there. For those situations where you don't have a spare USB port to accommodate the second cable, I asked Verenkoff if one could get the drives with an AC adapter and the answer was not durrently.


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