Hub w/ maximum no. of unpowered USB ports?

March 17, 2006 12:15 PM

I need an unpowered USB hub with the max number of ports available, for cheap. What is the max number of ports possible, and where might I find such a doozie?

I know some are stackable, but most of the stackables seem to require power -- and I've already loaded up too many outlet plugs. Brand recommendations?

posted by vanoakenfold to computers & internet (10 comments total)

7 ports seems to be about the limit for unpowered USB hubs. I imagine this is some sort of protocol limitation as 8 devices seems far too round to be a coincidence.
posted by public at 12:28 PM on March 17, 2006

When USB first came out, wasn't one of the points that you could daisy chain them? Couldn't you plug one hub into another?
posted by PurplePorpoise at 12:32 PM on March 17, 2006

"When USB first came out, wasn't one of the points that you could daisy chain them? Couldn't you plug one hub into another?"

USB supports up to 127 devices, I think, so as long as your devices are self-powered you could chain as many hubs together as you wanted until you could support 127 devices.

I think devices that draw power through the USB connection itself can draw up to 500mA (again, this is off the top of my head) so if you were going to use devices that aren't self-powered you'd need powered hubs.
posted by mr_crash_davis at 12:35 PM on March 17, 2006

Doozy refers to the Dusenberg automobile.
posted by fixedgear at 1:05 PM on March 17, 2006

You can't daisy-chain unpowered hubs. The child hub will ask to be permitted to draw power (for itself) and the parent hub will refuse. That means the child hub will not initialize and won't work.
posted by Steven C. Den Beste at 1:06 PM on March 17, 2006

You're probably not going to find a bus-powered (unpowered) hub with more than 4 ports for one very good reason: to do so would violate the USB specification.

Section 4.3.1 of the spec has the definition: "Each USB segment provides a limited amount of power over the cable. The host supplies power for use by USB devices that are directly connected. In addition, any USB device may have its own power supply. USB devices that rely totally on power from the cable are called bus-powered devices. In contrast, those that have an alternate source of power are called self-powered devices."

Section 7.2.1.1, paragraph 2, kills the idea of more than four unpowered ports: "Figure 7-42 shows the partitioning of power based upon the maximum current draw (from upstream) of five unit loads: one unit load for the Hub Controller and the non-removable function and one unit load for each of the external downstream facing ports. If more than four external ports are required, then the hub will need to be self-powered."

I can't guarantee that I read spec-ese 100%, but I don't see a way for the unpowered hub to support more than four ports and be in compliance. If a passing USB engineer wants to contradict me with a spec cite, I'm open to correction.

Anyway, the Amazon link listed earlier shows self-powered 7-port hubs. You can't see it with the first D-Link DUB-H7 product, but the setup manual shows it does have a power port. The devious buggers just don't show the power cord in any of their pictures.

I suppose it's possible a commercial company out there has a product which allows more unpowered hub ports than the USB specification allows, but if they do, do you really want to use their product?
posted by mdevore at 1:23 PM on March 17, 2006

fixedgear - The dusenberg was so unique and/or bizarrely-faceted (looks burdensomely large to me) that doozie/doozy also means just such an object, rather than specifically a dusenberg.

Steven - I've got a mobiblu mp3 player that charges thru the USB cable, but the hub I am now using (4 port) is unpowered and still provides charge, presumably transmitted from the USB port in the laptop. I was under the impression that the hubs themselves didn't need power (in the same way an audio splitter doesn't) except to power the devices they employ when the original parent (or grandparent, etc) doesn't.. am I mistaken?
posted by vanoakenfold at 1:32 PM on March 17, 2006

Hubs are not passive devices, they use some power.

Almost all hubs have a little DC power socket. Bus-powered hubs will run without needing extra power if the devices you want to plug into it don't need more than 500mA (minus however much the hub takes)

There is a trick where you can get a USB cable that plugs into two ports, one for data and power, one just for extra power, but I've only seen that used for an external hard drive, not a hub.

If you want more than four ports and the things you want to plug in don't have their own power supply, you'll most likely need to use a powered hub.
posted by krisjohn at 2:33 PM on March 17, 2006

Theoretically you can have up to 127 devices connected to one host PC but there are practical power and data bandwidth limitations that would prevent that. These 127 devices can be arranged in a tiered network of hubs up to seven levels, counting the PC host as level 1 and the farthest device as level 7. The PC supplies 5 units of power to a hub. Each unit is 100 mA and each port requires a minimum of 1 unit. The hub itself consumes one, leaving 4 units for up to 4 ports. So you will never find a bus-powered hub with more than four ports.

Bus-powered hubs have a further limitation. Since each port has only a single unit of power, they can only connect downstream to a single device. You cannot connect them to another hub for expansion because that hub requires an additional 5 units. And not all devices can run off a single unit. A low-power device like a mouse will work, but a high power device can claim up to 5 units. High power USB devices will only work when plugged directly into a PC host or a self-powered hub that can supply 5 units to the port.

When you connect a device, the PC queries the device for its capabilities and requirements. So, for example, the PC knows that it is connected to a 4-port bus-powered hub which means that each port can only drive one unit. If you try to plug a high power device into that hub, Windows will throw up an error message and refuse to enable the device.

Bus-powered hubs are generally a bad idea because of these limitations and are intended mainly for mobile, not desktop use.

On the other hand, if the devices that are plugged into the port are themselves self-powered, then they may not need more then one unit from the hub. For example external disk drives have their own power so should work okay on a bus-powered hub.

A self-powered hub will be able to supply 5 units to each port, allowing high-power devices and further levels of hubs. So you could plug in one 7-port self-powered hub and then seven more 7-port hubs into that, giving you 49 available ports at just the fourth tier.

Some self-powered hubs will allow you to use them also in bus-powered mode, but if you try to plug in too many devices, Windows will give you an error message.

Most hub controller chips have 4 ports so you commonly see 4-port hubs. When they want to build a bigger hub, they just use two 4-port chips. But they have to daisy chain them with the first chip connected to the host and the second connected to one of the downstream ports on the first chip. This leaves only seven ports left for the user, three on the first chip and four on the second -- hence the common 7-port hub.

USB is plug-and-play in the sense you don't have to know any of this stuff when you connect things up. Windows keeps track of the power budget of each hub and device, will tell you if the connection will not work, and that you need to use a self-powered port.
posted by JackFlash at 5:30 PM on March 17, 2006

JackFlash is right, four ports is the max for a bus-powered ('unpowered') USB hub. A USB device can draw 500mA max from the port it's plugged into. A bus-powered hub is a USB device and thus can only draw 500mA. A hub must supply at least 100mA per port and the hub itself needs power (say 100mA), so that means only 4 ports are possible (4port x 100mA + 100mA for hub = 500mA)
posted by todbot at 7:58 PM on March 17, 2006