Bom artigo que explica bem este tipo de problema que pode ocorrer no USB.
Não existem recursos de sistema suficientes para concluir o serviço solicitado.
Insufficient system resources exist to complete the requested service.
Bandwidth and power problems
Another common problem with USB devices is that you can only use so many USB devices simultaneously. This is because each USB 1.1 port is limited to 12 Mbps (megabits per second) of bandwidth (USB 2.0 allows for 480 Mbps) and 500 mA. If you exceed either of these limitations, then the USB devices will cease to function.
Let’s take a look at bandwidth limitations first. Unfortunately, I can’t tell you the exact number of USB devices that will exceed your bandwidth limitations and there is no Performance Monitor option to examine USB bandwidth. The type of USB device and how the device is used both affect the amount of bandwidth the device consumes. For example, scanners and digital cameras tend to consume a lot of bandwidth because they transfer large chunks of data to the computer. At the same time, though, a scanner will consume almost no bandwidth unless you are actively in the process of transferring data.
The best way to tell if you might be exceeding your USB bandwidth limitation is to take a look at the number of devices connected to a particular USB port, how those devices are being used, and then apply some common sense. If you have many USB devices connected or your devices tend to transmit and receive a lot of data, then you could be pushing the limits of a port’s available bandwidth.
Power consumption is a lot easier to figure out than bandwidth consumption. To figure out how much power you are consuming, there are a few things to keep in mind. First, each USB port on your computer supplies 500 mA. This 500 mA has to be shared among all devices connected to it. If you connect a single device to a USB port, power consumption isn’t a consideration, because most USB devices only consume around 100 mA.
Things start getting interesting when you start connecting USB hubs. If you connect a USB hub to a USB port, then the port’s 500 mA must be shared between the USB hub and all of the devices connected to the hub. You can also daisy chain USB hubs. If you daisy chain USB hubs together, you can have up to five hubs connected in a series. Remember, though, that 500 mA must be sufficient to power all connected hubs and devices. A USB hub does consume power, even if no other devices are connected to it.
To get around this problem, many manufacturers have started making USB hubs that have an external power supply. If you have a USB hub with an external power supply, the power supply will supply a total of 500 mA to the hub. This means that if you have multiple powered hubs daisy chained together, then each hub will be self sufficient and will consume almost no power from the computer’s USB port or from other hubs in the chain.
Even if all of your hubs are powered, though, there are still limitations that you must be aware of. First, the five-hub limit still applies whether the hubs are powered or not. The other limit is that you must still avoid overburdening any one hub with USB devices with excessive power consumption. Finally, external power sources do nothing to provide extra bandwidth. If you were to interconnect five hubs with four devices each, you could connect 20 USB devices to a single USB port, as long as none of the devices had excessive power consumption. At the same time, though, these 20 devices would have to share the 12 Mbps of available bandwidth.
When I first started talking about power consumption, I said that it is easier to determine whether you are using too much power than it is to determine if you are using too much bandwidth. Windows will actually tell you how much power your USB devices are using. Before I show you how to figure this out, though, there is one thing that you need to understand. The number that I am about to show you doesn’t actually measure power consumption, but rather how much of the USB port’s power is being used. Remember that you can have powered USB hubs connected to the computer. If a powered hub is connected, then the devices will be feeding off of the hub’s power, not off of the computer’s power. Therefore, the computer will show you that there is very little or no power being drawn from the USB port.
To view USB power consumption, return to the Device Manager, right-click the USB Root Hub entry, and click Properties to display the USB Root Hub Properties sheet. Select the Power tab and you will see how much power is being drawn from the USB ports, as shown in Figure C.
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| The Power tab shows how much power your USB devices are drawing. |
If a device (or a combination of devices) draws too much power, then the hub will usually turn off the port. To get the port to function again, you must disconnect the device and reattach it to the port. Depending on the type of hub that you are using, there may also be a dialog box that prompts you to reset the port.
