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How many USB devices can I connect to a PC?

USB allows you to connect a maximum of 127 devices at a time, of course this is via serveral hubs and interlinked connections.

USB devices are linked in series through hubs. There always exists one hub known as the root hub, which is built into the host controller. So-called "sharing hubs", which allow multiple computers to access the same peripheral device(s), also exist and work by switching access between PCs, either automatically or manually. They are popular in small-office environments. In network terms, they converge rather than diverge branches.

A physical USB device may consist of several logical sub-devices that are referred to as device functions. A single device may provide several functions, for example, a webcam (video device function) with a built-in microphone (audio device function).

USB device communication is based on pipes (logical channels). Pipes are connections from the host controller to a logical entity on the device named an endpoint. The term endpoint is occasionally used to incorrectly refer to the pipe. A USB device can have up to 32 active pipes, 16 into the host controller and 16 out of the controller.

Each endpoint can transfer data in one direction only, either into or out of the device, so each pipe is uni-directional. Endpoints are grouped into interfaces and each interface is associated with a single device function. An exception to this is endpoint zero, which is used for device configuration and which is not associated with any interface.

When a USB device is first connected to a USB host, the USB device enumeration process is started. The enumeration starts by sending a reset signal to the USB device. The speed of the USB device is determined during the reset signaling. After reset, the USB device's information is read by the host, then the device is assigned a unique 7-bit address. If the device is supported by the host, the device drivers needed for communicating with the device are loaded and the device is set to a configured state. If the USB host is restarted, the enumeration process is repeated for all connected devices.

The host controller directs traffic flow to devices, so no USB device can transfer any data on the bus without an explicit request from the host controller. In USB 2.0, host controller polls the bus for traffic, usually in a round-robin fashion. In SuperSpeed USB, connected devices can request service from host.

Actually, the realistic number is less than 127. If you think about it, unless you have a computer with 16 USB ports, you are limited on how many hubs you can attach to each port. Each of the hubs will take one of 8 ports available, so the real number is 7 you can use. So, if you do the math, 7*16 equals 112, still a big number but can't really reach the theoretical maximum number.

Lonman, what brand or type of audio interface do you use? Thanks. Nitecat

I too have not enough ports. I have my external hard drive, my interface, my dongle, my cooling fan, and a gooseneck light. I found a $4 hub at Walgreens that changes 1 to 4. Problem is if I try to hook up my dongle or hard drive to it, the laptop won't recognize them. When I plug them in, you can hear it on the speakers but still not recognized. Some nights I end up unplugging my cooling fan.

I plug my mouse and sound card into the back of my laptop. I plug my hard drive and dongle into a PCMIA usb card without any trouble. The card is a Buslink USB 2.0 cardbus for notebook.

http://cgi.ebay.com/Buslink-USB-2-0-Car ... 7C294%3A50

I paid more for it because I bought it at Officemax.

I plug my mouse and sound card into the back of my laptop. I plug my hard drive and dongle into a PCMIA usb card without any trouble. The card is a Buslink USB 2.0 cardbus for notebook.

http://cgi.ebay.com/Buslink-USB-2-0-Car ... 7C294%3A50

I paid more for it because I bought it at Officemax.