Common problems with USB ports
The term "peripheral" may be a less confusing term due to inconsistencies in terminology.
The term "peripheral" is not defined in the definition list in the USB specification, but is interchangeable with USB devices in the specification. The USB authentication check list for USB devices is also called the peripheral check list.
The definition of functionality in the USB specification refers to the ability of a USB device to provide to the host.
How many devices can a host connect to?
Each host controller can connect up to 127 devices. A host can contain multiple host controllers. If you have a hub and/or composite device, the total number of available physical devices may change depending on how they are implemented. Hubs also act as devices on the bus, but they also reduce the number of available interfaces on USB devices/peripherals.
What is a USB hub?
A USB hub is a type of USB device that allows other devices to connect to it, thereby expanding the network. The hub can be powered or self-powered by the bus. The bus-powered hub does not provide power to all USB devices connected to it, but the self-powered USB hub does. Since USB devices can receive a minimum of 100mA and a maximum of 500mA per USB port from a USB hub, the bus-powered USB hub can have up to four downstream devices.
8. How many hubs can be connected simultaneously?
Up to 5 hubs can be connected on a single chain.
9. Is there an organization that owns and controls the USB specification?
There is USB developer BBS, also known as usb-if. Its official website is http://www.usb.org, from which you can download the latest USB specification for free.
10. How is the data transmitted through USB protocol?
The USB specification defines four different data transfer mechanisms.
Control transmission
The USB host sends commands and inquiries to USB devices using control transmissions. When enumerating USB devices, control transfer USES endpoint 0 (EP0), which forces all USB devices to support EP0 without regard to speed. The maximum capacity of the control package is 8, 16, 32, or 64 bytes. The packet length to control transmission in low speed USB devices must be 8 bytes, full speed USB devices must be 64 bytes, and high speed USB devices allow 8, 16, 32, or 64 bytes.
Interrupt transfers
Interrupt transmission is the USB device to request a certain polling rate to the USB host. During enumeration, the USB device requests polling time from the USB host. The maximum polling rate for full speed devices is every millisecond and for low speed devices every 10 milliseconds. The maximum data load for low speed USB devices is 8 bytes and for full speed USB devices is 64 bytes. This puts the maximum throughput of full speed USB devices at 64KB/s and low speed USB devices at 800B/s. Confirm interruption of transmission to ensure data transmission. If the transport packet reception fails, it is resent
Mass transfer
Bulk transmission is the method by which a device transfers large amounts of data, but there is no guarantee that it will be sent in time. Bulk transfers have the lowest priority when the bus schedules transfers. When the other transfers are completed, the remaining bandwidth is reset and allocated for bulk transfers. As with interrupt transmissions, confirmation of bulk transmissions ensures data transmission. Only full speed and high-speed devices support bulk transmission. The maximum packet capacity can be 8, 16, 32, or 64 bytes in length for full speed USB device endpoints. For high speed USB device endpoints, the maximum packet capacity can be up to 512 bytes long.
Synchronous transmission
Synchronous transmission guarantees the transmission rate. Full speed synchronous transmission can send 1023 bytes of data per frame. No confirmation is required for synchronous transmission. Therefore, it is possible that the synchronization package did not arrive. The typical application of synchronous transmission is audio/video streaming, the most important of which is keeping video and audio in place at the expense of lost packets. The maximum transmission rate for full speed USB devices is 1023Kbps.
