In the event of an error after three retry attempts, the irp is cancelled and all further and outstanding irps to that pipe are ignored until the software responds to the error signal that is generated by sending an appropriate call to the usb. How exactly this is handled depends upon the type of device and the software. As suggested by the name Universal Serial Bus, data transmission in the bus occurs in a serial form. Bytes of data are broken up and sent along the bus one bit at a time, with the least significant bit first as illustrated by figure. The actual data is sent across the bus in packets. Each packet is a bundle of data along with information concerning the source, destination and length of the data, and also error detection information. Since each endpoint sets, during configuration, a limit to the size of the packet it can handle, an irp may require several packets to be sent. Each of these packets should be the maximum possible size except for the final packet.
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The use of this type of pipe is in some ways similar in purpose to the irq lines of the traditional peripheral system used in computers. Bulk Transfers : As the name suggests, the intended purpose is for transmitting large amounts after of data. This type of transfer gets the lowest priority, so pipes using this method are only allowed to transmit when there is available bandwidth. This means that a heavily loaded usb may have relatively slow bulk transfers compared to one with is servicing few devices. This transfer type would be useful for sending data from devices like digital scanners. Low Speed devices The description of usb devices so far has describe full speed usb peripherals, however there exists a special class of device known as low speed devices. These are simpler devices such as joysticks, and are the same as the full speed versions, except for the following restrictions: The maximum packet size for data transfers is 8 bytes They cannot use Isochronous nor Bulk Transfer pipes may only have 2 endpoints other. How is data sent across the usb? When the software requires data transfer to occur between itself and the usb, it sends a block of data called an I/o request Packet (IRP) to the appropriate pipe, and the software is later notified when this request is completed successfully or terminated by error. Other than the presence of an irp request, the pipe has no interaction with the usb.
Usb provides a special type of transfer for this data, giving it preference to guarantee a constant transmission rate with the required bandwidth. To ensure that the usb has enough time to handle the maximum data flow (1023 bytes) in each frame, a check is made during the initial configuration and the pipes will only be configured if this check is successful. This transfer method uses unidirectional pipes with no error handling procedures. Even though an error may be indicated in the status reply to a request, the pipe will not be halted and it is up to the software to decide what. Interrupt Transfers : These are used for small, infrequent transfers which require priority over other requests. As with Isochronous transfers, pipe configuration is granted on whether or not the system can handle the maximum packet size within the required time, with a further restriction that stops Interrupt and Isochronous Transfers from using more than 90 of any frame (discussed later) and. The endpoint tells the host during configuration how often it should be polled for interrupt requests, drinking and upon each polling returns a nak signal if there is nothing to send.
Control Transfers : These differ from the other types in that they are intended for use in configuring, controlling, and checking the status of a usb device. A request is sent to the device from the host, and appropriate data transfers follow in the appropriate pipes. At some later stage, a status indicator is returned to the host. The pipe used for this type of data may be bidirectional, but uses vegetarianism the same numbered endpoint for each direction. In addition, a device only handles one control request at a time, with the host withholding outstanding requests until a status is returned on the one in progress. For example, the default Control Pipe uses Control Transfers and accomplishes such tasks as initialising the device, and telling the host of the requirements of each of its endpoints. This type of pipe might also be used to control the operation of other pipes. Isochronous Transfers : These involve data whose accuracy thesis is not critical and which is sent at a rate corresponding to some timing mechanism. For example, 44100khz audio fits into this category since it doesn't have to be perfectly accurate and every 44100 samples indicates one second of audio.
For example, a simplistic model of a keyboard (figure 2) could have a keypad as output endpoint number 1, and the led key lock display as receiving endpoint. All usb devices have one of each of their 16 possible input and output endpoints reserved as " zero endpoints ". These are used for the auto-detection and configuration of the device when it is connected, and are the only accessible endpoints until this occurs. In addition each endpoint sets, upon connection, its own set of characteristic requirements concerning its requirements when accessing the bus. The combination of the address, endpoint number and direction are what is used by the host and software to determine along which pipe data is travelling. A pipe is simply a data path between an endpoint and the associated portion of the controlling software, such as between the keyboard leds and the bios routine which determines what leds should be lit. A special pipe is defined to connect to the zero endpoints, and is called the default Control Pipe. Types of data transfers Before we can understand any further detail about the communications process within a usb network, we need to be aware of the types of data that it must cope with. To accommodate the different types of data that needs to travel across the usb, each pipe can be configured as one of four transfer types.
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The host has a hub embedded in it called the root hub, and in practical implementations hubs are usually combined with one or more functions, such as keyboards or monitors. These are called compound devices and act like a hub with the functions permanently connected, along with any additional ports. Hubs may be connected to other hubs in a tiered arrangement, pdf but the bus topology still applies. How does Data get to the right Places? Even with this type of arrangement, we still have the same problem as with the traditional pc layout. Each function has to know when a piece of data is meant for it, and the host needs to know where signals are coming from, so numbers are assigned to each component on the usb.
However, rather than each using a fixed irq and I/O address, online the usb system takes a different approach. When a device is attached to the usb system, it gets assigned a number called its address. The address is uniquely used by that device while it is connected and, unlike the traditional system, this number is likely to be different to the address given to that device the last time it was used. Each device also contains a number of endpoints, which are a collection of sources and destinations for communications between the host and the device. Endpoints operate in simplex mode, meaning that they are either an input or output, but not both.
Up to 127 devices can be connected, and since it is a high speed system supporting up to 12 Megabits per second, it can accommodate the needs of a wide variety of peripherals. Other advantages include the ability to safely disconnect and reconnect items without switching off the computer, and the ability to use a usb device on any computer supporting the usb system. How the usb system is Arranged. The Universal Serial Bus is a network of attachments connected to the host computer. These attachments come in two types known as Functions and Hubs.
Functions are the peripherals such as mice, printers, etc. Hubs basically act like a double adapter does on a power-point, converting one socket, called a port, into multiple ports. Hubs and functions are collectively called devices. As far as the functions are concerned, hubs are furthermore like double adapters because although the entire system is physically in the star topology seen in Figure 1(a logically the system acts like a bus topology. This means that signals appear to travel along a single set of wiring, called the bus, to the host and is accessible by all functions, as illustrated in Figure 1(b). However, the host does keep track of the physical arrangements so that if a hub becomes disconnected, it is aware that all hubs and functions attached to it will consequently be disconnected too.
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The result is that neither card functions correctly. Many people find these potential problems rather daunting and this reduces the attractiveness of PCs to the market. An innovation known as "Plug and Play" aims to sort out the problem of clashing numbers, but there are still only a finite amount of these numbers available, with some being reserved for certain purposes. As well as all of the technical problems, where are other nuisances such as excessive paper cabling, the fact that you cannot disconnect devices while the computer is on without risking damaging something, and the different types of plugs required for different types of computers such. Even in the absence of problems, that back of a computer can be a daunting place for some, who often fear damaging something. This is where usb aims to simplify things by extending the trend of "user friendliness" to the hardware level. To the average computer user, it is a system where you can simply plug universities a device into any available socket and that device will instantly be available for use by the computer.
To examine why, we need to investigate the structure of the. Each of those back panel sockets is attached to a circuit board ( a card ) year which, like the internal devices, in turn plugs into a slot on the main circuit board. The computer needs to be able to distinguish which of these cards information is coming from, and similarly each card needs to know when out going data is directed. This is done by using special numbers called. Irqs Interrupt Request used for when the card wants to get something done, and. I/o input/Output addresses, used for transferring data between the computer and cards. When the card has something for the software to do, it generates a signal on the irq line, and communicates with the software via the I/O addresses. The irqs and I/O addresses are set in hardware on the card but, while these are usually adjustable, people have often found that when adding new hardware these numbers can clash with existing cards.
while printers usually use a parallel data cable, and monitors have their own special plug. This system was quite suitable when it was implemented and has been used for many years. However, recent technological developments have created problems for many users of PCs with this system. Today there is a huge range of peripheral equipment including scanners, digital cameras, specialised pointing devices, high speed modems, all of which need their own connection to the. While the above mentioned parallel and serial sockets can indeed be used by many different devices, they cannot be shared by more than one device at once, and so we can quickly run out of space to attach this new equipment. Then why not just add more places to plug things into? This is quite possible, and has previously been the solution in many cases by installing more sockets, and even connecting devices internally to bypass the need for these plugs altogether. However, there is a practical limit to how much this can be done.
However, it should still serve as a useful background water or introductory note. Where new technical terms are introduced and defined in the text, they are printed in italics like this, document Contents. Background : Why replace Existing Technologies with usb? How the usb system is arranged. How Data gets to the right Places. Types of Data Transfers, low Speed devices, how Data is Sent across the usb. Examples of Data Transactions, the role of the hub, the role of devices. In Conclusion, appendices, types of Packets, cyclic Redundancy Checks.
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Usb is resume a system for connecting a wide range of peripherals to a computer, including pointing devices, displays, and data storage and communications products. Although not a relatively new development in personal computing, it has only recently gained popularity due to increasing software support. This document discusses what the usb system does and how it is done. The technical detail covers the system's logical structure, rather than the electrical or software characteristics. About this note, this was originally a first year uni assignment, so it is limited in depth and the content is rather dated. Things have changed a lot since then (e.g. Usb2, and now 3).