Basic Computer Fundamentals

1. Lecture6 Prepared by Jalal Basic Computer Fundamentals 28-1-2013

2. Table of content • USB ports • SCSI • Sound card

3. Just about any computer that you buy today comes with one or more Universal Serial Bus connectors. These USB connectors let you attach mice, printers and other accessories to your computer quickly and easily. The operating system supports USB as well, so the installation of the device drivers is quick and easy, too. Compared to other ways of connecting devices to your computer (including parallel ports, serial ports and special cards that you install inside the computer's case), USB devices are incredibly simple.

4. Anyone who has been around computers for a while knows the problem that the Universal Serial Bus is trying to solve -- in the past, connecting devices to computers has been a real headache. • Printers connected to parallel printer ports, and most computers only came with one. Things like external storage media, which need a high-speed connection into the computer, would use the parallel port as well, often with limited success and not much speed. • Modems used the serial port, but so did some printers and a variety of odd things like personal digital assistants (PDAs) and digital cameras. Most computers had at most two serial ports, and they were very slow in most cases. • Devices that needed faster connections came with their own cards, which had to fit in a card slot inside the computer's case. Unfortunately, the number of card slots is limited and some of the cards are difficult to install.

5. The goal of USB is to end all of these headaches. The Universal Serial Bus gives you a single, standardized, easy-to-use way to connect up to 127 devices to a computer. • Just about every peripheral made now comes in a USB version. A sample list of USB devices that you can buy today includes: • Printers • Scanners • Mice • Joysticks • Digital cameras • Webcams • Modems • Speakers • Telephones • Storage devices • Network connections

6. USB Cables and Connectors

8. How USB Ports Work

10. The USB Process When the host powers up, it queries all of the devices connected to the bus and assigns each one an address. This process is called enumeration -- devices are also enumerated when they connect to the bus. The host also finds out from each device what type of data transfer it wishes to perform: • Interrupt - A device like a mouse or a keyboard, which will be sending very little data, would choose the interrupt mode. • Bulk - A device like a printer, which receives data in one big packet, uses the bulk transfer mode. A block of data is sent to the printer (in 64-byte chunks) and verified to make sure it's correct. • Isochronous - A streaming device (such as speakers) uses the isochronous mode. Data streams between the device and the host in real-time, and there is no error correction.

11. The USB process cont. The host can also send commands or query parameters with control packets. • As devices are enumerated, the host is keeping track of the total bandwidth that all of the isochronous and interrupt devices are requesting. They can consume up to 90 percent of the 480 Mbps of bandwidth that's available (USB 3.0 increases that speed to 4.8 gigabits per second). After 90 percent is used up, the host denies access to any other isochronous or interrupt devices. Control packets and packets for bulk transfers use any bandwidth left over (at least 10 percent). • The Universal Serial Bus divides the available bandwidth into frames, and the host controls the frames. Frames contain 1,500 bytes, and a new frame starts every millisecond. During a frame, isochronous and interrupt devices get a slot so they're guaranteed the bandwidth they need. Bulk and control transfers use whatever space is left.

12. USB Features The Universal Serial Bus has the following features: • The computer acts as the host. • Up to 127 devices can connect to the host, either directly or by way of USB hubs. • Individual USB cables can run as long as 5 meters; with hubs, devices can be up to 30 meters (six cables' worth) away from the host. • With USB 2.0,the bus has a maximum data rate of 480 megabits per second (10 times the speed of USB 1.0). • A USB 2.0 cable has two wires for power (+5 volts and ground) and a twisted pair of wires to carry the data. The USB 3.0 standard adds four more wires for data transmission. While USB 2.0 can only send data in one direction at a time (downstream or upstream), USB 3.0 can transmit data in both directions simultaneously.

13. USB features cont. • On the power wires, the computer can supply up to 500 milliamps of power at 5 volts. A USB 3.0 cable can supply up to 900 milliamps of power. • Low-power devices (such as mice) can draw their power directly from the bus. High-power devices (such as printers) have their own power supplies and draw minimal power from the bus. Hubs can have their own power supplies to provide power to devices connected to the hub. • USB devices are hot-swappable, meaning you can plug them into the bus and unplug them any time. A USB 3.0 cable is compatible with USB 2.0 ports -- you won't get the same data transfer speed as with a USB 3.0 port but data and power will still transfer through the cable. • Many USB devices can be put to sleep by the host computer when the computer enters a power-saving mode. • The devices connected to a USB port rely on the cable to carry power and data.

14. USB 2.0 and 3.0 • The standard for USB version 2.0 was released in April 2000 and serves as an upgrade for USB 1.1. • USB 2.0 (High-speed USB) provides additional bandwidth for multimedia and storage applications and has a data transmission speed 40 times faster than USB 1.1. To allow a smooth transition for both consumers and manufacturers, USB 2.0 has full forward and backward compatibility with original USB devices and works with cables and connectors made for original USB, too. • Supporting three speed modes (1.5, 12 and 480 megabits per second), USB 2.0 supports low-bandwidth devices such as keyboards and mice, as well as high-bandwidth ones like high-resolution webcams, scanners, printers and high-capacity storage systems. The deployment of USB 2.0 allowed PC industry leaders to forge ahead with the development of PC peripherals to complement existing high-performance PCs. In addition to improving functionality and encouraging innovation, USB 2.0 increases the productivity of user applications and allows the user to run multiple PC applications at once or several high-performance peripherals simultaneously.

15. USB 2.0 and 3.0 cont. • The USB 3.0 (SuperSpeed USB) standard became official on Nov. 17, 2008. USB 3.0 boasts speeds 10 times faster than USB 2.0 at 4.8 gigabits per second. It's meant for applications such as transferring high-definition video footage or backing up an entire hard drive to an external drive. As hard drive capacity grows, the need for a high-speed data transfer method also increases. • Adoption of the USB 3.0 standard has been slow. Chip manufacturers must design motherboard hardware that supports USB 3.0. Computer owners have the option to purchase cards that they can install in their computers to give USB 3.0 support. But hardware support is just part of the problem -- you also need support from your operating system. Even though Microsoft announced that Windows 7 would eventually support the USB 3.0 standard, the company shipped its operating system without USB 3.0 support. Recent distributions of the Linux operating system support USB 3.0.

16. How SCSI Works A computer is full of busses -- highways that take information and power from on­e place to another. For example, when you plug an MP3 player or digital camera into your computer, you're probably using a universal serial bus (USB) port. Your USB port is good at carrying the data and electricity required for small electronic devices that do things like create and store pictures and music files. But that bus isn't big enough to support a whole computer, a server or lots of devices simultaneously.

17. SCSI cont. For that, you'd need something more like SCSI. SCSI originally stood for Small Computer System Interface, but it's really outgrown the "small" designation. It's a fast bus that can connect lots of devices to a computer at the same time, including hard drives, scanners, CD-ROM/RW drives, printers and tape drives. Other technologies, like serial-ATA (SATA), have largely replaced it in new systems, but SCSI is still in use.

18. SCSI Basics SCSI is based on an older, proprietary bus interface called Shugart Associates System Interface (SASI). SASI was originally developed in 1981 by Shugart Associates in conjunction with NCR Corporation. In 1986, the American National Standards Institute (ANSI) ratified SCSI a modified version of SASI. SCSI uses a controller to send and receive data and power to SCSI-enabled devices, like hard drives and printers.

19. SCSI Basics cont. • SCSI has several benefits. It's fairly fast, up to 320 megabytes per second (MBps). It's been around for more than 20 years and it's been thoroughly tested, so it has a reputation for being reliable. It lets you put multiple items on one bus. SCSI also works with most computer systems. • However, SCSI also has some potential problems. It has limited system BIOS support, and it has to be configured for each computer. There's also no common SCSI software interface. Finally, all the different SCSI types have different speeds, bus widths and connectors, which can be confusing. When you know the meaning behind "Fast," "Ultra" and "Wide," though, it's pretty easy to understand.

20. RAID • SCSI is often used to control a redundant array of independent discs (RAID). Other technologies, like serial-ATA (SATA), can also be used for this purpose. Newer SATA drives tend to be faster and cheaper than SCSI drives. • A RAID is a series of hard drives treated as one big drive. These drives can read and write data at the same time, known as striping. The RAID controller determines which drive gets which chunk of data. While that drive writes the data, the controller sends data to or reads it from another drive.

21. SCSI Types SCSI has three basic specifications: • SCSI-1: The original specification developed in 1986, SCSI-1 is now obsolete. It featured a bus width of 8 bits and clock speed of 5 MHz. • SCSI-2: Adopted in 1994, this specification included the Common Command Set (CCS) -- 18 commands considered an absolute necessity for support of any SCSI device. It also had the option to double the clock speed to 10 MHz (Fast), double the bus width from to 16 bits and increase the number of devices to 15 (Wide), or do both (Fast/Wide). SCSI-2 also added command queuing, allowing devices to store and prioritize commands from the host computer. • SCSI-3: This specification debuted in 1995 and included a series of smaller standards within its overall scope. A set of standards involving the SCSI Parallel Interface (SPI), which is the way that SCSI devices communicate with each other, has continued to evolve within SCSI-3. Note: All of these SCSI types are parallel -- bits of data move through the bus simultaneously rather than one at a time.

22. Controllers, Devices and Cables • A SCSI controller coordinates between all of the other devices on the SCSI bus and the computer. Also called a host adapter, the controller can be a card that you plug into an available slot or it can be built into the motherboard. The SCSI BIOS is also on the controller. This is a small ROM or Flash memory chip that contains the software needed to access and control the devices on the bus. • Each SCSI device must have a unique identifier (ID) in order for it to work properly. For example, if the bus can support sixteen devices, their IDs, specified through a hardware or software setting, range from zero to 15. The SCSI controller itself must use one of the IDs, typically the highest one, leaving room for 15 other devices on the bus.

23. Controllers, Devices and Cables cont. • Internal devices connect to a SCSI controller with a ribbon cable. External SCSI devices attach to the controller in a daisy chain using a thick, round cable. In a daisy chain, each device connects to the next one in line. For this reason, external SCSI devices typically have two SCSI connectors -- one to connect to the previous device in the chain, and the other to connect to the next device.

24. The cable itself typically consists of three layers: • Inner layer: The most protected layer, this contains the actual data being sent. • Media layer: Contains the wires that send control commands to the device. • Outer layer: Includes wires that carry parity information, which ensures that the data is correct.

25. Different SCSI variations use different connectors, which are often incompatible with one another. These connectors usually use 50, 68 or 80 pins. SAS uses smaller, SATA-compatible connectors. Once all of the devices on the bus are installed and have their own IDs, each end of the bus must be closed.

26. Termination • If the SCSI bus were left open, electrical signals sent down the bus could reflect back and interfere with communication between devices and the SCSI controller. The solution is to terminate the bus, closing each end with a resistor circuit. If the bus supports both internal and external devices, then the last device on each series must be terminated. • Types of SCSI termination can be grouped into two main categories: passive and active. Passive termination is typically used for SCSI systems that run at the standard clock speed and have a distance of less than 3 feet (1 m) from the devices to the controller. Active termination is used for Fast SCSI systems or systems with devices that are more than 3 feet (1 m) from the SCSI controller.

27. Sound Cards • Sound cards are a kind of expansion card which deals with audio input and output. Sound cards generally fill two functions. First, sound cards enhance the sound processing capability of the computer, allowing for the creation and output of more complex sounds. Secondly, sound cards often possess input and output ports that allow for other audio devices such as microphones or speakers to be connected to it.

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