PC Architecture. A book by Michael B. Karbo

Copyright Michael Karbo and ELI Aps., Denmark, Europe.

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    Chapter 43. SCSI, USB and Firewire

    In this chapter I need to discuss three I/O buses. They are very different, but they still belong together.

  • SCSI is an older but advanced I/O bus which has especially been used for hard disks, CD-ROM drives, scanners and tape units.

  • USB is a modern bus which can be used for a host of devices, and which has had a powerful breakthrough on the PC front in recent years.

  • FireWire is a modern, high-speed I/O bus which is especially used for digital video cameras (DV), scanners and external hard disks.

    As I mentioned, the three buses are very different, but they also overlap with each other. For example, you can buy CD burners with all three types of interface.

    Figure 201. Adaptec 2940U SCSI controller.


    SCSI (Small Computer System Interface) is an advanced controller technology, which is especially used in high-end PC’s. These can be network servers or just powerful workstations, for which there are a number of different SCSI standards. The SCSI bus can transfer up to 160 MB/second, which is more than the PCI bus can deliver.

    A SCSI system is built around a central controller, called the host adapter, which is almost a tiny computer in its own right. The adapter can be quite expensive if, for example, it has to be used with very fast hard disks. However there are simpler SCSI adapters, for example, those sold with SCSI based scanners.

    Figure 202.
    A small SCSI controller. Note the wide cable.

    The best known manufacturer of SCSI adapters is Adaptec. It used to also be quite common for motherboards to have built-in SCSI controller – often of high quality.

    A host adapter can control a number of SCSI devices, which are connected in a long series (a chain). Every device is allocated an identification number, and a terminator has to be put in at both ends of the SCSI chain. This is done, for example, using a jumper on one of the devices.

    Figure 203. A typical SCSI chain.

    The SCSI controller has its own startup routine in which it identifies the devices which are connected. The startup procedure can be followed in the screen:

    Figure 204. This SCSI chain consists of a CD-ROM drive, a CD burner and a Zip drive. This can be seen during PC startup, when the SCSI controller identifies each device.

    SCSI is intelligent

    The big advantage of SCSI is the intelligent protocol which manages the devices and ensures optimal data transfer. Each device requests, for example, data from the host adapter, which then does all the work of acquiring this data. While each SCSI device is working, the controller has time to do other tasks.

    The SCSI controller actually manages an entire small network of I/O devices, and thus relieves the CPU of the burden of all the associated work. SCSI therefore works well in multitasking environments such as web servers. These can have scores of SCSI disks organised in RAID chains.

    Today, SCSI is used primarily for tape units (streaming tape) or hard disks in servers in large network servers (e.g. used by Internet service providers).

    SCSI disks have a greater transfer capacity than standard ATA drives, and are more robust and generally better quality. SCSI used to be the only option if you wanted a big hard disk. But in recent years, ATA drive specifications have greatly improved, and today they are comparable with SCSI disks.

    One consequence of the technological advances has been that the SCSI standard has lost a great deal of its significance. For scanners, the USB and FireWire I/O buses are used instead. These are much easier to configure, and have the advantage of small connectors and thin cables.

    Figur 205. SCSI hard disks anno 2004.


    RAID stands for Redundant Array of Inexpensive Disks. It is a disk technology that connects together a serious of standard hard disks to form an advanced, error correcting system, which is used in servers. 

    The system is virtually an extension of the SCSI standard, and was first used in 1987. Since then, ATA-based RAID systems have been developed which use the much cheaper ATA or SATA disks in an equivalent configuration (see the discussion later in the guide). The trick is, that you can spread your data over several disks. With a RAID chain of hard disks, you gain two advantages:

  • Greater security. The data is on several disks. If one disk goes down, the other disks contain the same data.

  • Faster data transfer. The RAID controller writes and reads from several disks at the same time. This means the transfer speed can be doubled or tripled using RAID. When the user has read or written his file, the controller finishes the job itself, so that the complete file is located on all the attached disks.

    There are several RAID categories. A RAID controller has to be used, which is a special SCSI adapter. Here is a brief description of some of the standards:



    RAID 0

    Two or more disks are connected together, and the files are split between them (striping). The goal is purely greater speed, as the controller can read from/write to several disks at the same time. There is no extra security.

    RAID 1

    Two hard disks are used. Purely provides increased security, as the data is written twice – first normally, and then to the mirror disk. If the first disk goes down, the second is immediately ready to replace it.

    RAID 0/1

    Uses four disks and combines the two techniques above.

    RAID 3

    Spreads the data over several disks and stores parity data on one of them.

    RAID 5

    Improves both performance and security. Uses at least three, usually four disks. Is considered to be the best principle.

    Figure 206. There are many different RAID systems.


    USB stands for Universal Serial Bus and is an I/O standard originally developed by seven companies – Compaq, Digital, IBM, Intel, Microsoft, NEC and Northern Telecom (see the USB Imple­menters Forum at www.usb.org).

    USB is a cheap serial I/O bus with an open specification. This means that anyone can produce USB products, without having to pay licences to anyone.

    USB has been the biggest and most welcome innovation in PC design seen for many years. It is an expansion bus which allows a vast amount of PC equipment to be connected. It’s suddenly possible to connect loads of different gadgets to the PC – and using just one type of connector! And USB devices can also be used both with Macintosh computers and PC’s – yet another advantage.

    USB has been advertised since 1994, and for many years it was called the Useless Serial Bus. But starting in 1999, production finally surged forward, and there are now thousands of different USB gadgets.

    USB unifies all the different connections for keyboard, mouse, scanner, joystick, digital camera, and perhaps printer, onto a shared bus – connected using a common connector type.

    Figure 207. A USB-based trackball – actually designed for Macintosh computers, but works fine on PC’s.

    USB – the technology

    From a technical viewpoint, the following can be said about USB:

  • The transfer speed is limited to a maximum of 12 Mbit/sec. in USB version 1.1. It is therefore primarily used for equipment which doesn’t require a large bandwidth.

  • USB version 2.0 has a bandwidth of 40 MB pr. second, and is used in all modern computers. USB version 2.0 is backwards compatible. The same type of connector is used, and old devices can be connected to the new controllers.

  • USB is a serial connection using just four conductors (in contrast to the 50 or so used for a PCI device). This makes manufacturing much easier and cheaper.

  • The USB cable can also supply power to the devices. This means that scanners, for example, don’t have to have their own power supply. The maximum cable length is 5 meters.

  • Up to 127 USB devices can be connected to the PC using USB hubs.

  • There are no IRQ’s to be configured or terminators etc. USB devices can be connected ”On the fly”, without restarting the PC.

    There has to be a USB host controller in the PC in order to be able to connect the devices. This controller can be bought separately, as an adapter, but most motherboards have one built into the chipset’s south bridge. There are typically two or four USB connectors on the motherboard, but you can have many more USB devices than this if you connect an extra hub (e.g. integrated in a screen, as in Fig. 209.

    Figure 208. An in-expensive USB 2.0-based hard disk box. You can use any old hard disk in it. Great for backup!

    A flexible bus

    I am personally very grateful for USB. I find use for the following devices:

  • Card reader for CF and other RAM cards

  • Trackballs and Wacom drawing pen (tablet).

  • Flatbed scanners and ISDN-modems.

  • Newer printers. A color laser printer is faster using the USB 2.0 interface than the old LPT-interface.

  • External CD- and DVD-drives.

  • External harddisks (like the one in Fig. 208.

  • USB-based speaker or microphone systems.

  • Interface to digital cameras and MiniDisc.

    USB has thus made the serial ports (COM1 and COM2) redundant, and we can be very happy about that. The technology has also opened the door for the development of a number of new cheap and fun products.

    Figure 209. This screen has a built-in USB hub in its base. That means that a further four USB devices can be connected to the screen, which is itself connected to the PC’s USB connector.

    IEEE 1394 – FireWire

    Another, more SCSI type, interface is called IEE 1394. It doesn’t look a lot like SCSI, as we know it, but the standard is an extension of the SCSI standard. IEEE 1394 (FireWire in everyday language) is a high-speed serial bus which has a maximum transfer rate of 400 Mbit per second.

    Figure 210. A FireWire cable

    Some computers are born with FireWire ports being integrated on the motherboard. On other PC’s, you can install a cheap little adapter; then you can have direct access to your digital DV video camera. IEEE 1394 connections are “hot”, i.e. you can connect and disconnect devices without having to restart your PC, just as for USB.

    A summary

    I have described three expansion buses in this chapter which, from a technological perspective, are very similar. For many years, SCSI has been a ”luxury bus” which made it possible to use a number of more sophisticated devices with a standard PC. But as I mentioned, the significance of SCSI is declining.

    From the beginning, USB was planned to particularly replace the PC’s serial COM ports. This has been very successful, and USB has become an indispensable element of the modern PC architecture.

    At the time of writing, USB 2.0 is a completely new standard, but there is no doubt that it will be a big success, due to its good bandwidth and backward compatibility. As I mentioned, FireWire is used particularly on Macintosh computers and for DV cameras.

    All digital cameras (as far as I know) have a FireWire port, which makes it very easy to download video footage for further processing on the PC. Windows XP is setup for this, as I described in my guide, ”Do it yourself Windows XP”.

    Figure 211. In the middle of the picture you can see the small DV connecter on this digital video recorder.

    For external disks (hard disks and CD/DVD drives), both USB version 2.0 and FireWire can be used. Maxtor, for example, supplies large, cheap, external hard disks using both interfaces.



    USB 1.1

    1.2 MB/sec.


    40 MB/sec.

    USB 2.0

    40 MB/sec.


    Up to 160 MB/sec.

    Figure 212. Four, partially competing, expansion buses.

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