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This article discusses a type of hard disk that is more popular in high-end machines (servers) than in personal desktop computers. First, I will discuss SCSI and some of its benefits. Then I will take a look at the steps involved in configuring SCSI devices. SCSI overview SCSI is an acronym for Small Computer System Interface. The important part of this term is “small computer.” SCSI environments use a SCSI controller that is responsible for managing all SCSI devices and controlling the conversation on the SCSI chain. SCSI technology has many advantages over IDE technology, such as: Types of devices supported Number of devices supported in a single SCSI chain Performance of SCSI over IDE devices SCSI supports a multitude of devices, including hard drives and CD-ROMs. Remember that IDE devices typically only support hard drives and CD-ROMs. SCSI devices come in the following flavors: Hard drives CD-ROMs Scanners Printers Tape drives You could have a SCSI chain that contains any of those devices, whereas IDE could probably support only two types of devices. The other major benefit of SCSI is the number of devices that are supported in the SCSI chain, or what is known as the SCSI bus. Original SCSI supports up to eight devices in the chain, but one of those devices is the SCSI card that’s added to the computer to give you the capability to use SCSI. Remember that IDE only allowed two devices in the chain, and EIDE supports four devices, so with SCSI you are not only allowed to have more types of devices, but you are also allowed to have more of those devices! Original SCSI supports up to eight devices in the chain, but if the exam asks how many devices can be attached to a SCSI bus, the answer is seven (the test makers assume that the SCSI card is already there). Remember to watch the wording of the questions closely. The last benefit of SCSI is performance. Original SCSI devices do not compare in the performance category with EIDE devices, but some of the latter SCSI technologies—such as SCSI-2 and SCSI-3—can outperform IDE and EIDE. Host adapter Assume that we are dealing with original SCSI, also known as SCSI-1. When you install SCSI devices, you first need to install the SCSI host adapter. The SCSI host adapter is an expansion card that you add to the computer so you can chain SCSI devices off the adapter. In essence, the SCSI host adapter is the brains of the SCSI bus; it acts as the controller for the SCSI bus. The SCSI controller (adapter) is responsible for sending and receiving all information to and from the SCSI bus, just like the IDE controller. When the system has information for one of the devices in the SCSI bus, the system will hand the information over to the SCSI controller, which will then hand the information to the appropriate device in the chain. The beauty of the entire setup is that you have installed a SCSI adapter in the computer, which is a card that will be assigned resources like an IRQ and an IO address. Each device in the SCSI chain is not assigned these resources, because the processor will pass information to the SCSI controller and the controller passes the information to the device. This means that the system never talks to the devices directly, which means each device does not require an IRQ and an IO address. When you go out to purchase the SCSI adapter, you will first have to look inside your system to figure out what type of expansion slots are free. Today, you will typically have some PCI slots, but you may have an ISA or an EISA slot, as well. The difference between these expansion slots is performance—PCI runs at 33 MHz, while ISA and EISA run at only 8 MHz. Also, PCI and EISA are 32-bit technologies, while ISA is only 16-bit. The bottom line is that if you have some PCI slots free, you will probably end up purchasing a PCI SCSI adapter. When the controller receives information for a particular device, how does it send the information to that device? Addressing Each device is assigned an internal address in the SCSI bus, and the SCSI controller knows the address of each device. When the SCSI controller receives information for a particular device, the controller references that device by its ID in the SCSI bus. This way, there’s no confusion as to who the data is destined for. You assign the SCSI IDs when you connect each device to the SCSI chain. You will assign an ID either by jumpers or DIP switches if the device is an internal device, or by a “ticker” (yes, I made that term up) if the device is external. A ticker is an indicator on the back of the external SCSI device whose value you can change by pressing the button to increase or decrease the SCSI ID. If you are installing an internal SCSI device, you will most likely need to assign the SCSI ID by using jumpers. The internal SCSI devices will have a jumper set with three pairs of jumper pins. The decimal values of these jumpers, although probably not shown on the drive, are 4, 2, and 1 (from left to right). Jumper Block Settings
J2 J1 J0
If you would like to assign the device a SCSI ID of zero, then you would not jumper any of the jumper pins. This is shown in Table 9-3 by having the off state (represented by zero) at each jumper location. Another example of setting a jumper ID would be if you wanted to assign the device a SCSI ID of 3, you would jumper the J0 pins and the J1 pins, but not the J2 pins. This would enable decimal values of 1+2. Table 9-3 also shows what would happen if you wanted to set the SCSI ID to six. You would enable jumper J1 and J2, which enables the decimal values of 2+4, while the first jumper has an off state. The IDs you assign to each device are completely up to you, but note that the higher the number, the more important the device is to the SCSI bus. For example, if two devices need to send information through the bus at the same time, which device will the SCSI controller service first? The device with the higher SCSI ID is always given priority. This is why the SCSI host adapter is usually assigned the highest number in the chain (usually seven if the bus supports eight devices—the ID numbers start with zero). So the host adapter is assigned an ID of seven, and a SCSI bootable hard drive is assigned a SCSI ID of zero. Outside of that, you decide what the ID numbers are for each device. As a rule of thumb, we give slower devices higher ID numbers so that they may get serviced when they need to. Cabling Different types of cabling are used to chain SCSI devices to the SCSI adapter. If the device is an internal device, a 50-wire ribbon cable will be used, which is very similar to the 40-wire ribbon cable used for an IDE device. If the device is an external device, you will use a thick Centronics cable to connect to the Centronics 50 (typically used by SCSI-1) or the Centronics 68 (typically used by SCSI-2) connector on the back of the device. Termination Both ends of the SCSI bus must be terminated so that when a signal is sent down the SCSI bus, it is absorbed at the end of the bus by the terminator, which is acting as a resistor. The first device in the chain must be terminated along with the last device in the chain, the first device being the host adapter. If the device is an internal device, terminating usually involves playing around with jumpers. If the device is external, a terminator will be added to the back of the device. Types of SCSI Over the last number of years, SCSI has increased the performance of the devices over other types of devices, such as IDE and EIDE. This is one of the reasons why you will find network servers using SCSI hard drives instead of EIDE devices. Original SCSI (SCSI-1), introduced in 1986, was an 8-bit technology and had a transfer rate of 5 Mbps—a slow speed compared to today’s IDE devices, which run at about 10Mbps. A few flavors of SCSI-2 were introduced when SCSI-2 was developed as a standard in 1994. The first flavor was called fast SCSI-2, which increased the transfer rate from 5Mbps to 10Mbps, but kept it as an 8-bit technology. The other version of SCSI-2 was called wide SCSI-2, where the 8 bits were increased to 16 bits and the transfer rate is also increased to 10Mbps. When developing the standards for SCSI-2 not only was there a Fast SCSI-2 and a wide SCSI-2 but a third version of SCSI was implemented that combined the features of fast SCSI-2 and wide SCSI-2. Fast/wide SCSI-2 is a 16-bit technology that has a transfer rate of 20Mbps. There is also an Ultra SCSI, which has an 8-bit bus width and a transfer rate of up to 20Mbps. Ultra SCSI has a wide version, which increases the bus width to 16-bit; as a result, Ultra-Wide SCSI can transfer information at 40 Mbps. Types of SCSI
Description Bus Width Transfer Rate (Mbps) Number of Devices Installing SCSI devices Once you understand the issues with SCSI, installing a SCSI bus is fairly simple. First, you want to assign a unique ID number to each device. I usually perform this step at the beginning so that when everything is connected, you won’t have to play around figuring out how to change the ID of the devices. When assigning the ID numbers, remember that you want to assign the bootable drive the ID of zero. You also want to enable the SCSI BIOS on the SCSI controller if you are booting off a SCSI hard disk. When you enable the SCSI BIOS, you will not be required to install a driver for the card, because the PC will recognize the device on startup. If you are booting off an IDE drive and using the SCSI disk as an additional drive, you should have the SCSI BIOS disabled and install a driver in the operating system. Now that you have the ID numbers assigned to each device, insert the SCSI host adapter into the expansion slots of the PC. After inserting the SCSI card, chain all of the devices together. Once you have the devices chained together, you want to make sure that each end on the SCSI bus is terminated. If the last device is an external device, you will need to put the terminator on the end of the device. If you are installing internal devices, you need to check the documentation on the internal devices to find out what jumpers to set. At this point, the IDs are configured for each device, the SCSI card is inserted into the PC, and the devices are connected to the card to create a SCSI bus. We also terminated the SCSI bus at either end. Before installing the driver for the SCSI card in the operating system, I’ll review the steps to install a SCSI device one last time. To install a SCSI bus: 1. Assign unique IDs to each device. Windows NT 4.0 Windows NT 4.0 is a non–plug and play type environment, so installing the device is a little different than in Windows 95 or Windows 98. With Windows 95 and 98, you could detect new hardware and take a chance on whether or not the operating system will see the SCSI card. With Windows NT 4.0, there is no hardware wizard that will install the device; you have to do it manually. To install a SCSI host adapter in Windows NT 4.0, follow the steps outlined below. STEP BY STEP: Installing a SCSI host adapter in Windows NT 4.0 1. Select Start - Settings - Control Panel.
Installing the SCSI host adapter in the Windows 2000 environment is a little bit different than installing the driver in Windows NT 4.0. A wizard will start up, asking whether you want to detect the device or install the driver. The following Step By Step shows you how to install a SCSI adapter in Windows 2000. STEP BY STEP: Installing a SCSI host adapter in Windows 2000 1. Select Start - Settings - Control Panel.
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