CHAPTER 14 SCSI DRIVES

	Introduction :
	The Small Computer Systems Interface (SCSI) pronounced as scuzzy enables you to attach hardware peripherals to the system. The SCSI system consists of the SCSI controller that communicates with the system. The SCSI cables connect the SCSI drives to the SCSI controller and the SCSI devices.
14.1 Advantages of SCSI
	SCSI enables fast communication between the system and the device. The SCSI connects many devices but has only few slots on the motherboard. The advantages of SCSI are : _ The SCSI devices have data transmission speeds that can reach about 320 Mb per second between the system and the device. _ It allows you to attach upto 15 devices to a single SCSI port. _ The SCSI controller on the SCSI devise boosts the signals and enables the devices to be connected at a distance of about 25 meters from the system. _ The SCSI devices reduce the processing load on the microprocessor, as they have a microcontroller built into the device. _ The communication problems between the system and the SCSI device due to data signals reflecting back when cables are left open are reduced, as the ends of the SCSI have terminators
14.2 Types of SCSI Interface
	The SCSI standards specify the features that a SCSI device manufacturer can include on the SCSI hardware. These standards specify the cable length and the data transfer speed between the system and the device. There are three standards SCSI-1, SCSI-2 and SCSI-3.
	14.2.2 SCSI-2
	The SCSI-2 standard was introduced in 1994. It uses a 16-bit bus to transfer data at a maximum speed of 20 MB per second. You can connect upto 16 devices using the SCSI-2 standard with a cable length of 3 meters. The devices built using SCSI-2 standards can also be used on SCSI-1 buses. However, the devices will work with the features applicable for SCSI-1, as SCSI-1 does not support the enhanced features of SCSI-2 such as command queuing. Command queuing enables the device to store, execute and change the order in which the commands from the system to the devices are executed. The SCSI-2 bus must be terminated with an active terminator to prevent data signals from reflecting back into the SCSI bus.
	14.2.3 SCSI-3
	The SCSI-3 standard was introduced in 1995. It uses a 16-bit bus to transfer data at a maximum speed of 40 MB per second. You can connect upto 32 devices using the SCSI-3 standard with a cable length of 3 meters. The SCSI-3 standard also specifies the rules that devices must follow while communicating with each other. The SCSI-3 bus must be terminated with an active terminator to prevent data signals from reflecting back into the SCSI bus.
14.3 SCSI Variations
	The different variations of SCSI specify the number of devices that can be connected, the bus speed and the bus width at which the SCSI devices communicate with each other. The bus speed specifies the amount of data that can be transferred in one second. This bus width is the amount of data that the cable can accommodate.
	14.3.1 Narrow
	The Narrow SCSI transmits data over an 8-bit bus. This uses the 50-pin connector cable known as the A cable. The A cables are the standard 8-bit cables used with SCSI-1 and SCSI-2 devices. If you use Narrow SCSI device you must ensure that the devices are set with SCSI ID less than 7.
	14.3.2 Wide
	The Wide SCSI transmits data over a 16-bit bus. The Wide SCSI uses a 68-pin connector cable known as the P cable, to connect to devices. The P cable is a 16-bit Wide SCSI cable used to connect SCSI devices.
	14.3.3 Fast
	The Fast SCSI can transfer data at a speed of 10 MB per second using the Narrow SCSI and 20 MB per second using the Wide SCSI.
	14.3.4 Ultra-wide
	The Ultra-wide SCSI transmits data at a speed of 20 Mbytes/sec for the Narrow SCSI. It enables you to use a maximum cable length of 1.5 meters. The Ultra-wide differential SCSI enables you to use a cable length of 25 meters.
14.4 Types of SCSI Bus Signaling
	_ SCSI bus signaling specifies the method used by the SCSI devices to communicate with each other. You must be careful not to connect the devices implementing Single-Ended (SE), Low- Voltage Differential (LVD) or High-Voltage Differential (HVD) together directly, or you may damage the hardware. You can connect the SE and DE using a SCSI expander called a Single-ended to Differential Converter and the LVD and HVD using the LVD to HVD connector.
	14.4.1 Single-Ended (SE)
	In SE, the system sends the instructions that must be transmitted to the device to the SCSI controller. The controller generates the signal and sends it to all the devices over the SCSI cable. The maximum length for the cable using SE is 6 meters. The devices implementing the SE signaling have a symbol on them. The SE symbol appears.
	14.4.2 Low-Voltage Differential (LVD)
	In LVD, the SCSI controller sends signals to the device using the SCSI cable. The SCSI device adapter has a small transceiver built into it. The transceiver accepts the signals and resends it to the next SCSI device. This continues till the appropriate device receives the signal. The transceiver boosts the signal and enables the maximum length for the cable to be 12 meters. The devices implementing the LVD signaling have a symbol on them.
	14.4.3 High-Voltage Differential (HVD)
	In HVD, the SCSI controller sends signals to the device using the SCSI cable. The SCSI device has a transceiver that accepts the signals and resends it to the next SCSI device. This continues till the appropriate device receives the signal. The transceiver boosts the signal and enables the maximum length for the cable to be 25 meters. The devices implementing the HVD signaling have a symbol on them.
	14.4.4 Low-Voltage Differential / Single-Ended Multi-mode (LVD/SE)
	The device that uses this signaling is compatible with both the LVD and the SE signaling. However if there is a device that uses the SE signaling on the SCSI bus, then this will be set to use the SE signaling. The devices implementing the LVD/SE signaling have a symbol on them. The LVD/ SE symbol.
14.5 Working with SCSI Identification (ID)
	The SCSI ID is used to specify the priority of the SCSI device. When two or more devices try to use the SCSI bus at the same time, the device with the higher priority gets to use the bus first.
	14.5.1 Jumper Block/DIP Switch Settings (Binary Equivalents)
	The SCSI identification (ID) can be set using the software available with the SCSI device, using the jumpers or switches on the SCSI device. The Switch 4 is available only on 16-bit drives that support upto 15 devices. The SCSI ID can be specified by setting the switches as specified, in Table 14.1.
	SCSI ID Switch 4 Switch 3 Switch 2 Switch 1
	0 0 0 0 0 1 0 0 0 1 2 0 0 1 0 3 0 0 1 1 4 0 1 0 0 5 0 1 0 1 6 0 1 1 0 7 0 1 1 1 8 1 0 0 0 9 1 0 0 1 10 1 0 1 0 11 1 0 1 1 12 1 1 0 0 13 1 1 0 1 14 1 1 1 0 15 1 1 1 1
	Table 14.1 - SCSI ID Switch Settings
	You can set the switch value to 0 by placing the switch in the OFF position or leaving the jumper slot vacant. You can set the switch value to 1 by placing the switch in the ON position or by inserting a jumper in the jumper slot. the different values and the jumper settings.
	Hands on exercise
	To set the SCSI ID using switches : 1. Decide the SCSI ID that you will set the device to. 2. Locate the jumpers or the switches on the device. The switches appears.
	14.5.2 Resolving Id Conflicts
	If two devices try to use the bus at the same time a conflict can arise that can cause the device to malfunction or the system to hang. The SCSI ID specifies the priority in which the device has access to the bus. The SCSI ID and the priority order are .
	Priority SCSI ID
	1 7 2 6 3 5 4 4 5 3 6 2 7 1 8 0 9 15 10 14 11 13 12 12 13 11 14 10 15 9 16 8
	Table 14.2 - SCSI ID Priority
	The Host SCSI controller on the system is generally given the highest SCSI priority, 7 to ensure that it gets first access to the system resources. The SCSI hard drives are factory set to have the
14.6 Working with SCSI Connectors
	SCSI Connectors enable you to affix the SCSI cable to the SCSI adapter or the SCSI device. There are many different connectors to connect the SCSI bus to the device. You must check that the SCSI bus and the device are compatible.
	14.6.1 Internal Connectors
	Internal connectors are located on SCSI devices such as hard disks and CD drives that are installed inside the system case. The different internal connectors .
	Connector Number Description of Pins
	50 Connects to SCSI-1 and SCSI-2 internal devices 68 Connects to SCSI-3, Ultra-Wide and LVD internal devices
	14.6.2 External Connectors
	The external connectors are located on the SCSI adapters and the SCSI devices such as printers, scanners and external tape drives. The frequently used external connectors .
	Connector Number Description of Pins
	50 Narrow 8-bit SCSI for external devices. It has wire latches to secure the device to the system. 25 8-bit connector used on Apple systems. This connector looks similar to the systems parallel port. 50 Also known as Alternative 1 and used for 8-bit SCSI-2 devices
	Connector Number Description of Pins
	68 Used for 16-bit and 32-bit SCSI-2 devices. It has a squeeze-torelease latch to secure the device to the system. 80 Also known as Alternative-4. Used for attaching Hot SWAP drives.
14.7 Using SCSI Cables
	The SCSI cables are used to connect the system, the SCSI controller and the SCSI devices together. The cable must also be used according to the signaling method used to communicate with the SCSI devices. You must try to avoid using cheap and ineffective cables as they can degrade the speed and quality of the data signals. The length of the SCSI cable must be limited to the maximum length as recommended by the SCSI standards and the data signaling method used.
	14.7.1 Types of SCSI Cables
	The internal connectors connect the internal SCSI devices such as the hard disk and the CD drive to the SCSI controller. The internal SCSI cable is a flat ribbon connector and appears.
	Internal SCSI Cable
	The external connector enables you to connect the external SCSI devices such as the printer and the scanner to the SCSI controller. The external connector is a thick round cable and appears.
	14.7.2 Termination Requirements
	The SCSI terminator is used to terminate the SCSI chain of devices. This ensures that the signal transmitted by the last device does not reflect backwards into the SCSI daisy-chain and disrupt data signals. The SCSI devices use passive and active terminators. Some devices also have the terminator built into them or have jumpers to activate the terminator. The SCSI bus requires two terminators one at either end of the SCSI bus. However, you must not attach a terminator directly on the host controller. The Terminators on the daisy-chain .
	Terminators on a Daisy-chain
	Terminator SCSI Host Adapter Terminator
	Passive Terminator
	Passive terminators are used to terminate the SCSI chain, if the distance between the last device on the SCSI chain and the SCSI controller is less than 1 meter. The Passive terminators use a resistor to terminate the SCSI signals. This terminator is used on HVD devices. The Passive terminator.
	Active Terminator
	Active terminators are used to terminate the SCSI chain, if the distance between the last device on the SCSI chain and the SCSI controller is more than 1 meter. The Active terminators have a resistor and also a voltage regulator. This ensures that the signals are properly terminated and there is no voltage fluctuation while terminating the signal. The Active terminator.
14.8 Working with SCSI Drives
	SCSI devices transfer data between the device and the system at great speeds. This ensures that you can fully utilize the increased speeds that the device can reach. SCSI enables you to connect multiple hard disks together to form a single unit by implementing RAID. You can connect a device to the SCSI chain without switching off the system. This enables you to attach and remove devices such as external hard disks from the system while the system is on.
	14.8.1 Redundant Array of Inexpensive Disks (RAID)
	RAID is a collection of hard disks that appear to the system as a single disk. For example, if you create a RAID with two hard disks, disk one having a storage capacity of 20 GB and disk two having a storage capacity of 40 GB. It will appear to the system as one hard disk with a capacity of 60 GB. RAID increases the data retrieval speed as the system reads the different blocks of the data from two different disks. It also keeps you data safe by enabling you to recover data due to the hard disk crashing.
	RAID Level 0
	This level enables you to treat a collection of two or more hard disks as a single unit. The data to be saved is spread out on the hard disk collection. This speeds up the process of reading and writing data as multiple disks are used to store data.
	RAID Level 1
	This level uses disk mirroring. One disk stores the data whereas the mirror disk stores a copy of the data. This ensures that if one disk fails the data can be retrieved from the mirrored disk.
	RAID Level 4
	This level stores the data on multiple disks and stores the sum of the data on a parity disk. If one disk crashes the parity disk can be used to recover the data. It is preferable to use hard disks of the same storage capacity to implement RAID Level 4. If the storage capacities of the hard disks are different, then RAID Level 4 must be implemented using the storage capacity of the smallest hard disk.
	RAID Level 5
	This is the frequently used method to RAID hard disks. This level stores the data on multiple disks, the parity information is not stored on a single hard disk but on different hard disks. It is preferable to use hard disks of the same storage capacity to implement RAID Level 5.
	Linear RAID
	Linear RAID combines the multiple hard disks together to form one drive. While storing data it first fills the first hard disk in the series and after the first disk is full it stores data to the next disk in the series.
	Best Practice
	1. Use disks having the same storage capacity. 2. Use disks located on different controllers if possible.
	14.8.2 Hot SWAP Drives
	Hot Swap Drives enable you to connect and disconnect devices to the system without shutting down or restarting the system. To connect the device to the system you just plug the device into the system. The operating system automatically recognizes the device, assigns it a SCSI ID and loads the drivers required by the system to communicate with the device. Hot SWAP drives are ideal for use on servers that must ideally not be shut down.
14.9 Installing SCSI Drives
	Installing the SCSI drive enables the system to use the drive. You can install an internal or an external SCSI device.
	Best Practices
	1. Read the documentation thoroughly to check if the motherboard supports the hardware. 2. Install only one device at a time, to ensure that a device is working properly before installing a new device. 3. Check if there is a spare expansion slot on the system to install the SCSI controller. 4. Check if the SCSI controller, cables and device are compatible with each other. 5. Ensure that the controller and the device use the same signaling procedure to communicate with each other. 6. Set the SCSI ID of the host adapter to 7, this ensures that it gets the first priority over the SCSI bus. 7. Ensure that the SCSI bus is terminated at both ends of the drive. 8. Format any new hard disks before using it. 9. Maintain a SCSI device worksheet while installing. A sample worksheet appears in Table 14.5.
	Precautions
	1. Wear an antistatic wristband. 2. Unplug the main power supply and the external device cables. 3. Ensure that the color marking on the SCSI cable faces the power socket while inserting it in the drive.
	14.9.1 Installing the Host Adapter
	The host adapter enables the SCSI devices to communicate with the system. This hardware is usually given the highest priority over the SCSI bus so that it can serve the other devices properly.
	Hands on exercise
	To install a SCSI host adapter : 1. Open the system case. 2. Insert the SCSI host adapter in the empty expansion slot and screw it in place.
	Inserting the SCSI Host Adapter
	3. Disconnect the LED from the motherboard and connect it to the SCSI host adapter.
	Inserting the LED in the SCSI Host Adapter
	4. Close the system case.
	14.9.2 Installing the Internal SCSI Device
	You can install internal SCSI devices such as the SCSI hard disk, or the SCSI CD drive in the system. The internal SCSI device connects to the host adapter bus using the SCSI cable. Host Adapter 2-pin LED from motherboard 4-pin LED from motherboard
	Hands on exercise
	To install an internal SCSI device : 1. Open the system case. 2. Set the SCSI ID for the device using the jumpers or the switches. 3. Locate an empty drive bracket in the system case to install the device.
	Empty Drive Bracket in System Case
	4. Insert the device in the drive bracket and insert screws to hold the device in place. 5. Insert one end of the SCSI cable into the host adapter.
	Inserting the SCSI Cable in the Host Adapter
	Drive Brackets 6. Insert the other end of the SCSI cable at the back of the device.
	Inserting SCSI Cable in the Device
	7. Insert the power connector in the device.
	Inserting Power Connector
	8. Terminate the ends of the SCSI cable, if the terminators are not already installed. 9. Close the system case. 10. Install the operating system on the device if it is the only hard disk on the system, or start the system and configure the device using the hardware wizard. Color Marking Power Connector
	14.9.3 Installing the External SCSI Devices
	You can connect external SCSI devices such as SCSI scanners and printers to the system. The first external SCSI device in the daisy-chain connects to the host adapter through a SCSI port at the back of the system case. The second SCSI device is in turn connected to the first device. The last device in this chain must be terminated using a Terminator.
	Hands on exercise
	To install an external SCSI device : 1. Connect one end of the SCSI connector to the SCSI port on the computer. 2. Secure the device plug to the system using the screws or wire pins located at the sides of the plug. 3. Connect the other end of the SCSI connector to the port on the back of the device. 4. Secure the device plug to the device using the screws or wire pins located at the sides of the plug. 5. Set the SCSI ID of the device attached to the system. 6. Connect one end of the power cord to the device and the other end to the power outlet. 7. Terminate the SCSI bus if the device is the last one in the daisy-chain. 8. Start the system. 9. If Windows does not recognize the new hardware, install it using the Device Manager.
14.10 Troubleshooting
	Troubleshooting enables you to correct the problems with the SCSI devices. Any problems with the SCSI device may cause your system to hang up or display error messages. The daisy-chain is a link of devices so any problem with any one device in the chain may cause all the devices to malfunction.
	14.10.1 Problem with the SCSI Daisy-Chain
	If there is any problem with the SCSI daisy-chain, the devices will not function and the system may also not be able to recognize the devices that are connected to the SCSI daisy-chain. 1. Check that the power supply is switched on for all the devices. 2. Check that the cables in the daisy-chain are properly connected and not loose. 3. Check that both ends of the SCSI daisy-chain are terminated. 4. Check that a SCSI ID is not repeated for two or more devices. 5. If you are using a Narrow device, ensure that the SCSI ID of all the devices is below 7. 6. Disconnect all the devices and reconnect them one by one. 7. Reinstall the device drivers.
	14.10.2 The Connected Device is not Functioning Properly
	When the device connected does not function properly, the device may not perform the requested task properly, or may display errors when you try to access the device. 1. Check if the devices are connected properly. 2. Ensure that the terminators are secured properly at the ends of the SCSI daisy-chain. 3. Change the SCSI ID of the device. 4. Reinstall the device driver.