1. 1 PC Peripherals for Technicians PC Peripherals for Technicians Chapter 2.4 - Chapter 2.4 - Storage: SCSI Systems Manufacturing Training and Employee Development Copyright © 1998 Intel Corp.

2. 2 Storage: SCSI OBJECTIVES: At the end of this section, the student will be able to do the following: l Describe the components of a basic SCSI sub-system. l Discuss the features of a SCSI-bus system. l Explain common terms used with SCSI interfaces. l Discuss SCSI IDs & LUNs, Terminations, and ASPI. l Discuss trouble-shooting SCSI sub-systems. l Discuss advantages of SCSI & EIDE drives.

3. 3 The SCSI Sub-system l SCSI (pronounced "scuzzy") stands for Small Computer System Interface.  SCSI is a computer bus interface standard that enables different kinds of peripherals to interface with the host computer by means of a SCSI-bus cable. » SCSI is used to interface to a number of different device types, such as Hard Drives, Tape Backups, CD-ROM drives, Printers, Scanners, etc. l Most desktop PC's have built-in support for IDE disks.  A SCSI disk controller can be used as a replacement or to provide supplemental storage.  SCSI is the standard hard disk interface for servers and the Apple MAC, but is not as common in desktop PC’s.

4. 4 SCSI Sub-system Components See SCSI Sub-system drawing on next slide l A typical PC SCSI sub-system consists of a SCSI Host Adapter and one or more SCSI devices connected to the host adapter by electrical cables (the SCSI Bus).  The SCSI Bus transfers data and commands at high speed to and from the peripheral device(s). l The SCSI Host Adapter provides a SCSI bus interface to the SCSI devices installed in a computer.  The SCSI host adapter is linked to the PC system bus, so it can transfer data between SCSI devices and the PC's main memory.  The host adapter usually has an onboard BIOS, both internal and external SCSI connectors, and terminators.

5. 5 SCSI Sub-system Components SCSI devices are daisy chained from device to device ID0 ID1 ID7 Host Adapter ID4 ID5 Internal Devices Device ID0 & ID1 External Devices Device ID4 & ID5 The SCSI Host Adapter may be a PCI, ISA, or EISA adapter board or a SCSI controller “chip” integrated on the motherboard in server systems. Devices should form a single chain from the device at one end to the device at the other. (No 'T's are allowed)

6. 6 SCSI Sub-system Components l SCSI may include both internal & external devices.  SCSI devices located inside the PC are connected to the adapter card by a flat ribbon cable.  External SCSI devices are typically connected via a special "Centronics" like connector. l The SCSI device interface is designed to accommodate intelligent devices attached to the bus.  All devices, including the host adapter, are intelligent devices with their own memory and can exchange data. » SCSI devices interpret & perform high-level commands.  Each SCSI device has a ROM which contains the device parameters (e.g. - Heads, Cyl, SPT, etc.).

7. 7 SCSI Sub-system Features l A common use for SCSI hard drives in PCs is in Network File Servers with large capacity hard disks.  Large/fast hard disks are available first as SCSI drives. » Disks with the high capacities & high performance intended for servers aren't as common with an EIDE interface.  SCSI can provide a boost in performance when used on a server with multiple drives attached or when used with a multitasking operating system. » Supports queuing controller requests: the O/S can request more data while it is waiting for the first data to be returned. » Several devices can use the bus at the same time. l Note: SCSI is more complex to implement and more expensive to manufacture than EIDE.

8. 8 SCSI Sub-system Features l PC SCSI adapter cards contain extra logic to convert the first two SCSI disks into emulated ST506 disks. » The standard PC BIOS allows for only 2 drives.  If system has only SCSI drives, the ROM inserts the first SCSI disk (under ST506 emulation) as the boot device. » An O/S and its device drivers are loaded using ST506 emulation by the standard BIOS (INT 13h). > The advanced SCSI feature support is then loaded from the SCSI disk and the system switches over and uses the more advanced support (e.g. higher performance, multitasking, additional device types).  If both IDE & SCSI are present, the SCSI controller defers to the IDE support & boots from the first IDE disk.  If IDE only, then the 1st IDE disk is the boot device.

9. 9 SCSI Definitions Overview Note: Terms defined in more detail on following pages!  The SCSI interface comes in several types » 8-bit (50 wire data cable - SCSI 1) > The "A" Cable is the "original" or "standard" cable. » 16-bit (68 wire cable - Wide SCSI) > The 68-wire "P" Cable is now the standard for Wide SCSI implementations.  The SCSI Clock can be 5, 10, 20, or 40 MHz SCSI bus clock8-bit (50 pin)Wide 16- bit (68 pin) 5 MHz (SCSI 1) 2-5 MBytes/s N/A 10 MHz (Fast SCSI) 10 MByte/s 20 MByte/s 20 MHz (Fast-20, Ultra SCSI) 20 MByte/s 40 MByte/s 40 MHz (Fast-40, Ultra-2) 40 MByte/s 80 MByte/s

10. 10 SCSI Definitions  Asynchronous or Synchronous protocols can be used by SCSI devices for communication. » Asynchronous is a classic Req/Ack handshake. > The time between successive data bytes is NOT fixed > Maximum transfer rate ~2MB/sec » Synchronous mode boosts performance for long cables. > Allows several REQs before receiving an ACK > An “offset” determines the max number of outstanding REQs with no corresponding ACK. > Can increase the transfer speed by a factor of three.  Note: SCSI-2 and SCSI-3 devices must start as follows to ensure that initial communications will work. » 8-bit (not wide) mode; 5 MByte/S transfer (not fast) mode; Asynchronous (not synchronous) mode

11. 11 SCSI Definitions  SCSI-1 (1986): » The original standard that defined the SCSI interface, a command set, & the electrical characteristics of SCSI bus. » Transmits either command or data bytes on 8 wires running in parallel at a 2-5 MB/sec transfer rate. > 1986 SCSI-1 (asynchronous) 2 MB/sec > 1990 SCSI-1 (synchronous) 5 MB/sec  Fast SCSI (1993): Doubles synchronous data transfer rate (to 10 MB/sec) on the 8 bit bus by increasing the data bus clock speed from 5MHZ to 10MHZ. » Synonymous with "10 MByte SCSI" » Both controller and device must be able to accommodate this mode (often uses differential SCSI buses).

12. 12 SCSI Definitions  Wide SCSI (1993): A SCSI-2 enhancement that allows data to be transferred 16 bits at a time » Can have up to 16 devices & be coupled with Fast SCSI. » 32 bit data supported but not common due to requirements for special cable & connector.  SCSI-2 (1993): Adds features to the SCSI-1 standard. » Fast SCSI; Wide SCSI; More Devices per Bus; Active Termination; Command Queuing; Additional Command Sets; Command Set Enhancements > Support for new SCSI devices: (tape drives, CD-ROM, etc.) > Note: 32-bit SCSI not common due to connector requirements » SCSI-2 supports both standard SCSI devices & SCSI-2 devices attached to the same bus.

13. 13 SCSI Definitions  Differential SCSI: An enhancement to make SCSI less susceptible to noise (max length up to 25 meters). » Uses 2 wires (normal & inverted signals) and the voltage difference determines whether the signals are asserted or de-asserted--Noise on lines cancels at differential receiver. > Note: Do NOT mix Differential devices & Single-Ended devices or Differential & Single-Ended terminators on the same bus.  SCSI-3 (1997): Adds features to SCSI-2. » Ultra SCSI is the SCSI-3 parallel solution (20 MHz & 16 bit). > "Fast-20 Wide" SCSI, also referred to as "Ultra" SCSI, > Formally defines 16-bit wide 68-pin (.025 pitch) P-Connector. » Adds support for a serial channel interface: Fiber channel & Twisted pair will provide faster transfer rates, more devices per bus, longer cables (~1km), and simplified connectors.

14. 14 SCSI Addresses: SCSI ID’s l Each device on the SCSI bus must have a unique 2-part address. l The SCSI address consists of an ID & a Logical Unit Number. l A SCSI ID is an identifying label (from 0 to 7) assigned to each device on the SCSI bus, including the host adapter itself. ID0 ID1 ID7 ID4 ID5

15. 15 SCSI Addresses: SCSI ID’s l Each device on the SCSI bus must have a unique 2- part address consisting of an ID and a LUN.  Each SCSI device must be uniquely identified when it is trying to use the SCSI bus or when another device is sending a command to it.  A SCSI ID is a unique identifying number (SCSI address), ranging from 0 to 7 (or 0 to 15 for Wide SCSI)  SCSI ID’s are commonly set by DIP switches or jumpers » The ID is transferred on the data bus and formed by bytes where LSB bit 0 corresponds to ID0 and MSB bit 7 to ID7. » ID's are logical assignments and have nothing to do with where they go on the bus or if they should be terminated. > Note: If two or more devices on the SCSI bus are set to the same ID, you must change one of them.

16. 16 SCSI Addresses: SCSI ID’s l The SCSI ID decides which device has priority when two or more are trying to use the bus at the same time.  The higher the SCSI ID number, the higher the priority » The sequence is 7, 6, 5, 4, 3, 2, 1, 0, 15, 14, 13, 12, 11, 10, 9, 8. l Most host adapters are set by default to SCSI ID 7.  Host adapter has highest priority for using the SCSI bus. l Recommended SCSI ID Assignments  Leave the host adapter SCSI ID at the default of ID 7.  Set the first two SCSI hard drives in your system to ID0 and ID1  Assign SCSI IDs 2 through 6 to CD-ROM drives, tape drives, and other SCSI peripherals in your system.

17. 17 SCSI Addresses: SCSI LUNs Host Adapter Initiator SCSI Bus Target (e.g. Disk) Logical Unit (LUN 0) CONTROLLERCONTROLLER Initiator SCSI Bus Target (e.g. Dual CD-ROM) Logical Unit (LUN 0) Logical Unit (LUN 1) CONTROLLERCONTROLLER Most SCSI devices have one Logical Unit

18. 18 SCSI Addresses: SCSI LUNs l Logical Unit Number (LUN)  Each device on the SCSI bus must have a unique 2-part address consisting of an ID and a LUN. » SCSI devices require a unique LUN from 0-7 (up to eight LUNs) which must be set for each device on the bus. » Most SCSI devices have a single embedded controller with a LUN of 0.  A SCSI initiator (usually a host adapter) can address each LUN separately through the target. » An example of a LUN is a hard disk. » An example of a device with multiple LUNs is a “juke-box” CD-ROM drive. > The CD changer takes up only one SCSI ID and each of the "platters" uses a separate LUN.

19. 19 SCSI Characteristics: Terminations l Terminators are a set of electrical resistors installed, or enabled, at each end of a SCSI bus. l When terminators are installed in a host adapter or other device, the device is said to be terminated. l The SCSI bus needs exactly two terminators Not Terminated Not terminated Not Terminated If you have only internal or external devices, the host adapter is terminated; otherwise it is not.

20. 20 SCSI Characteristics: Terminations l Terminators are electrical components that provide impedance matching which results in clean signals without reflections or ringing.  The significance of termination becomes greater as cable lengths and data transfer rates increase.  IF an incorrectly terminated system happens to work now, don't count on it continuing to do so. l The device at each end of the SCSI bus must be terminated, all other devices must be unterminated.  Terminators must be installed, or enabled, on the devices at each end of the SCSI bus.  Terminators must be removed, or disabled, on devices in the middle of the SCSI bus.

21. 21 SCSI Characteristics: Terminations l There are two types terminations which are electrically the same.  Passive termination is a 220-Ohm pull-up resistor to a ~5-V supply (TERMination power) and a 330-Ohm resistor to ground, for each signal line.  Active termination has a voltage regulator that provides a 2.85-V output and a 110- Ohm resistor from each signal line to this 2.85-V source. » Active termination (with termination power provided by the host adapter) maintains a constant 2.85 V and provides less line noise and more reliable data-transfers for long buses.  SCSI-2 defined the active terminator (it is optional)  SCSI-3 requires active termination.

22. 22 SCSI Characteristics: Terminations l Each device can provide it’s own terminating power.  Note: Terminator power is often fused.  External devices must be switched on.  Newer designs may use auto-restoring fuses which ‘reset’ themselves after a period of time. l There are several ways to change the termination, depending on what kind of device you have.  Physically remove terminating resistors from their sockets or insert them in the sockets.  Change a switch setting on the device's switch block.  Switched electronically (by running a configuration program supplied with the SCSI adapter)

23. 23 Advanced SCSI Programming Interface l The Advanced SCSI Programming Interface (ASPI) is a standardized software protocol developed by Adaptec and adopted by other vendors.  ASPI has become a de facto standard that enables programmers to develop applications and device drivers that work with all ASPI-compatible SCSI adapters. » Each host adapter-specific device driver and each hardware-specific device driver is written according to the same ASPI protocol standards. » Note: A Device Driver is a hardware-specific software program that acts as an interface between a computer's O/S and the hardware devices attached to the PC.  An alternative interface protocol is called Common Access Method (CAM) is not as widely used as ASPI.

24. 24 Troubleshooting SCSI l Check these potential problems if SCSI devices do not work properly after setting up the sub-system:  Are all SCSI bus and power cables properly connected? » Is pin-1 orientation correct for the internal SCSI cable?  Are all SCSI devices powered? » The host adapter cannot detect a device on the SCSI bus at boot up unless the device is powered up.  Do all devices on the SCSI bus have unique IDs? » Note: If two or more devices on the SCSI bus are set to the same ID, you must change one of them.  Is the SCSI bus correctly terminated? » IF an incorrectly terminated system happens to work now, don't count on it continuing to do so. Fix the termination.

25. 25 SCSI vs. EIDE l Enhanced IDE represents a good low-cost solution, while SCSI retains a significant performance advantage. l Advantages of SCSI:  Can link multiple (up to 7 or 15) SCSI devices together (e.g. scanner and a drive, or multiple drives). » Enhanced IDE can support a maximum of four.  SCSI devices can be daisy-chained together, whereas all IDE devices must be directly connected to the controller. » This can save IRQ's, DMA channels, and/or ISA/PCI slots  SCSI will coexist with any other type of controller (e.g. IDE) or host adapter (other SCSI cards). » SCSI host adapters do not take up one of the two available hard drive controller port addresses (1FX,3FX,17X,37X).

26. 26 SCSI vs. EIDE l Advantages of SCSI: (Cont.)  SCSI allows devices to be physically external. » SCSI cables can be up to 6 meters long. » IDE allows a maximum cable length of 46cm (18”) - insufficient to support external devices.  SCSI supports different types of devices simultaneously with the same host adapter. » For multitasking operating systems, SCSI allows multitasking and command queuing: multiple I/O requests can be outstanding and be completed out of order. > With EIDE, only one of the two devices on each channel (cable) can be active at a given time. » Support for almost any type of peripheral--Scanners and CD-writers aren't commonly available with EIDE (1997).

27. 27 SCSI vs. EIDE l Advantages of SCSI: (Cont.)  Automatically configures device type, geometry (size), speed and manufacturer/model number. » Set up as "not installed" in the CMOS.  Superior bus-mastering DMA support. » SCSI supports device to device transfers on the bus without host computer interruption. » Bus-mastering DMA decreases the amount of CPU time required to do I/O, leaving more time to work on other tasks.  Software portability - drivers are written for the host adapter, not the specific device. » The CD-ROM driver written for your host adapter will work with any brand or speed SCSI CD-ROM drive.

28. 28 SCSI vs. EIDE l Advantages of EIDE:  EIDE controllers are cheaper than SCSI host adapters.  Less overhead per command than SCSI. » In general, DOS/Windows users will experience as good or better disk performance with Enhanced IDE in single user, non-multitasking evnvironments.  SCSI configuration can be more complex: » Loading drivers, setting IDs, installing terminations.  SCSI requires device drivers to take advantage of advanced and high performance SCSI features. l High price of SCSI justified for high-end servers and power users running multitasking operating systems!

29. 29 REVIEW & SUMMARY WE HAVE DISCUSSED THE FOLLOWING: l The components of a basic SCSI sub-system.  A typical PC SCSI sub-system consists of a SCSI Host Adapter and one or more SCSI devices connected to the host adapter by electrical cables (the SCSI Bus).  All devices, including the host adapter, are intelligent devices with their own memory and can exchange data. l The features of a SCSI-bus system.  Large/fast hard disks are available first as SCSI drives.  SCSI boosts performance on servers with multiple drives or when used with a multitasking operating system.  PC SCSI adapter cards contain extra logic to convert the first two SCSI disks into emulated ST506 disks.

30. 30 REVIEW & SUMMARY l Common terms used with SCSI interfaces. SCSI bus clock8-bit (50 pin)Wide 16- bit (68 pin) 5 MHz (SCSI 1) 2-5 MBytes/s N/A 10 MHz (Fast SCSI) 10 MByte/s 20 MByte/s 20 MHz (Fast-20, Ultra SCSI) 20 MByte/s 40 MByte/s 40 MHz (Fast-40, Ultra-2) 40 MByte/s 80 MByte/s l SCSI IDs & LUNs, Terminations, and ASPI.  Each device on the SCSI bus must have a unique 2-part address consisting of an ID and a LUN.  The device at each end of the SCSI bus must be terminated to provide impedance matching which results in clean signals without reflections or ringing.  ASPI enables programmers to develop apps and drivers that work with all ASPI-compatible SCSI adapters.

31. 31 REVIEW & SUMMARY l Trouble-shooting SCSI sub-systems.  Bus & power cables properly connected; All SCSI devices powered; Unique Ids; Correctly terminated. l Advantages & disadvantages of SCSI & EIDE drives.  SCSI links multiple SCSI devices (up to 7 or 15) together and allows devices to be physically external.  SCSI supports different types of devices simultaneously with the same host adapter (Printers, Scanners, etc.)  SCSI auto configures device type, geometry (size), etc,  EIDE controllers are cheaper than SCSI host adapters and have less overhead per command than SCSI.  High price of SCSI justified for servers and power users running multitasking operating systems! End of Chapter 2-4