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CET2176C Server+ Lecture #14 - SCSI: Small Computer System Interface

Lecture Only
The origin and evolution of SCSI,
The general description of SCSI,
The practical applications of SCSI,
The components of a SCSI bus,
Installation and configuration of a SCSI bus,
The terminology of SCSI.
The student will become familiar with the SCSI technologies and know the basic concepts of what the SCSI bus is and how it works. The student will know how to select SCSI components; being able to identify and discuss the features and how they correspond to the performance and design of the computer system. The student will learn the SCSI generations and be able to identify them. The student will be able to upgrade and service existing SCSI based systems as well as be able to design and implement new SCSI based computer systems.
SCSI - Origins
  1. SCSI was chosen by Apple Computer as their solution to long term high capacity internal storage for their systems. It is the Apple PCs principal HDD controller, although ATA interfaces especially on the PCI bus, which was adopted by Apple as well as the PC, have emerged.

  2. Since the technology evolved independently of the PC market two major differences are immediately apparent from the end user perspective: the SCSI controller can handle more devices than the PCs ATA controllers and it can communicate with a much more diverse set of devices.

  3. The SCSI bus has evolved faster than any of the PC industry’s buses because it is not under the same backwards compatibility pressures as the PC. This does not mean that the technology is not backwards compatible at all, it does mean that technologies are rendered obsolete much faster than in the PC industry. It was introduced to the PC as an eight bit bus clocked at 5mhz with an effective data transfer rate of 4MB/s. Because the technology is privately owned (licensed) by Adaptec, they are free to improve it as they wish. They are currently offering on "FiberSCSI" also called "fiber channel SCSI" which attaches devices using fiber optic cables that run at 1GB/sec and now 2GB/sec fiber channel SCSI is available.

SCSI - An Expansion Bus
  1. Physically SCSI is a parallel bus topology. This means that the control and data lines run in parallel uninterrupted throughout the SCSI subsystem. Devices attach to these lines via "T" branches to the lines. The primary engineering obstacle to such a physical topology is the handling of the ends of the bus lines. Left unattended electron transmissions on the bus lines will reflect off of the ends of the bus lines back toward the devices. These reflections will collide with actual data and be mistaken for data and the bus will fail. To prevent this, the ends of the wires are attached to ground through resistors. If they were attached directly to ground without the resistors all of the electrons would fly straight to ground (the path of least resistance) and not be caught by the devices on the bus. Each end of each parallel bus wire must be attached to such a resistor and then on to ground. This set of parallel resistors that connect the ends of the bus to ground are called terminators.

  2. The SCSI bus is managed by a SCSI host controller or adapter in the case of the PC. Because SCSI did not evolve within the standard PC architecture, this device attaches to one of the PC motherboard expansion buses so that it can attach SCSI devices to the computer and communicate with the processor and operating system. It translates requests and data from the motherboard's standard buses onto the SCSI bus and directs commands and data to and from the target device specified by the PC's request. So the SCSI bus is in effect an isolated and independent fully functional bus like the ISA or PCI buses, yet it interfaces to the system through an expansion slot attached to one of these buses as well.

  3. SCSI supports both internal and external devices and depending on the maximum bus length restrictions from end to end the SCSI bus can support any number of internal and/or external devices up to the number of supported logical device ID's that type of SCSI bus can address. This is another major advantage over ATA controllers. The host adapter can reside anywhere on the bus physically, it can support external devices and many more devices. The host adapter can be placed in the middle of the bus between groups of both internal and external devices or at either extreme end of a group of either internal or external devices.

  4. On the 8-bit SCSI bus 7 devices may be attached, this does not count the host adapter. The 8-bit bus has 8 SCSI IDs, one is always claimed by the controller. The Adaptec standard is for the host adapter to claim ID = 7. Some older controllers adhering to the old specifications might claim ID = 0. Adaptec also defines ID#7 as the highest in priority which the bus controller obviously needs. This means that order of priority descends with #7 as highest, and zero as lowest. When the bus was expanded to 16-bit data transfers, 8 new IDs were introduced to the bus 8 – 15. In order to preserve existing host adapters as the highest priority device on the bus a tiered priority scheme was developed. The highest priority set or tier of IDs is 7 – 0. The highest ID = 7, the lowest = 0. The next tier down in priority is 15 – 8. With highest priority ID within this tier = 15 and the lowest = 8. If another tier were added it would have IDs 23 – 16 and this whole group would be lower in priority than the preceding group (15 – 8). Within this tier the highest ID would be 23 and the lowest would be 16.

    Internal SCSI Hard Drive Jumper Bank Location

  5. With 8 possible IDs 3 binary bits are required to convey the ID through the bus. In the 8-bit bus, there are three ID lines as part of the control lines of the bus. Further a device’s ID is selected by setting jumpers on three possible positions each representing one bit of the 3-bit SCSI ID. The 16-bit Wide SCSI bus allows for 15 devices (plus the host adapter) and this requires one more ID control line in the bus (4 bit ID numbers) which leads to "SCSI Wide" devices having 4 device ID jumpers that must be set. No two SCSI devices can ever share the same ID. If this happens the bus will not function properly. This ranges from sporadic crashes and data loss, to total failure of the bus not to mention the guaranteed inability to detect one or both of the devices that are attempting to use the same device ID.

SCSI - Signaling
  1. SCSI devotes 2 wires to each signal line so in a 50-wire/50-pin internal ribbon cable there are actually only 25 signals each using two lines. One of the two wires used for each actual signal is used for the data signal itself and the other line is connected to ground. In ribbon cables the ground lines lie between the data lines; an idea copied from SCSI and used in the UDMA 80-wire EIDE cable. Sometimes the two wires are twisted adding a little more signal stability. This type of transmission method is called unbalanced signaling. Another term for this type of data signaling is "single ended". This is the term that SCSI uses, but it means the same thing.

  2. Another type of physical signaling used in the wires of the SCSI bus is balanced signaling. If the data voltages are transmitted on one wire and the opposite voltages are transmitted on the other wire, then the receiver needs only to respond to the detected difference in the voltages. This means that the voltages can be dramatically reduced or the cable can be dramatically lengthened. This is the signaling method used by USB as well. Another term for balanced signaling is voltage differential signaling. This is the term that SCSI uses.

  3. SCSI developed two different voltage differential technologies refered to as LVD or Low Voltage Differential and HVD or High Voltage Differential. SCSI also features unbalanced signaling devices so there are three separate signaling technologies used by SCSI devices: single ended devices (SE), high voltage differential devices (HVD), and low voltage differential devices (LVD). HVD devices operate at higher voltages than what most devices operate at in the PC. Attaching one to a bus with an SE or LVD controller and devices will result in catastrophic damage to all devices on the bus and possibly even the host computer. It is completely incompatible with all other SCSI systems. LVD devices can be safely connected to SE buses, although they may not function. HVD devices were obsolete as of SCSI-3.

  4. Many LVD devices are backwards compatible by design and support SE operating mode and can revert to this method of signaling if they detect it on the bus. If even one SE device is present on the bus, all LVD devices that are capable of it will revert to SE operation and run at the slower SE bus speed, since SE is the slower and older signaling method. Any LVD device that cannot revert to SE mode will fail on this bus. It is most significant if the controller itself is the LVD device and does not support SE mode. In this case all SE devices will fail, and their signaling attempts may cause other devices to attempt to revert to SE mode and cause them to fail as well. If the LVD controller is SE capable and a single SE device is attached and there are other LVD devices that are not SE capable, the controller can revert to SE and communicate with the new device and effectively strand all of the other existing devices since they cannot revert. The important point to bear in mind is that the bus signals themselves can only be one type, either SE or LVD never both. When mixing SE and LVD devices it is important to make sure that all of the LVD devices can run in SE mode and that in this mode they will all run slower since it is a much older and slower technology.

SCSI - Symbols
  1. These are the standard SCSI device symbols used. A SCSI device should have the appropriate symbol printed on it somewhere. If it does not, then the technician will be responsible for researching the device on the Internet before attaching the wrong type of SCSI device to the bus and running the risk of destroying all devices on that bus:

    Single Ended

    Low Voltage Differential

    SE/LVD Compatible

    High Voltage Differential

  2. If a SCSI device's drivers install successfully into a Windows system a SCSI symbol icon will appear in Device Manager as a device category. This category is built-in to the operating system. Opening this category will reveal the new SCSI device probably using the same or a similar SCSI symbol icon.

  3. Sometimes a SCSI device icon will appear in the Device manager even though there are no actual SCSI physical devices or SCSI host adapters in the PC. This is due to the way a particular set of drivers are built. Even though the physical bus is not SCSI, a virtual SCSI bus can be established over any media even wireless. This is the case if you see the icon but there are no physical SCSI devices in the system, the drivers have been built in such a way that a virtual SCSI bus is set up in RAM and the OS sees it as a SCSI bus and communicates with it through SCSI driver interfaces. This interface technology is called ASPI – Advanced SCSI Programmers Interface. CD-ROM burners and parallel port scanners often load an ASPI driver even though they are using a non-SCSI physical bus in interface with the PC.

SCSI - Cables and Connectors
  1. Internal SCSI cables are either 50 pin ribbons that look like wide EIDE cables. This is an 8 bit technology. The internal 68-pin ribbon cable is high density in design so it is actually less than half the width of the 50-pin cable but it is much stiffer due to a thicker protective layer of PVC added to the insulation of the wires for this purpose because the conductors are very fine. External 8-bit cables resemble thick printer cables and are called "A" cables referred to in the chart below. External 68-pin cables are sometimes thinner but stiffer for the same reasons as the internal cables and are referred to as "P" cables.

    Comparison of Internal SCSI (top) vs ATA (bottom) HDD
    showing Male IDC50 connector vs ATA Male IDC40 connector

    Internal Device Female High Density HD68 connector

    Internal Cable Male High Density HD68 connector

  2. There are two common external "A" cable connectors. A 50-pin Centronics connector which looks like a very large printer connector (at the printer's end of the cable, which is in fact a 36-pin Centronics connector). There is also a high density 50-pin connector which looks like a double rowed DB connector but is noticeably narrower and the pins in the two rows are orthogonal (directly above and below and side by side like sides of a square) rather than staggered like the old DB connectors. External 68-pin connectors are usually high density resembling slightly wider high density 50-pin connectors. Female connectors of all types reside on the device, Males reside on the ends of the cables.

    External Device Female Centronics-50 Connectors with a SCSI ID Roller Switch

    External Device High Density HD50 Connector

    External Device High Density HD68 Connector

    External Cable High Density HD68 Connector

  3. The maximum SCSI bus length depends on the technology and the quality of the cables and the connectors. A SCSI bus using a SE signalling has a maximum length from terminator to terminator that depends completely on the clock speed of the bus (NOT the throughput). The HVD bus achieved its length through overpowering crosstalk and EMI/RFI. The same is true of LVD buses but they are not as long. The HVD bus can be 25M long. LVD buses can be 12M long. 5mhz SE bus max. length = 6M. 10mhz SE bus max. length = 3M. 20mhz SE bus max. length = 3/1.5M (3M if less than three devices according to the specification but 1.5M is much more realistic).

SCSI - Termination
  1. There exist 3 different types of terminator. Passive terminators have a bank of 132Ω resistors connecting the lines to ground. Active resistors devote a small voltage regulating type of transistor circuit to each line that actively suppress excess electronic signals in the line. And Forced Perfect Terminators which have superior circuits than active terminators that mop up the line to perfection leaving no detectable stray electrons wandering about. All three technologies are SE level terminators. Passive terminators are obviously the cheapest and least effective. FPTs are the most expensive and effective. FPTs are available in FPT-3, FPT-18, and FPT-27. The FPT-3 terminates the three most active signal lines in the bus. The FPT-18 forces perfect termination on all 8 bit bus data and control lines and the FPT-27 terminates all control and dat lines on the 16-bit bus.

    Internal 68-pin Active Terminator

    External 50-pin Passive Terminator

  2. The SCSI bus can be physically terminated two different ways. A separate plug-like device that is attached to the ends of the cables (the next connectors physically beyond the farthest devices are the best places for this). Or a device may have a terminator built in to it. In this case the device may either be capable of autotermination (only available on the very latest SCSI Wide LVD devices), or have a jumper that must be set in order for it to be used. Obviously the documentation is necessary in order to determine how the device terminates and what type of terminator it has built in to it (i.e. passive, active or FPT).

SCSI - Generations of the Technology
  1. Here is a condensed table of the evolutionary generations of SCSI technology that have been produced:

StandardNameTermClock  Width   Transfer RateDevicesCables
SCSI-1AsyncAsynchronous5mhz 8-bit 4MB/sec7A
SCSI-1Fast-5Synchronous5mhz 8-bit 5MB/sec7A
SCSI-2Fast-5/WideWide5mhz 16-bit 10MB/s15P
SCSI-2Fast-10Fast10mhz 8-bit 10MB/s7A
SCSI-2Fast-10/WideFast/Wide10mhz 16-bit 20MB/s15P
SCSI-3(SPI)Fast-20Ultra20mhz 8-bit 20MB/s7A
SCSI-3(SPI)Fast-20/WideUltra/Wide20mhz 16-bit 40MB/s15P
SCSI-3(SPI-2)Fast-40Ultra240mhz 8-bit 40MB/s7A
SCSI-3(SPI-2)Fast-40/WideUltra2/Wide40mhz 16-bit 80MB/s15P
SCSI-3(SPI-3)Fast-80DTUltra3(Ultra160)40mhz 16-bit 160MB/s15P
SCSI-3(SPI-4)Fast-160DTUltra4(Ultra320)80mhz 16-bit 320MB/s15P

  1. There will not be any more ANSI SCSI standards after SCSI-3. Since it moves faster than technology bureaucracies the new technologies are all addenda to the original SCSI-3 referred to as SPI – SCSI Parallel Interface. The current copper standard available for highend user systems and workstations is SPI-4 which has a 320MB/sec transfer rate.

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