This is a document titled: SCSI general information It consists of an overview and description of SCSI.
SCSI overview
Typical consumer SCSI interfaces fall into three categories for SCSI signaling and termination:
- Single-ended
- Differential (HVD)
- Differential (LVD)
Single ended
This is the most common form of SCSI signaling. Most SCSI devices are single-ended. Many removable drives, scanners, and almost all 50-pin SCSI devices fit into this category. Often you don't see “SE” or “single-ended” written on the documentation; if a device does not specifically say LVD, Ultra2 Wide, differential, or some similar definition, then it is probably single-ended. Typically, single-ended devices support a total bus length of 1.5 meters (5 feet) or less. IBM high performance SCSI attached tape drives (that is, IBM 3590 or Magstar, IBM 3570 or Magstar MP, and LTO Ultrium drives) are all differential attached drives and not single-ended. They must be attached to supported differential adapter cards in the host machine; these are usually additional external adapter cards and are not integrated on the base machines.
Differential (HVD)
HVD (often referred to as just “differential”) uses differential signaling. The idea behind differential signals is that each bus signal is carried on a pair of wires. The first wire of the pair carries the same type of signal as single-ended SCSI. However, the second wire of the pair carries its logical inversion. The receiver of the signals takes the difference of the pair (hence the name), which makes it less susceptible to noise and capable of supporting greater cable lengths. HVD and single-ended SCSI are completely incompatible with each other. In general, IBM HVD tape devices support an overall bus length of 25 meters, using point-to-point or multi-drop interconnection.
Differential (LVD)
This is a new implementation of a differential interface: LVD uses less power than the current differential interface (HVD), is less expensive, and will allow the higher speeds of Ultra-2 SCSI. LVD requires 3.3 Volts DC instead of 5 Volts DC for HVD; the LVD transceiver is integrated into the SCSI chip using CMOS technology. LVD is sometimes referred to as Ultra-2 Wide SCSI, which is a general marketing term for “16-bit Fast-40” or “80 MB/s”. Only LVD and HVD could potentially run in Ultra2 Wide mode, and in current commercially available products it is LVD only. The IBM LVD tape devices support a bus length of 25m point-to-point, and 12m using multi-drop interconnection. The terms “fast” and “wide” and “ultra” indicate characteristics that are separate from those implied by “differential”, “single-ended”, and high or low voltage. The table below shows the SCSI terms used to describe different host and device adapters and what they imply in terms of bus width and speed.
SCSI terms, types and characteristics
|
SCSI terms
|
Bus width (bits)
|
Speed (MB/s)
|
Notes
|
| SCSI |
8 |
5 |
. |
| Fast SCSI |
8 |
10 |
Fast signifies faster timing on the bus: 10MHz instead of 5MHz |
| Fast Wide (F/W) SCSI |
16 |
20 |
Wide uses an extra cable (or 68-pin P cable) to send the data 16 or 32 bits wide |
| Wide Ultra (W/U) SCSI |
16 |
40 |
Ultra means it allows up to 20MHz signals on the bus |
| Wide Ultra2 SCSI (LVD) |
16 |
80 |
Ultra-2 (or Ultra2) allows up to 40 MHz |
The newest SCSI features
| Ultra160 SCSI (LVD) |
16-bit |
160 MB/sec |
Ultra160 SCSI hard drives and Ultra160 SCSI capable tape units
|
Features added with Ultra160 SCSI
- 160 MByte/sec performance
- Cyclic Redundancy Checking (CRC) checks all transferred data, adding significantly to data integrity
- Domain Validation intelligently verifies system configuration and automatically sets reliable transfer speeds
Connectivity
- Connectivity for internal and external SCSI devices
- Single SCSI card can connect up to 7 or 15 devices per channel
- Backward compatible with earlier versions of SCSI
Increased performance
As data transfer rates grow, bus speeds must also increase to prevent data from overrunning the bus or to avoid limiting the number of drives that can be connected to the bus. The traditional ratio holds that bus speed must double every two years to keep available bandwidth at four times the data transfer rate of one drive. This suggests that as systems continue to become heavily loaded in 2000, SCSI's 80MB/s data transfer rate may fall short of system needs.
Ultra160 uses double transition clocking to send 2 bits of data per clock cycle instead of one, doubling Ultra2's data transfer rate of 80MB/s to 160 MB/s. As drive caches approach the 2 MB range, Ultra160 will grow to meet demands much as its predecessors Wide Ultra and Ultra2 SCSI evolved over the last three years. And as faster network I/O such as gigabyte Ethernet evolves, storage subsystems can be expected to require data transfer rates of up to 200 MB/s.
Full backward compatibility
A hallmark of SCSI evolution has always been seamless backward compatibility to protect existing hardware investments. Adding Ultra160 into a system only involves the installation of a simple card; Ultra160 uses all the existing connectors, and allows a wide range of attached peripherals of all levels of SCSI to continue to be used.
Ultra160 uses several new features to ensure data integrity and reliability critical to overall system performance.
Domain Validation intelligently verifies system configuration, testing during the boot cycle or on introduction of a new device for the maximum speed the target can maintain.
- Management software can also use Domain Validation to monitor performance and suggest tuning of the bus for better throughput.
Cyclic Redundancy Checking (CRC) ensures that in the high speed environment of Ultra160, all data reaches its intended destination properly.
- Each data block contains extra code letting the receiver validate the contents.
Commands built into the current SCSI standard are also supported by Ultra160.
- Disconnect/Reconnect allows a SCSI device to disengage from the host adapter's bus while performing a complex task
- Tagged Command Queuing optimizes transfer speed by reordering blocks of data moving along the bus
The information above is courtesy of Adaptec and IBM Redbooks.
Additional information
IntelliStation Pro:All operating systems listed
Older System x:All operating systems listed
BladeCenter:Operating system independent / None
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