1. What is SCSI ? What is SASI ?
SCSI is a parallel bus architecture, it is a protocol(standard) and also its command set is called SCSI command set
SCSI - Small Computer System Interface
SASI - Shugart Associates System Interface was the earlier name for SCSI
2. What are the advantages of SCSI compared to ATA or IDE ?
IDE/ATA had below limitations which are overcome in SCSI
- Limitation of Cable length(ATA/IDE supports less than 0.5 mtrs but in SCSI maximum 24 meters cable length)
- Limitation of no. of drives connected ( Maximum 4 in IDE/ATA but in SCSI maximum 126)
- No Cache support (SCSI has good cache support )
- No command queuing support in IDE/ATA while its there in SCSI
- IDE/ATA has narrow data bus but SCSI supports 8 or 16 or 32 bit bus
- High performance in SCSI & Longer life (better MTBF: Mean time before Failure) in SCSI compared to IDE/ATA
3. Explain SCSI ID usage & its usage in narrow SCSI ?
In SCSI each device connected in the SCSI bus is assigned a unique id called "SCSI-ID"
In narrow SCSI since it has 8 bit data bus maximum 8 devices are supported including the HBA/Controller Card
4. Give SCSI Evolution ( Mention different types of SCSI )
SCSI 1
SCSI 2
SCSI 3
5. What is CDB ?
Command Descriptor Block is the format in which SCSI commands are sent across the SCSI bus
6. What are different SCSI command phases ?
arbitration
selection
message out
message in
command data out
data in
status
bus free
7. Mention device types in SCSI-3 ?
Device types supported in SCSI3 are given unique id's as given below
1 Block devices ( Hard disk drives)
2 Stream devices
3 Graphic devices ( Plotters & Printers)
4 Medium Changer ( Tape libraries & Juke boxes)
5 RAID
6 CD ROms / DVD roms
7 Enclosures
8. What is the difference between LVD & HVD ?
LVD : Low voltage differential
Gives better performance & bandwidth as its based on TTL logic ( transistor logic)
HVD : High voltage differential
Since its based on resistor logic it has low data throughput compared to LVD
8. What is the difference between SE & Differential (HVD) SCSI ?
SE : Single Ended
Obsolete technology which is very low on data throughput compared to differential types
HVD : High voltage differential
better performance & bandwidth compared to SE
9. What is a LUN ?
Each Target device can be subdivided into logical units which are identified by an unique no. which is called Logical Unit Number or "LUN"
10.Mention differences between SCS-1,2 and 3 ?
Another attachment has this information which is accompanied along with this file.
11. What is a terminator ?
The SCSI interface must be correctly terminated to prevent signals echoing on the bus. Many SCSI devices have built-in terminators that engage when they are needed; with some older SCSI devices you have to add an external SCSI terminator that plugs into the device's SCSI connector.
12.What are the differences between synchronous & asynchronous SCSI ?
Asynchronous transfers mean that every single byte must be
acknowledged before the next can be transfered. Synchronous means that
the device sending data can drop a series of transfers onto the bus,
toggling REQ or ACK (as appropriate), and then sit back and wait for
the corresponding pulses to return from the other device.
Async transfers, involving much more waiting, are correspondingly
slower. 2-4 MB/sec are good values for async transfers.
Sync transfer speeds are established during a negotiation between the
initiator and target, but devices are not required to use the full
speed they negotiate for. This speed represents the maximum burst rate
your device will use. Common values are 5 and 10 MB/sec.
In practice, virtually every modern device supports synchronous
transfers, but some implementations are better than others.
13.What is the default ID of SCSI HBA ?
Answer is 7 , since 7 SCSI-ID is given highest priority compared to other ID's SCSI HBA is assigned this id so that it can gain bus control most of the time
14.How you will install device drivers for HBA during OS installation ?
in windows os installation we press F6 & follow on screen messgs install drivers using Driver floppy
in Linux we need to type : linux dd in the boot screen & provice the floppy/CD containing the device drivers
15.What is parallel SCSI daisy chaining ?
We can daisy chain ( Connect one to another in daisy chain manner )
We can add devices to the SCSI bus by daisy chaining cables from device to device. The first and last devices in the SCSI daisy chain must be terminated
16.Explain arbitration & Selection process ?
ARBITRATION Phase
The ARBITRATION phase is entered when a SCSI device attempts to gain control of the SCSI bus. Arbitration can start only if the bus was previously in the BUS FREE phase. During arbitration, the arbitrating device asserts its SCSI ID on the DATA BUS. For example, if the arbitrating device's SCSI ID is 2, then the device will assert 0x04. If multiple devices attempt simultaneous arbitration, the device with the highest SCSI ID will win. Although ARBITRATION is optional in the SCSI-1 standard, it is a required phase in the SCSI-2 standard.
SELECTION Phase
After ARBITRATION, the arbitrating device (now called the initiator) asserts the SCSI ID of the target on the DATA BUS. The target, if present, will acknowledge the selection by raising the -BSY line. This line remains active as long as the target is connected to the initiator.
17. What are the maximum no. of devices supported in SCS-1,2 and 3 ?
SCSI 1 supports maximum 8 devices ( 7 devices excluding 1 HBA/Controller Card)s
SCSI-2 Narrow 8 devices , wide 16 devices
SCSI-3 Narrow 8 devices , wide 16 devices, even 32 devices are also supported in Ultra SCSI3
18. What is the maximum length of cable supported in SCS-1,2 and 3 ?
SCSI 1 - 6 mtrs
SCSI2 - 12 mtrs
SCSI3 - 24 mtrs
19. What is the difference between a SCSI HBA & SCSI RAID Controller card ?
HBA or Host Bus Adaptor is just acting as an interface between the System internal bus & external interfaces ( Example PCI to SCSI) it does not have extra intelligence & wont have any cache memory.
RAID controller has extra intelligence compared to HBA. RAID controllers have RAID algorithms embedded in a CHIP & might also have cache memory onboard. RAID controllers support many features like Raid creations & management, RAID migration , Hot spares etc.
20. What is the Disk speed of different SCSI drives ?
SCSI 1 - 5000RPM (Revolution Per Minute )
SCSI 2 - 6000RPM
SCSI 3 - 7200RPM
FC - 15000RPM
SAS - 15000RPM
SATA - 7200RPM
21. What is the difference between single-ended and differential SCSI?
* Answer: Single-ended and differential are two methods of placing SCSI signals on the cabling. Single-ended uses one wire driven against ground and the signal is the voltage difference between that wire and ground. The differential interface drives two wires. The signal is the voltage difference between the two wires. Single-ended and differential are not directly compatible. (It should be noted that HVD and LVD are also not directly compatible). They can be interconnected by the use of a SCSI expander called a Single-ended to Differential Converter. Single-ended cable lengths are 6 to 1.5 meters (20 to 5 ft), decreasing with increasing data throughput, while differential (HVD and LVD) offers cable lengths to 25 meters (82 ft), regardless of the speed of the bus.
22. What is meant by "Narrow" SCSI?
* Answer: Narrow SCSI is the term that is used for 8-bit SCSI. It can usually be identified by 50-pin connectors.
23. What is meant by "Wide" SCSI?
* Answer: Wide SCSI is the term that is used for 16-bit SCSI. It can usually be identified by 68-pin connectors. From SCSI-2 until the SPI-3 document in SCSI-3, this term also applied to 32-bit SCSI. SPI-3 obsoleted the 32-bit SCSI bus.
24. What is HVD SCSI?
* Answer; This is the "old" differential SCSI using TTL voltage levels that was originally defined in SCSI-2, offering 25 meter (82 ft) cable length. It was functionally replaced by LVD (Low Voltage Differential) SCSI in the SPI-2 document of SCSI-3 and obsoleted in the SPI-3 document of SCSI-3. HVD and LVD SCSI are not directly compatible but can be interconnected by the use of a SCSI expander called an LVD to HVD Converter.
25. What is Wide Ultra SCSI?
* Answer: Ultra SCSI, defined in the SPI-2 document of SCSI-3 offers a maximum data throughput of 20 Mbytes/sec for Narrow (8-bit) SCSI. Ultra Wide SCSI is the 16-bit version that offers 40 Mbytes/sec data transfers. Ultra Wide single-ended SCSI has a maximum cable length of 1.5 m (5 ft) with more than 4 active IDs and 3 m (10 ft) with 4 or fewer active IDs. Ultra Wide differential SCSI has a maximum cable length of 25 m (82 ft).
26. What is LVD SCSI?
* Answer: LVD, which stands for Low Voltage Differential, was introduced in the SPI-2 document of SCSI-3. It is also called Ultra 2 or Fast-40 SCSI. It uses 3 volt instead of 5 volt logic level and is not directly compatible with the "old" differential (HVD) SCSI. LVD again doubles SCSI data throughput to 40 Megatransfers/sec. Cable lengths are 12 m (40 ft). Single initiator-single target applications may use up to 25 m (82 ft) of cable. The "multimode" implementation of LVD is backward compatible with single-ended SCSI. However, connecting one single-ended peripheral to a multimode LVD bus will cause the entire bus to switch to the single-ended mode with the single-ended limitations on data throughput and cable length. LVD can be interconnected with HVD by the use of a SCSI expander called an LVD to HVD Converter.
27. What is "multimode LVD" or LVD/MSE SCSI?
* Answer: Multimode LVD and LVD/MSE (Multimode Single-Ended) are terms for the same interface. It is an implementation of SCSI that automatically switches between the LVD and the single-ended mode. When a single-ended device is connected to a multimode LVD/MSE bus, the entire bus switches to the single-ended mode. Otherwise LVD/MSE devices operate in the LVD mode.
28. What are the benefits of LVD SCSI?
* Answer: In addition to the obvious benefits of longer maximum cable length than single-ended and a doubling of data throughput, there are a number of other benefits. LVD/MSE and single-ended offer some compatibility. The lower operating voltage of the LVD bus means lower power dissipation, so the differential drivers can be included on the LVD ASIC rather than having to mount them external to the chip. This results in smaller boards, less heat dissipation, higher reliability and lower cost. Also, manufacturers will no longer have to design and build devices with both single-ended and differential interfaces. This results in lower costs.
29. Is LVD SCSI backward compatible?
* Answer: LVD is backward compatible through the single-ended interface if it is multimode LVD. It is doubtful that anyone will build LVD devices that are not multimode. Remember that connecting a single-ended device to a LVD/MSE bus will cause the entire bus to switch to the single-ended mode with its data throughput and cable length limitations. To add a single-ended peripheral to an LVD bus and preserve the data throughput and cable length of LVD, you can use a SCSI expander called an LVD to SE or LVD/MSE to LVD/MSE converter. This converter divides the SCSI domain into two bus segments - one segment will operate at the LVD data throughput and cable length and the other bus segment will operate at the single-ended data throughput and cable length.
30. What is Ultra160 or U160 SCSI?
* Answer: Ultra 160 is defined in SPI-3. It offers data throughput of 80 Megatransfers/sec or 160 Mbytes/sec for Wide (16-bit) SCSI which is the only defined bus width. For this speed, clocking on both the rising and falling edges of the REQ and ACK clock is required. This is called Double Transition (DT) clocking. Also called Fast-80 or Ultra 3 SCSI.
31. I have heard of U160/m SCSI. What is it?
* Answer: The SPI-3 document defines 5 new features for SCSI: Double Transition Clocking, CRC, Domain Validation, Quick Arbitration and Select (QAS), and Information Units (Packetization). In order to be compliant with the SPI-3 U160 specification, at least one of these features must be implemented. A group of industry leaders agreed to incorporate three of these features in order to speed up introduction of U160 products. These three features are Double Transition Clocking, CRC and Domain Validation. U160 devices with these three features are called U160/m.
32. Is Ultra160 SCSI backward compatible?
* Answer: Ultra 160, also called Ultra 3 is backward compatible through the single-ended interface, if it is multimode Ultra 160. It is doubtful that anyone will build Ultra 160 devices that are not multimode. Remember that if a single-ended device is placed directly on a multimode Ultra 160 bus the entire bus will switch to the single-ended mode with its limitations on data throughput and cable length.
33. Is Ultra 160 SCSI better than fibre channel?
* Answer: This is a discussion that will go on for some time and there is no simple answer. It depends on the application. At least until now, fibre channel (FC) implementations of SCSI have been FC-AL or FC-Arbitrated Loop, so I will limit my comments to FC-AL. At the time of writing this FAQ, SCSI is beginning to ship devices that are capable of 160 Mbytes/sec data transfer rate while fibre channel (FC) is stalled at a maximum of 100 Mbytes/sec and the only peripherals with true FC interface are disk drives -- and only one manufacturer makes them. Mark up the data throughput advantage to SCSI.
FC proponents say that connectivity is more important than data throughput and that FC can have up to 126 nodes. If that were true why would every increase in SCSI data throughput be immediately adopted? Data throughput rules in nearly every serious application we have encountered. Anyway, on a practical basis, FC is limited to only a couple dozen nodes which is very similar to what SCSI can handle. Call the connectivity issue a draw.
Well then, FC can have up to 10 kilometers of fiber. Well, there are a number of manufacturers of SCSI Extenders that offer the same or longer fiber cable lengths. Call this one a draw.
Again, at the time of this writing, an issue that FC proponents do not raise is interconnectivity. There are great problems in getting FC devices from different manufacturers to work together. Sometimes this is true with products from the same manufacturer. SCSI went through these problems many years ago. There are now very few problems with SCSI interconnectivity. Of course, these FC interconnectivity problems will eventually be solved, but for now, score this one for SCSI.
Cost is always a factor. FC is more expensive. As FC gets wider application its cost will go down, but for now there is no contest. Score this one for SCSI.
34. Is Ultra 160 SCSI better than EIDE?
* Answer: Again, the answer depends on the application. SCSI is an intelligent interface that can perform data transfers with no intervention from the host CPU. SCSI is multi-tasking. An initiator can issue a command to a target. The target can then disconnect from the bus to perform the task and free the bus up for another task. Ultra 160 SCSI can have up to 16 devices connected to the bus and they can be any of a large variety of peripherals, including hard drives, floppy drives, tapes, CDs, scanners, printers, etc. The number of devices can be substantially increased through the use of LUNs. EIDE can have two internal drives connected. Your PC probably has two EIDE buses, so it may have up to four peripherals. Ultra 160 SCSI allows up to 12 m (40 ft) of cabling, which may be internal or external to the computer. For point to point applications you may have up to 25 m (82 ft) of cable. EIDE is for internal cabling only and the maximum cable length is only 18 inches. And, don't forget that 160 Mbytes/sec is much faster than any EIDE bus.
For a home user with a single hard drive, EIDE is probably better as it is less expensive and almost as fast as Ultra SCSI. In a compute-intensive or storage-intensive application, however, SCSI is the clear choice.
35. What is Fast-20 [or Fast-40 or Fast-80] SCSI?
* Answer: The term "Fast-xx" refers to the maximum data throughput that a particular version of SCSI is capable of, expressed in Megatransfers/sec. For example, Fast-20 is 20 Megatransfers/sec which is 20 Mbytes/sec for 8-bit (Narrow) SCSI and 40 Mbytes/sec for 16-bit (Wide) SCSI.
* Fast-10 is the same as Fast SCSI
* Fast-20 is the same as Ultra SCSI
* Fast-40 is the same as Ultra 2 (uses LVD transmissions)
* Fast-80 is the same as Ultra 3 or Ultra 160 SCSI (uses LVD transmissions)
36. Can I connect an Ultra 2 Wide (LVD) disk to an Ultra Wide adapter?
* Answer: The answer is definitely yes. However, exactly how you do it depends on the type of Ultra Wide host adapter you have. If it is single-ended SCSI and the LVD disk is multimode LVD, you can connect the disk directly to the host adapter. The multimode LVD interface on the disk will switch to the single-ended mode.
If the Ultra Wide adapter is differential (HVD), they can still be connected, however, you will have to use a SCSI expander called an HVD to SE or an HVD to LVD Converter.
37. What is Double Transition clocking?
* Answer: Double Transition (DT) clocking is used to double the data transfer rate from Ultra 2 (Fast-40) to Ultra 3 (Ultra 160 or Fast-80) SCSI without having to increase the clock speed. That means that both edges of the REQ and ACK signals are used to clock data. The REQ and ACK signals run at 40 MHz on Ultra SCSI, so double clocking increases the rate at which data is clocked to 80 MHz. This provides data throughput of 160 Mbytes/sec for Wide SCSI.
38. What is "Domain Validation"?
* Answer: Domain Validation is a method used in Ultra 160 (Fast-80 or Ultra 3) to test for the optimum rate for data exchange. Once the host adapter (initiator) has located a peripheral and negotiated a data transfer rate, the initiator sends a Write Buffer command to the target at that negotiated data transfer rate and then reads it back to determine if what it reads is what it wrote. If not, it will resend the Write Buffer command at the next lower data transfer rate. This will continue until a speed is reached where the test is successful. This is all accomplished automatically.
39. What is CRC?
· Answer:Cyclic Redundancy Check is a means of detecting errors that is much more effective than the simple parity check that SCSI has used for years. CRC detects all single bit errors, all two bit errors, all errors with an odd number of bit errors, and all burst errors up to 32-bits long. CRC uses a 32-bit polynomial checksum to test data integrity. A similar process is used in Fibre Channel, Ethernet and other buses.
40. Name some of the SCSI hardware components & their vendors ?
SCSI Hard drives : IBM,Fujitsu, Hitachi
SCSI HBA / Host Bus adaptors : LSI, Adaptec
SCSI Enclosures
SCSI Connectivity cables
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