· Q. 11. 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.
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.
· Q. 12. 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).
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).
· Q. 13. 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.
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.
· Q. 14. 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.
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.
· Q. 15. 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.
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.
· Q. 16. 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.
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.
· Q. 17. 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.
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.
· Q. 18. 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.
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.
· Q. 19. 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.
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.
· Q. 20. 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.
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.
§ Overall, at this time, FC has a place in large scale storage and backup systems but there is almost no reason for the home user and very little reason for the small business to consider FC.
· Q. 21. 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.
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.
· Q. 22. 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.
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)
· Q. 23. 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.
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.
· Q. 24. 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.
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.
· Q. 25. 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. (Cntd...)
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. (Cntd...)
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