Random Problems Encountered When Mixing SE and LVD SCSI Standards (285013)

CAUSE

These problems are caused by the changes made to the SCSI standard, starting with SCSI-3. Up to, and including SCSI-3, SE drives are known as Fast, Wide, FastWide, Ultra, and UltraWide. Starting with SCSI-3, LVD was added to the standard. These two standards take their names from the way they transfer data on the SCSI bus. An SE device transfers data by sending a single data bit on a single wire. An LVD device reads the difference in voltage between two wires for each bit that is sent. This makes LVD less prone to cross talk, and allows for a longer SCSI bus at higher data rates. The LVD standard is being implemented to allow for both SE, and LVD devices to work together on the same SCSI bus, at the SE standard, to allow backwards compatibility. This implementation is totally dependent on the ability of the LVD device to autosense the line, and to run as an SE device. Some of the early LVD devices are truly LVD only, but these devices are rare.

Note: Although UltraWide (UW) and LVD (U2W) both fall under the SCSI-3 standard, there is a large difference in the way SE and Differential function. When SE and LVD SCSI are mixed on the same SCSI bus, the specifications for the bus are in SE. This is true even if the bus is currently running as Differential, and then you add an SE device. This causes the bus to revert to the SE standard. This is especially important for termination, and bus length, where an LVD bus can be as long as 12 meters, and an UltraWide bus can only be 1.5 meters. Some manufacturers make an Active Terminator, which will autosense, and adjust the terminator for the correct impedance.

Single Ended (SE) SCSI
In addition to consideration for SE and LVD devices, you should take care in situations where mixing narrow and wide devices occurs. See Table 1.1 for data rates, and cable requirements. A Narrow device runs at 8 bits where a Wide device runs at 16 bits. If you use a Narrow device to terminate a Wide SCSI bus, only the Lower 8 bits will be terminated, and the upper 8 bits will not be terminated, which causes corruption and inconsistent behavior of SCSI devices. A Wide SCSI bus must be terminated with a Wide Active Terminator. This can be a separate terminator at the end of the bus, or a terminated hard drive. Active SE Termination has an impedance of 90 +/- 6 Ohms.

LVD SCSI
Ultra2 SCSI, also known as LVD (Low Voltage Differential) SCSI, uses the Differential standard for cable lengths and termination. On an LVD bus, the drives generally do not include a terminator, and adding an Active Terminator to the end of the SCSI cable terminates the bus. Again, remember that adding an SE device to an LVD SCSI bus causes the bus to revert to the SE standard. Active LVD termination has an Impedance of 125 +/- 10 Ohms.

Ultra 160 (Ultra3)
All SCSI-3 Standards beyond LVD will only be Wide, but will follow the LVD standard for cable length and termination.

Note: It is important to verify that your SCSI configuration adheres to the following industry standards for SCSI hard disks:

Table 1.1

Definitions

• Standard: The name of the SCSI standard as defined by the American National Standards Institute (ANSI).
• Bit Width: The number of bits that are transferred by the SCSI bus during the data transfer phase.
• Max Transfer Rate (MB/sec): The number of bits transferred over the SCSI bus in one second.
• Pin Count: The number of pins in the cable.
• Max SCSI Devices: The maximum number of devices that you can connect to the SCSI bus with one host adapter installed.

Standard	Bus Width	Max Transfer Rate	Max Bus Length (Meters)	Pin Count	Bus Speed	Max SCSI Devices	Description	Termination
SCSI-1 Narrow	8	5	6	50	5 Mhz	8	Synchronous	SE 90 +/- 6 Ohms Low 8-bits
SCSI-2 Fast	8	10	3	50	10 Mhz	8	Fast	SE 90 +/- 6 Ohms Low 8-bits
SCSI-2 Fast Wide	16	20	3	50+68	10 Mhz	8	Fast / Wide with two cables	SE 90 +/- 6 Ohms
SCSI-3 Ultra	8	20	3	50	20 Mhz	8	Fast	SE 90 +/- 6 Ohms Low 8-bits
SCSI-3 Ultra Wide	16	40	1.5	68	20 Mhz	16	Fast / Wide with one cable	SE 90 +/- 6 Ohms
SCSI-3 Ultra2 (LVD)	8	40	12	68+68	40 Mhz	32	Fast / Wide	LVD (Differential) 125 +/- 10 Ohms
SCSI-3 Ultra2 Wide (LVD)	16	80	12	68+68	40 Mhz	32	Fast / Wide	LVD (Differential) 125 +/- 10 Ohms
SCSI-3 Ultra 160 (Wide)	16	160	12	68+68	40 Mhz	32	Fast / Wide	LVD (Differential) 125 +/- 10 Ohms