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When you mix SCSI drive types, Single Ended (SE) and Low Voltage Differential (LVD), on the same SCSI bus, several problems can occur. The symptoms may include, but are not limited to, the system not starting, data corruption, blue screens, and drives becoming unstable.
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.
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:
Collapse this tableExpand this table
To avoid these problems, when possible, put all SE devices on one SCSI bus, and all LVD devices on a separate SCSI bus. This allows all devices to run at their optimum performance, and prevents any issues with cable lengths, and termination. This is also true for Wide and Narrow devices where improper termination can cause difficulties.
Best PracticesVerify that you have the latest drivers and firmware for your adapter. This is especially true if you are having problems with the adapter from the beginning. In some cases, the driver and firmware will need to be certain versions to work together. You need to check with the original equipment manufacturer.
Always read the user's guide, and release notes before you install an SCSI device. It may also be good to check the manufacturer's Web site for the most current information. This will let you know of any issues that the manufacturer has discovered. This is especially true of Hardware Raid adapters, which are more sensitive to certain configurations, and certain hardware.
Always low-level format all hard drives with the SCSI adapter with which they are going to run, this includes new, moved, or questionable hard drives. In some cases, this may also be necessary after upgrading the firmware for the SCSI adapter.
SCSI adapters can have single, or multiple channels. A single channel adapter will have three sockets, but only two of them can be used at a time, or the bus will not be terminated correctly. Using all three may work for a time, but eventually corruption or a heavy load will cause the hard disk to fail. If you are not sure if this is a single or dual channel adapter, check the documentation.
For additional information, click the article numbers below to view the articles in the Microsoft Knowledge Base:
(http://support.microsoft.com/kb/168094/EN-US/ )Problems Attaching SCSI-2 Devices to SCSI-3 Adapter
(http://support.microsoft.com/kb/141832/EN-US/ )How to Change the Boot Disk Order with Multiple SCSI Adapters
Article ID: 285013 - Last Review: November 1, 2006 - Revision: 1.2