Sun is now selling and supporting SCSI Low Voltage Differential devices and controllers, otherwise known as LVD. Examples include Sun's PCI Dual Ultra3 SCSI Host Adapter (X6758), Dual FastEthernet + Dual SCSI PCI Adapter (X2222), Sun StorEdge[tm] S1 Array, and the Sun StorEdge[tm] D2 Array. For that reason, we need to know what SCSI Low Voltage Differential is, and how it differs from Single Ended, S/E, and High Voltage Differential, HVD.

Low Voltage Differential, first introduced with Ultra2 SCSI, supports transfer rates of 80 Mbytes/sec in WIDE mode, and 40 Mbytes/sec in NARROW mode, across a bus length of 12 meters. Ultra3 SCSI (Ultra160) doubles the transfer rate of Ultra2 SCSI, and Ultra320 doubles it yet again, while still allowing 12 meter bus length with up to 16 devices, including the controller. LVD is compatible with single ended scsi devices, but with limitations. If a single-ended device is added to an LVD bus, the LVD controller switches to Single-Ended Ultra SCSI mode, with a maximum transfer rate of 40 Mbytes/sec in wide mode, with a maximum bus length of only 1.5 to 3 meters, depending on the number of devices attached.

High Voltage Differential, or HVD, currently has a maximum transfer rate of 40 Mbytes/sec in Ultra-Wide mode, across a bus length of 25 meters (Note: maximum cable length that sun makes is 12 meters). HVD is incompatible with both single-ended and LVD devices. Many Sun differential controllers and storage devices currently being sold are HVD. A good rule of thumb is, if it is labeled differential, but does not say low or high voltage, it is most likely HVD. When in doubt, check the device documentation.

Single-Ended, or S/E SCSI, supports a maximum transfer rate of 40 Mbytes/sec in Ultra-Wide mode, across a bus length of 1.5 to 3 meters, depending on number of attached devices. Sun's implementation of Ultra-Wide S/E supports cables up to 2 meters.

LVD SCSI Controller Termination Options

There are three types of terminators that can be used on a LVD bus, each with their own advantages / limitations:

LVD only termination: LVD only termination will only work in LVD mode. If you put any SE devices or adapters on your bus, it will no longer work. LVD only termination is good for when you want to make sure the bus will only run in LVD mode.

LVD/SE termination: LVD/SE termination will allow the bus to be scanned and if any SE devices are found, the entire bus will revert to SE mode. This type of terminator offers the greatest flexibility because if you need to use SE devices on the bus, you can temporarily revert the bus to SE mode for emergencies. However in this case SE mode cable length and bus speed limitations will apply.

Active SE termination: You can run your LVD/SE devices in SE mode, which means that regular SE termination and cabling will work for your LVD/SE devices, but you have to adhere to the cable length & bus speed limitations of SE. If you want LVD mode, Active SE termination will not work.

Will an LVD disk or tape work with a Sun Single-Ended scsi controller card?

In theory, yes. LVD devices have circuitry that can sense a single-ended controller, and should configure themselves accordingly. However, the LVD device will be restricted to the maximum transfer rate and cable length of the single-ended controller. A single-ended terminator is required in this configuration. As always with third party hardware, the manufacturer or seller of the LVD device is responsible for testing and supporting their product in Sun configurations.

Will an LVD disk or tape work with a Sun Differential (HVD) SCSI card?

No. High Voltage Differential is not compatible with LVD or Single-Ended devices.

Can a third party LVD controller be used in a Sun system?

Again, in theory, yes. At a minimum, the card must support Sun's Open Firmware standard, and the card manufacturer or distributor must supply a software driver that is compatible with the Solaris version to be used. Beyond that, the manufacturer or seller of the LVD controller is responsible for testing and supporting their product in Sun configurations.

External sources of information used to research this infodoc:

http://www.adaptec.com/worldwide/product/markeditorial.html?cat=%2fTechnology%2fSCSI&prodkey=ultra160_vs_ultra2_wp&type=Technology

http://www.seagate.com/support/kb/disc/ultra2.html

Email address: thomas.quatrale@Sun.com