Six Generations of SCSI Working Together.

PART 2

This article is the second in a two-part series. The first part appeared in the September issue of CTR.

LVD SCSI devices are generally multimode, which allows them to run in LVD or in SE mode. The SCSI bus or device will switch to SE mode if it detects the Diff Sense line below 0.5 volts. The bus must be capable of running with the single-ended rules at Ultra SCSI speeds. SE devices ground the duff sense line; LVD devices monitor the diff sense line. The enabled terminators on an LVD SCSI bus drive the diff sense line to 1.3 volts. HVD devices pull the diff sense line high. If LVD or multimode devices see the diff sense line pulled above 2.4 volts, they will turn off the transceivers. A HVD device plugged into an LVD bus will shut down the bus. LVD Devices attached to a single-ended bus will switch to single-ended mode and work under the Ultra SCSI rules for cable distance and speed.

LVD or multimode devices do not have termination. The cable system or another device such as a host adapter must provide termination.

HVD Devices

HVD devices have a different connector pin out than LVD or SE and are not electrically compatible with SE or LVD. HVD devices require a HVD host adapter card (which are not sold anymore) or a SE to HVD expander, which cost about the same as a 9GB drive. Using old HVD drives on a system is only practical if a HVD adapter is already in the system.

Wide Devices On A Narrow Bus

A wide device can be used on a narrow bus to pull up the high byte and parity signals or by terminating the high byte and parity bit 1 at the adapter for the wide to narrow bus. If the wide device is used at the end of the narrow bus and the termination is enabled, the high byte will be terminated.

Hot Plug Drives--SCA-2 Connectors

The SCA-2 connector was designed for hot plug backplane systems. These systems allow the disk drive to be hot plugged during system operation. The SCA-2 connectors are normally used for servers, RAID systems, and high reliability systems. This is an 80-pin connector interface with an early ground connection, precharge power pins, power pins, and the signal pins. The backplane position sets the SCSI ID; there are no ID switches on the drives. The drives do not have termpwr or terminators since they can be removed from the backplane of a running system. There are mating pins that notify the system and the drive when the drive has made contact on both ends of the connector.

Many users try to put the SCA-2 drive into PCs where they have adapters with 68 pin connectors. A special adapter is required that supplies power, the ID, and termination when the drive is at the end of the bus. These SCA-2 adapters add a lot of capacitance, doubling the load of the drive. No more than two of these can be used on a system without degrading the signals. If more than two drives are required, a hot swap backplane should be used that does not require the adapters.

The SCSI bus flexibility allows systems to be built with six generations of the high performance drives available today and still support legacy devices and slower devices. SCSI is the connectivity and performance champion of the computer interfaces. No other system bus allows connections inside or outside the box with the range of peripherals, distance, and speed.

Paul D. Aloisi is the applications engineer of Unitrode (Merrimack, NH).