The Old Guard In The New Era - SCSI - Technology Information

Computer Technology Review, Dec, 1999 by Marc Brooks

Supporting HVD systems with LVD devices

For years, SCSI has had two defined physical interfaces: Single Ended and Differential, now called HVD (High Voltage Differential) SCSI. Each had their strengths and weaknesses. Whereas single ended SCSI devices were less expensive to make, differential SCSI devices could communicate over longer cables and were less susceptible to external noise influences. Both interfaces carved out their places in the market. The single ended SCSI interface became the interface of choice for internal applications and low-end systems, where price was the deciding factor. The robust nature of differential SCSI and the longer cable lengths associated with it made it the interface to use in applications where SCSI devices were to be external to the computer.

Differential SCSI always had a higher cost associated with it. The 64 milliamp drivers required for differential systems drew too much current to be able to drive the bus with a single chip. Single ended SCSI required only 48 milliamp drivers and could be implemented within a single chip. Most device manufacturers produced single ended versions of their devices before they made differential versions, if they even bothered to make differential versions. All this made differential hard to get and expensive compared with single ended. There is, however, still a demand for differential SCSI, from many mid-range and large systems manufacturers and from those that service the large base of differential SCSI systems in the field. The high cost and low availability of differential SCSI devices also created a market for devices that converted single ended SCSI to differential SCSI so both device types could coexist on the same bus.

Increased SCSI Speeds Required A Change

The SCSI interface, as with the rest of the computer industry, has continually been faced with the issues of increasing speeds and higher capacity. SCSI went from a bus that had 1.5MB data transfers to one that maintained 40MB transfers on wide SCSI, and was under pressure to go faster. Differential SCSI along with its single ended alternative had reached the limits of what would be physically reliable in transfer rates, even though the flexibility of the SCSI protocol allowed for implementing much faster communications. Another problem has been the confusion of having two physical interfaces to choose from in implementing a SCSI system, since single ended and differential devices were not directly compatible with each other.

Low Voltage Differential SCSI (LVD) is the answer to both problems associated with the physical SCSI interface. The 14 milliamp maximum LVD drivers can be implemented within a single chip and have the low cost elements of single ended interfaces. LVD SCSI is also able to drive the bus over distances comparable to differential SCSI. LVD SCSI adds to this by being able to support communications at faster data rates, so SCSI can continue to increase in speed without having to change from the LVD SCSI physical interface. With LVD SCSI, there are now wide Ultra2 speed devices, which have 80MB transfer rates. Products have also been released that support Ultra 160 SCSI, which provides 160MB transfer rates.

The advent of the Multimode implementation of LVD SCSI, which is being called LVD/MSE, allows single ended SCSI applications to smoothly upgrade to LVD SCSI devices. LVD SCSI has such made an impact that the SPI-3 specifications for SCSI do not even define the old differential (HYD SCSI) bus anymore. SPI-2 may have the last complete definition of the differential (HVD SCSI) bus. Almost all hard disk drive manufacturers have discontinued their line of HVD SCSI drives, with many of them standardizing on LVD/MSE drives as their only SCSI interface type. There are still manufacturers of differential HVD SCSI tape drives and other backup devices, but the trend is for the new versions of these devices to also be LVD/MSE in the near future.

Systems based on differential HVD SCSI have no direct way of using the new LVD SCSI interfaces without adding a single ended or LVD SCSI host adapter, which may not be available for that system. This leads to a number of problems for those who are selling, using, maintaining, or upgrading these systems.

Differential Converters Are The Answer

The Differential Converter is a SCSI Expander that is designed to resolve such problems. The current class of differential converters separate the SCSI bus into two segments so one segment is differential and the other segment is LVD/MSE. This allows for adding single ended and/or the new LVD SCSI devices to a differential system.

Example: There is a requirement to upgrade the speed and capacity of a drive package in a differential SCSI based system, but the fastest and highest capacity hard disk drives have only come out in LVD SCSI. The new drive package is added to the system by mounting a differential converter inside the package. This allows the system to access the drive package as if it was a differential one. (Fig 1)