Bring Legacy Storage Interfaces into the Modern World

Storage bridging and new drive technologies can extend the lifetime of storage system interfaces, increase reliability and add security features while reducing system size, weight and power.

Just gimme that old-time SCSI interface. Such is the call from many folks in the embedded industry who have very reliable—and heavily relied upon—systems that still use a SCSI interface and drive technology. Ranging in applications from ship-board weapons control to semiconductor manufacturing, thousands of systems designed years ago, and in operation today, still rely on SCSI hard drives for their storage subsystems and will continue to do so for the foreseeable future.

The problem is that SCSI drive supplies are drying up. More and more hard drive manufacturers are dropping out of the business so the availability horizon is uncertain. Making investments to improve drive reliability and performance—improvements that would be beneficial even in older systems—are not always applied to a technology that is widely viewed as past its prime.

SCSI, small computer system interface, is a standard describing the physical and electrical characteristics for a parallel-attached, multi-drop, computer-to-computer or computer-to-peripheral interface. Though originally intended for connecting many types of peripheral devices, including printers and scanners, the interface has evolved over time to be almost exclusively applied to storage devices. With the architectural shift from parallel to serial I/O in embedded systems, any device that is natively connected via parallel interface is on the down side of the availability slope.

Coping with Change

In the age of shrinking budgets, greater system performance expectations and reduced downtime requirements, cost-efficient solutions are in high demand. Making the changeover from SCSI to SATA, or even PATA, drives entails hardware redesign to accommodate the new interface and its accompanying connector and form factor types.

SCSI connector pin counts range from 50 to 80, while SATA connectors contain just 7 pins, forcing board design changes to accommodate the new connector. Many currently deployed SCSI drives are 3.5 inch while SATA drives with few exceptions are 2.5 inch or smaller.

When every dollar is measured no matter the application, buying a boat load of drives for stock pile as a hedge against impending scarcity is often not an option. The real heartache occurs, however, when changes to the operating system and device drivers necessary to support the new host interface are considered because new hardware often means new software.

Software changes carry system-wide implications that impact other processes and communication subsystems that end users simply don’t want to touch. As a result there is an emerging need for seamless methods that bridge legacy systems to new storage technologies without fixing what isn’t broke and with only the slightest impact to the budget.

Conquering the Great Divide

Bridging solutions allow the use of modern storage drives to be connected via the legacy host storage interface using form, fit and functional board-level replacement products designed to insert into specific applications. SCSI-to-SATA bridges form elegant solutions that present the legacy interface to the host, while masking any indication of the underlying SATA technology and thereby eliminating the need for software changes.

As you may guess, many legacy systems are Eurocard-based and for the most part specifically VME or its derivatives (with the exception of VPX) along with some cPCI. Bridging solutions can be applied to any board size, however, and modular solutions make it easy to adapt bridges mechanically and electrically for fast development cycles.

Solutions have been developed that use current devices such as SSDs, compact flash and CFAST media to replace large rotating hard drives, and tape drives that support removable cartridges. Board-level storage product suppliers must cover a short—but very important¬—list of considerations when solving a legacy storage bridging problem. (See Table 1.)

Table 1. Considerations when solving a legacy storage bridging problem.

Creative Designs Ease the Transition

Embedded board designs aimed at solving this challenge use advanced bridging devices to create custom solutions that address it in several ways. Relatively simple designs consist of dual SCSI interfaces with non-removable drives. An example is a board that supports one wide SCSI-to-SATA and one Ultra320 SCSI-to-SATA II connection captured on a VME board where the original design consists of dual straight SCSI connectivity. (See Figure 1.)

Figure 1. Dual SCSI interfaces support one wide SCSI-to-SATA and one Ultra320 SCSI-to-SATA II connection on a VME board where the original design consists of dual straight SCSI connectivity.

More advanced conversion challenges include interfaces where the legacy hardware includes not only SCSI hard drives but also removable media like DAT drives attached via SCSI. Where removability is a carry-forward requirement, a SCSI-to-ATAPI bridging solution is necessary. ATA Packet Interface is a protocol that transports packetized SCSI commands and also carries additional capabilities including a media eject command.

DAT tape drives, much like rotating hard drives, are inherently failure-prone in relation to rugged and lightweight solid state alternatives, so given the chance, users should opt for making the solid state transition. Considerations in this case must be given to the data offload method, since removable drives must be compatible with hardware at the data offload point. (See Figure 2.)

Figure 2. Removable drives must be compatible with hardware at the data offload point.

Many older deployed systems will remain unchanged throughout their usable life and so a fixed bridging solution—such as those described above—will do the trick. But in cases where a transition to a new host system is planned, it may be beneficial and cost-effective to address the needs of both the old and the new systems in a single configurable manner.

Moving to a new system architecture is a staged and carefully managed transition that may require a significant period of overlap where both system types are deployed and supported for years. To cover this transition period, creative board designers make use of simple switching devices embedded on the doorstep of the drive that enable either a pass through of the SCSI command set or the SATA command set.

Front or rear panels would include I/O connectors for both the legacy interface and for the new SATA interface, allowing the single design to be applicable to both systems. In each case described above, custom transition modules can be designed to support rear I/O where the need arises. The dual-purpose board may be deployed as part of either the legacy system or the new system, reducing spares requirements with support for both interface options.

Interface conversion applies not only to cases involving SCSI-to-SATA but also SCSI-to-PATA and finally PATA-to-SATA and the basic framework of the solutions remains similar with the exception of the specific bridge device used. SCSI-to-PATA bridging solutions although easy to implement, are not recommended since PATA drives, like SCSI and other parallel I/O devices, are also on the downside of the usage slope with fewer and fewer suppliers in the business.

Interface speed mismatches are inevitable, with the overriding requirement being that the bridged solution does not carry with it a reduction in data transfer rates. Many older storage subsystems support data rates lower than those attainable using today’s interfaces and drives, so data-flow constraints are determined by the legacy interface. Currently embedded storage products make use of Ultra 320 SCSI-to-SATA II bridge adapter solutions, which match up theoretical bandwidths at around 3 Gb/sec…enough bandwidth to support most entrenched systems.

A Chance for Improved Performance

Beyond resolving the immediate need to keep your system obsolescence-proof into the future, there are benefits that you may take advantage of when upgrading to the latest drive technology.

Rotating hard drives make up the vast majority of all drives found in older systems. They also have the dubious distinction of being the only wear item, the least reliable and the most delicate component in the system, accounting for most of the failures and repair costs in deployed systems. With that in mind, any transition in drive technology is also the chance to move to more rugged solid state drives, thereby increasing overall system mean time between failure (MTBF) and improving long-term repair costs and environmental toughness.

Though more expensive—low-cost MLC solid state drives cost around eight times more than rotating hard drives on a per gigabyte basis—total cost of ownership approaches parity when considering improvements in equipment up time and reduced replacement costs.

Data security is more important today as compared to when most legacy equipment went into service. In addressing that new reality, solid state drives now offer multiple levels of secure erasure plus write protection and data encryption and these options can easily be accommodated when designing replacement boards. Power-hungry rotating hard drives dissipate roughly three times the wattage as SSDs and weigh up to eight times more.

Since many legacy SCSI systems use 3.5 inch drives, another way to reduce weight results from transitioning to 2.5 inch drives—either rotating or solid state. In this case a complete solution would consider any mechanical mounting changes necessary to support the smaller drive form factor as it is nested on the board.


Storage solutions can be replaced transparently, while maintaining backwards compatibility with operating system software and associated drivers. Implementing storage bridging technology and replacing SCSI and PATA drives with cutting-edge SATA drives can extend the lifetime of the storage system interface by five or more years.

End users will find additional immediate benefits in making the jump to SATA drives by taking advantage of the opportunity to move to more reliable solid state versions and adding security features while possibly reducing system size, weight and power. Embedded storage manufacturers, such as Elma, with a long history of innovative designs spanning multiple interface variants are in the best position to address legacy storage configuration requirements.

For more information please contact

SSteve Gudknecht is product marketing manager at Elma Electronic. He has held positions in field applications and marketing in high technology industries for nearly three decades. Steve’s responsibilities include product development, product marketing, training and sales support.

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