SCSI Converters Information
SCSI converters enable interoperability between different SCSI bus types. Also known as controllers or host adapters, they facilitate reliable communication between separate interfaces without data throughput losses. The vast difference between the SCSI standards and cabling created the need for versatile adapters that enable connections in a heterogeneous environment. Such settings also include non-SCSI components such as IDE, Parallel ATA, or Serial ATA that are different competing solutions.
Successful facilitate the successful creation of heterogeneous environments to leverage the best of what standards and technologies have to offer while allowing for backward compatibility. This eliminates the need for existing equipment replacement. The SCSI converters are internal (fixed inside the computer system) or external (connecing to devices outside the computer).
The simplest example is an external SCSI to USB converter. All modern external hard drives come in a case with a USB connector. It is wired up through an electronic circuit that links to the hard drive interface. This piece of hardware, starting from the USB connector to the interface, is the converter. A SCSI-1 to SCSI-2 adapter enables joining the older generation version to a newer generation controller; inversely, a SCSI-3 to SCSI-1 adapter links a more recent generation product to an older bus type.
A computer system communicates with a SCSI controller when it needs to write or read from a component. One advantage of this option is the ability to link multiple items using the same data cable, referred to as daisy-chaining. The controller exists on an add-on adapter card that communicates with a PC through the ISA or PCI expansion slots, physically on the mainboard. When the SCSI controller is on the circuit board, the SCSI units are directly attached through connectors available on the mainboard.
SCSI adapters have evolved over time extending data rates and functionality. Specific types include:
- SCSI-1 is the original version using an 8-bit parallel bus. It was designed for data rates of 3.5 and 5 MB per second in asynchronous or synchronous modes. This enabled support for up to 7 elements on the same data bus.
- SCSI-2 introduced Fast SCSI and Wide SCSI variations in 1994. Fast SCSI offered 10 MB per second data rate while using the 8-bit parallel bus. WIDE SCSI used a 16-bit bus and data rates of 20 MB per second. The 16-bit bus extended the capabilities by allowing connection with up to 15 components.
- In 1996, SCSI-3 version of SCSI-2 was released with double the data rates of 20 MB/s for 8-bit and 40 MB/s for 16-bit systems. The 32-bit recommendation presented with SCSI-2 was discontinued in this version.
- In 1997, low-voltage differential (LVD) bus was added to develop the Ultra-2 SCSI unit that enhanced the data rate again to 80 MB/s.
- The Ultra-3, also known as Ultra-160, upped the data rates to 160 MB/s. It introduced error checking using CRC (cyclic redundancy check) and performance management through domain validation.
- In the second quarter of 2002, the Ultra-320 solution reached data rates of 320 MB/s.
- The Ultra-640 was standardized in 2003 and maxed out the data rates for this adapter at 640 MB/s. The Ultra-640 or Fast-320 was limited by hardware to up to two instruments, and manufacturers are now focusing on the Serial Attached SCSI.
- Serial attached SCSI (SAS) protocol is a high-data throughput technology that currently supports up to 12 Gigabit/s under SAS-3.
The SCSI devices have adopted several cabling alternatives over time:
- The SCSI Parallel Interface uses a ribbon cable relying on at least two 50-pin, 68-pin or 80-pin connectors. The hot-pluggable instruments use a single 80-pin connector.
- Exceptional data rate demands of supercomputers and storage area networks employ Fiber Channel supporting rates of up to 16 Gigabit/s.
- The Serial attached SCSI is a modified Serial ATA (Serial AT Attachment or SATA) and is designed for hard drives and tape drive storage media.
- iSCSI (Internet SCSI) relies on networking cables and connectors. The SCSI RDMA (remote direct memory access) protocol (SRP) was developed for generating remote access to data stored on other machines.
- USB Attached SCSI links the tools using a USB cable.
The most frequent use of SCSI is for enhanced availability and redundancy storage and fault tolerance. The SCSI controller can be configured to coordinate data read and write operations across multiple hard disks. The higher levels of RAID (redundant array of independent drives) are leveraged through the multi-unit control capability provided by these products. Furthermore, if hot-swappable or hot-pluggable connectors are deployed in conjunction with specific RAID configurations, an option to switch out the failing drives without powering down the servers or data losses exists.