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AMI Elite 1600 SCSI RAID Review

Subject: Storage
Manufacturer: AMI
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SCSI vs. IDE

This content was originally featured on Amdmb.com and has been converted to PC Perspective's website. Some color changes and flaws may appear.

SCSI vs. IDE:


SCSI is an interface that can perform data transfers with no requirement from the host CPU. SCSI is multi-tasking. An initiator can issue a command to a target. The target can then disconnect from the bus to perform the task and free the bus up for another task. This is referred to as Connect/Disconnect. Ultra 160 SCSI can have up to 15 devices connected to the bus (30 for a dual channel) and they can be any SCSI device including hard drives, CD-RW’s, scanners, printers, etc. The number of devices can be substantially increased by Logical Unit Numbers. EIDE can have two internal drives connected. Your PC probably has two EIDE buses, so it may have up to four peripherals. Ultra 160 SCSI allows up to 12 m (40 ft) of cabling, which may be internal or external to the computer. For point to point applications, you may have up to 25 m (82 ft) of cable. EIDE is for internal cabling only and the maximum cable length is only 18 inches. Moreover, do not forget that 160 Mbytes/sec is much faster than any EIDE bus.


Differences between SCSI and IDE:


SCSI is superior to IDE in nearly every application. IDE is less expensive than SCSI. In other words, IDE is probably the best choice in most home applications. If you have a system with compute-intensive or storage intensive applications, SCSI is a better choice.


Logic:


Speed: The latest EIDE systems are nearly as fast as Wide Ultra SCSI for single disk drive applications. However, in systems with a "busy" bus and a multitasking OS, SCSI will win the race. During a data transfer, when data is not actually being moved between devices, SCSI will free the bus for use by other devices. This makes more efficient use of the bus. IDE will hold the connection between the two devices until the transfer is complete. This is a major frustration for IDE users.


Number of Devices: An IDE bus can have two peripherals attached (your PCI bus computer has two IDE buses for four peripherals). U160 can have up to 15 devices on each channel, for up to 30 devices.


Loading of CPU: IDE requires the CPU to manage every aspect of the transfer of data between two devices, so the CPU is tied up while data transfers are taking place. On the other hand, SCSI is an intelligent bus that manages data transfers and does require the intervention of the CPU. Once the task is complete, the SCSI host adapter notifies the CPU. Command queuing in SCSI allows the host adapter to list up to 256 tasks for a peripheral to perform. The peripheral keeps working off each task until the list is completed. These features free the computer for other tasks.


Data integrity: For years, SCSI has had a parity check to minimize the possibility of undetected errors in data transmission. Ultra160 SCSI has added a CRC check that results in a tremendous increase in the ability of SCSI to detect data transmission errors of many different types. Cyclic Redundancy Check is a means of detecting errors that is much more effective than the simple parity check that SCSI has used for years. CRC detects all single bit errors, all two-bit errors, and all errors with an odd number of bit errors, and all burst errors up to 32-bits long. CRC uses a 32-bit polynomial checksum to test data integrity.


SCSI or IDE?


This is best answered by how you use your computer. Generally, home users with limited devices and requirements may be best suited for IDE. Whereas, those of us who demand the best in performance and require the use in a multi-tasking or multi-threaded environment, should opt for SCSI. If you have a single drive and a CD-RW, IDE is most likely better and more cost effective for simple applications. However, your applications are robust such as Adobe PhotoShop or 3D Studio Max, SCSI is a superior choice.






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