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Siemens Industry Online Support
Siemens AG
Entry type: FAQ Entry ID: 24508350, Entry date: 01/31/2007
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Which hard disks are used in SIMATIC PCs and what are the differences between SCSI and ATA hard disks?

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Configuration Notes:
Both 2.5" and 3.5" hard disks with memory capacities of over 40 GB are used in the SIMATIC PCs. Here both parallel ATA hard disks (PATA) and increasingly serial ATA hard disks (SATA) are used. These have a higher data transfer rate that the PATA hard disks and the slim, shielded SATA cables provide much greater security than the broad, unshielded PATA cable.

Differences between SCSI and ATA hard disks:
SCSI hard disks have a specified MTBF value that is up to 50% higher than ATA hard disks and thus seem even more reliable. However, you should note that SCSI hard disks are four times higher and the SCSI controller required swallows more costs and space on the motherboard and can cause loss of performance. Apart from this, the MTBF of SCSI hard disks is specified at lower operating temperatures than for PATA/SATA hard disks. So the MTBF value of an SCSI hard disk of 1 million hours at an operating temperature of 50°C drops to 550,000 hours at 60°C and those comes very close to the MTBF value of an ATA hard disk (400,000 hours at 60°C). SCSI hard disks are usually implemented in the IT/server segment where robustness and temperature requirements are considerably lower than in the industrial segment.

Keywords:
MTBF (Mean Time Between Failure)

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