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SCSI Drives - The Exception

Building Your Own PC, Part 1: Know-How for Do-It-Yourselfers
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Although the SCSI (Small Computer Systems Interface) bus system offers greater flexibility, it's also much more expensive. SCSI is only used for workstations and servers. Ultra2 or Ultra 160 SCSIs are typical standards. A SCSI ribbon cable has 68 pins. All SCSI standards have one thing in common: you can run at least seven drives on one adapter. "Wide" models even allow 14 devices to be operated.

It is important to know how it works. SCSI is an open bus system and allows cable lengths of well over a meter. However, the bus must be closed with a terminal resistor at each end, so that the signals don't reflect. Termination can mostly be activated by a jumper on the last device. LVD cables have their terminator as a plug-on module. The position of the individual devices on the SCSI cable, by the way, is up to you. The drives are distinguished by way of so-called SCSI Ids that run from 0 to 7 or 0 to 15. ID7 is usually the host adapter, 0 or 1 is usually used for the hard drive(s). The rest of the configuration is up to you. Jumpers are used to define the ID address from 0 to 7. In the following example, the manufacturer has named its SCSI address IDs DAS0 to DAS3.

Description of the SCSI jumper block for auxiliary connectors. DAS0 to DAS3 are the SCSI address bits.

Jumper table for setting addresses.


Connector blocks on an SCSI hard drive: power supply, jumper blocks (auxiliary connector), SCSI ribbon cable (from left to right).

In this example, termination can be activated by bridging pins 9 and 10. This is called "Enable SE SCSI Terminator" in the picture.

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