Adaptec USB2Xchange User Manual

Contents

25Understanding SCSIYou generally set the SCSI ID on external SCSI devices by setting aswitch on the back of the device. However, for specific instructionson setting the SCSI ID of your devices, refer to the devices’documentation.Terminating the SCSI BusYour SCSI bus is a chain of devices with electrical signals runningback and forth between them. If the SCSI bus is not properlyterminated, any signal travelling down the bus bounces back whenit hits the end of the bus, and interferes with itself. By terminatingboth ends of the SCSI bus, you ensure reliable communication onthe bus by preventing this interference.Termination is permanently enabled on the USB2Xchange adapter,since it is always at one end of the SCSI bus. You must enabletermination on the device at the other end of the SCSI bus anddisable termination on each device connected between the physicalends of the bus.Most SCSI devices come from the factory with terminationenabled. You usually control termination on external SCSI devicesby installing or removing a SCSI terminator module (similar to theone shown in the figure below), or by setting a switch on the backof the device. However, methods for setting termination vary andyou should refer to each SCSI device’s documentation for detailedinstructions.Note: Some SCSI devices require that termination power besupplied to the SCSI bus. The USB2Xchange adapter does notsupply termination power. To use these devices with theUSB2Xchange, a device which does supply termination powermust be added to the SCSI bus.Terminator