CD-ROM Tower SCSI ID Switches

Kenn's
Tech
Notes

19980408: CD-ROM Tower SCSI ID Switches

April/1998
Tech Note #8

When assembling a CD-ROM tower, an essential component of the process is connecting the SCSI ID switches on the back of the case to the individual drives. Every SCSI device has 3 (or 4, in the case of wide SCSI) address jumpers, usually labelled A0, A1, A2. When jumpered, they add 1, 2, and 4, respectively to the SCSI ID of the device. In a CD-ROM tower, it is convenient to be able to change the SCSI IDs of the drives without having to open the case. Most towers have ID switches mounted on the back of the case. Inside, these switches connect to the drives' ID pins.

There is a potential problem when first connecting these switches. Although obviously three pairs of pins requires six connections, most towers' ID switches have only four wires. This is because a common ground connection is used for the three individual switches. Likewise, the three sets of pins on the drive also share a common ground connection. What this means is that, if you connect the pins which are common on the switches to the pins which are not common on the drive, you are shorting out all the SCSI ID pins! The drive will not function, and you will get incorrect device enumeration on your SCSI bus.

If you have a multimeter, you can easily establish which pins are common on the drive and on the switch by using the continuity function. If you don't have a meter, you will have to use trial and error.

Once the switches are correctly connected, ensure each device has a unique SCSI ID (the SCSI adapter usually uses ID 7). Remember that the device at each end of the bus must be terminated, and all devices in between must have termination disabled.

DISCLAIMER: This document is intended for the reference of computer support personnel within Winnipeg School Division No. 1. There is no warranty or liability if procedures recommended here have an adverse affect on any systems. Use them at your own risk. Any trademarks mentioned are the property of their owners, none of whom have certified any information provided here. Opinions expressed here are personal only and do not represent the policy of Winnipeg School Division No. 1 or any other organization anywhere.

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