How to Connect SCSI Devices to a Mac

SCSI is one method that peripheral devices, such as disk drives and scanners, can use to communicate with your computer. (Other methods include USB.) To connect a SCSI device to your computer, you need a SCSI port. All older Macintoshes have built-in SCSI ports. Most newer Macintoshes do not come with SCSI ports; they are optional on the higher-end machines. You can add a SCSI card to a Macintosh that uses the PCI bus (not covered here).

Instructions

Difficulty: Moderate

Things You’ll Need:

• SCSI Terminators

1. Install the SCSIProbe control panel if you don't already have it in your system.
2. Shut down the computer.
3. Connect the SCSI cable to the SCSI port on your computer (each end of the cable may be different). Securely tighten hand screws.
4. Connect the other end of the cable to the input SCSI port on your SCSI device. Securely tighten hand screws or fasten clips.
5. Connect the cable for a second SCSI device to the output SCSI port on the first SCSI device. Repeat these steps for up to seven devices.
6. Put a SCSI terminator on the last device in the SCSI chain (unless the device has built-in termination).
7. On each SCSI device, look for some sort of box with a number on it, usually on the back of the device, near the ports. You can change the number, usually by pressing a button near the number box. Make sure each device in the SCSI chain has a unique number.
8. Connect power cords for the SCSI devices to a surge suppressor.
9. Turn on the devices.
10. Insert removable media into any SCSI devices that use removable media.
11. Turn on the computer. Your devices should appear on the desktop.
12. Open SCSIProbe.
13. If a device doesn't appear on the desktop, select it in SCSIProbe and click Mount.

Tips & Warnings

• SCSIProbe is shareware that you can download from the Internet.
• Your computer's hard disk is usually assigned the SCSI number 0 (zero), and an internal CD-ROM drive is usually assigned the number 3. Don't assign your external devices these SCSI numbers. You can open SCSIProbe to check the numbers of any internal SCSI devices before you set up the chain.
• If you have a removable media drive, such as a Zip drive, put it at the end of the SCSI chain if possible.
• You can connect up to seven external SCSI devices to your Macintosh, but the total length of cables cannot exceed 18 feet.
• The most common problem in attaching SCSI devices is not assigning a unique number to each device in the chain. If you don't, your computer may behave erratically, or might not even start up. Shut down the computer, reassign any duplicate numbers, and start it up again.
• The other common problem is in not terminating the chain properly. The output port on the last device in a chain must have a terminator, but no other device can have one. Some SCSI devices are self-terminating. These have to be last in the chain. Check the documentation that came with your device.

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