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Product
All, Premiere
Platform
Macintosh
Last Edited
06/06/2002
Filename
ba02.htm SCSI Chain Troubleshooting Guide
Issue
One of the following occurs:
-- Your Macintosh doesn't start up when a SCSI device (e.g., external hard drive, removable drive, CD-ROM drive, scanner, film recorder) is connected.
-- An external hard drive, removable drive, CD-ROM drive, or other desktop mountable hardware does not show up on the desktop.
-- A scanner, film recorder, or other SCSI connected device does not seem to be connected to the Macintosh, and you are unable to access it using its software.
-- A system error (e.g., Bus Error, ID=1) occurs only when a SCSI device is connected to your Macintosh, and you have determined that software is not the cause of the system error.
-- Random lines appear through scanned TIFF images.
Solutions
Do one or more of the following:
Solution 1
Check the SCSI Devices Number and Connection:
1. Make sure all power is off to the Macintosh and all devices. Failure to do so before changing the SCSI chain can result in voltage spikes that can damage the equipment.
2. Verify that all connectors are fastened tightly, and correctly (i.e., ensure the cable connector's pins are not bent). The first and last devices on a SCSI chain should be terminated. (The Macintosh IIfx requires a special black terminator).
3. Check the SCSI address on the device (usually on the back of the device next to the SCSI port).
No two devices may have the same SCSI ID number. Most devices allow you to choose a SCSI ID number from 1 - 6. SCSI ID number 0 is reserved for the internal hard drive, SCSI ID number 7 is reserved for the Macintosh itself.
4. Ensure all SCSI devices are turned on before starting the Macintosh computer to allow time for the SCSI devices to warm up and be accessible by the time the Macintosh checks the SCSI chain. Some SCSI devices require a system Extension to properly access or mount the SCSI device.
Solution 2
Remove termination.
Solution 3
Use a shorter or longer cable. The longest recommended cable length is 19.6 feet for the entire SCSI chain. Excessive cable length may result in "echoing," or signals not terminating.
Solution 4
Connect one SCSI device at a time. Check for a conflict between each combination of the SCSI devices.
Soution 5
Change the physical order of devices on the SCSI chain in an organized way. For example, move the first device to the end of the chain (changing the termination). If that doesn't eliminate the problem, move the current first device to the end of the chain, and so on.
Solution 6
A SCSI device may be damaged (e.g., loose cable connectors, improper or damaged terminators, excessive SCSI cabling). Remove one of the devices from the SCSI chain. One of the SCSI devices may be allowing static electricity to enter the Macintosh and cause errant behavior.
Solution 7
Move all SCSI devices physically away from each other to ensure electromagnetic fields created by power supplies are not entering the SCSI chain.
Additional Information
SCSI stands for Small Computer System Interface (pronounced "Scuzzy"). The SCSI port is used to connect peripheral devices to a Macintosh. Each device is connected in a series to the Macintosh, called a "daisy chain" or "chain." Apple recommends that the first and last devices are terminated.
The Macintosh looks at SCSI ID #7 first and then goes in order from SCSI ID #5, #4, #3, #2, and #1, with ID #0 being last. Shareware programs like SCSI Probe, SCSI Tools and SCSI Evaluator enable you to view what is connected on the SCSI chain. They are useful programs for making SCSI devices mount. Some SCSI devices do not have a switch for setting the SCSI ID # and are either hard coded with a SCSI ID # or have software that comes with the device for setting the SCSI ID #.
All Macintosh computers since the Macintosh Plus have an Apple standard SCSI port (25 pin to 50 pin wide SCSI interface), Apple PowerBooks use a special SCSI cable called HDI30 (50 pin to 50 pin tiny square connector). Many SCSI devices can be connected together with a standard 50 pin to 50 pin peripheral SCSI cable.
SCSI terminators are plugs that fit into SCSI ports. Their function is to keep signals from echoing back and forth along the SCSI cabling. There will be problems with echoing with a faster Macintosh, increased number of connected SCSI devices, longer cable lengths, and SCSI devices located too near another. Some SCSI devices have a termination switch for terminating the SCSI device. These devices do not need SCSI terminator plugs for termination. SCSI devices that are self terminating must be the last SCSI device on the chain. The Macintosh IIfx requires a special black external terminator at the end of the SCSI chain. No device in the SCSI chain maybe internally terminated, except for the internal hard drive.
Removable hard disk drives and tape drives may not be connected on the same SCSI chain. The SCSI interface only allows for one backup channel. If you connect both only one is likely to work correctly.
To choose the startup disk, select the Startup Disk Control Panels in System 7. In System 6, choose Set Startup under the Special menu.
Communication signals along the SCSI chain are often disrupted by noise. Terminators have resistors placed at either end of the SCSI chain to supress noise. Most Macintosh computers have an internal hard drive that is already actively terminated. Adding SCSI devices requires an additional terminator to be placed at the end of the SCSI chain.
The 100- and 500-series PowerBook models have internal terminators that aren't powered like most desktop Macintosh models, where the terminator is only effective for the internal hard drive termination. Adding external SCSI devices to PowerBooks requires the use of two external terminators instead of one. Many SCSI devices have internal active or "powered" termination that can substitute for the second terminator.
When none of the SCSI devices provide powered termination, a powered terminator must be used. Powered terminators usually have lights to indicate the status of the connection. APS technology has a powered terminator called SCSI Sentry. PowerBook Duos have an active termination (powered) in the docking bay, and don't need a second terminator for connecting to external SCSI devices.
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