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SCSI Printer Prevents System Boot

Q: Our printer uses SCSI as one of its interfaces and operates normally with all interfaces except SCSI. When the printer is connected to a PowerMac 7100 using the SCSI interface, the computer won't boot. If we turn the printer off, the Happy Mac shows up on the screen, and the Power Mac boots normally. How do we troubleshoot this problem?

A: The most likely causes are termination and bus-ID conflicts. If your device is using the same bus ID as some other SCSI device on the chain, it can prevent the boot sequence from preceding. This problem could also occur if your device has a permanent hard-wired terminator and there is another terminator on the bus.

There is one other possibility that relates to the Macintosh boot sequence. When the Mac bootstrap sees a device, it tries to read block zero. If your printer has a long warm-up period before it can fully respond to a read request, it may be stalling the bootstrap. You can check this by booting the Macintosh and waiting to see if the problem clears itself when the printer is fully initialized.

The best way to troubleshoot the problem is to use a SCSI-bus analyzer, so you can see the actual bootstrap sequence.

[Jul 01 1995]

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