I've got a 5420 whose SCSI subsystem blew when I was trying to resurrect a flaky internal CDROM drive, swapping it when the machine was powered up. That's a no-no; these machines aren't designed to handle it. Probably something to do with the active terminator that these things (54/5500s and similar) have on the logic board.

The active terminator is supposed to function automatically, depending on what's connected to the bus and how the devices are configured. The Apple Developer Notes make mention of the presence and function of the thing, though I'm not sure which chip the terminator is, or how to test it. Of course, the Apple Service Manuals only say "replace logic board" when a problem like this occurs.

There could also be a fuse squirreled away somewhere that I couldn't see. Many machines (particularly PowerBooks) had protection for the SCSI bus in case somebody mistakenly plugged a PC parallel device into it or something, but with the cheapness of many of these consumer machines, they may have decided to save the .001 cents required and omitted it.

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Found my stash of developer notes, so more info:

The 5260 uses passive termination, actually, but it's still automatic. The SCSI subsystem is part of the F108 chip, originally introduced in the LC 630 and its siblings. From the Developer Notes for the 5260:

The internal end of the SCSI bus is terminated by a 220/330 passive terminator. Theterminator is located on the main logic board near the portion of the internal chassis

connector that contains the signals for the internal CD-ROM drive.

Click to expand...

Other machines that use passive termination include those based on the Curio chip, such as the 75/85/9500 series.

Models utilizing the O'Hare chip (54/5500 and similar, PowerBook 2400/3400 thru WallStreet) have active termination. From the 5400 dev note:

The internal end of the SCSI bus is terminated by an active terminator. The terminator islocated on the main logic board near the portion of the internal chassis connector that

contains the signals for the internal CD-ROM drive. On enclosures with only one

internal SCSI device located close to the logic board, the active termination is

automatically enabled. On enclosures with multiple SCSI devices, the active termination

is disabled, and a positive terminator is located at the end of the internal bus.

Click to expand...

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