From my notes:
The IIfx introduced a new implementation of Mac SCSI that was faster than previous versions. Unfortunately, IIfx SCSI introduced a set of quirks that are unique to this model. A full description of IIfx SCSI can be found in Apple's Technical Note DV15, wittily named "Why Is The Terminator After Sarah Connor?".
To achieve faster performance, the IIfx requires that special terminators are attached at each end of the SCSI chain. Each IIfx was shipped with the required terminators although these may have been separated from the Mac by now. Externally, you need to disable SCSI termination on all devices and use Apple's Black Terminator II on the final device. A blue filter device was supplied for the internal SCSI bus. If a hard drive is fitted, termination should be set on the drive using resistor packs or jumpers in the normal way. Additionally, the filter is installed between the ribbon cable and the disk drive. If no hard disk is installed, fit the blue filter on the logic board in place of a ribbon cable and insert an inline terminator.
When using modern disks with built-in active termination, you can usually ignore Apple's termination advice for the IIfx. Active termination provides similar functionality to Apple's special filter and terminator. For reliable operation with disks of the same age as the IIfx, you must observe Apple's termination rules.
The IIfx introduced a new implementation of Mac SCSI that was faster than previous versions. Unfortunately, IIfx SCSI introduced a set of quirks that are unique to this model. A full description of IIfx SCSI can be found in Apple's Technical Note DV15, wittily named "Why Is The Terminator After Sarah Connor?".
To achieve faster performance, the IIfx requires that special terminators are attached at each end of the SCSI chain. Each IIfx was shipped with the required terminators although these may have been separated from the Mac by now. Externally, you need to disable SCSI termination on all devices and use Apple's Black Terminator II on the final device. A blue filter device was supplied for the internal SCSI bus. If a hard drive is fitted, termination should be set on the drive using resistor packs or jumpers in the normal way. Additionally, the filter is installed between the ribbon cable and the disk drive. If no hard disk is installed, fit the blue filter on the logic board in place of a ribbon cable and insert an inline terminator.
When using modern disks with built-in active termination, you can usually ignore Apple's termination advice for the IIfx. Active termination provides similar functionality to Apple's special filter and terminator. For reliable operation with disks of the same age as the IIfx, you must observe Apple's termination rules.