Apple's new SCSI terminators for the Mac IIfx have bewildered a number of new owners who have attempted to daisy chain SCSI devices from their machines. The new terminators work with the IIfx's new SCSI DMA controller to provide high speed data transfer between SCSI devices and the 68030 chip. Only A/UX takes advantage of this controller now, though future versions of the MacOS and future, faster SCSI devices (up to 3 megabytes per second) will use the controller to increase the data transfer speed of the IIfx.
For those of you unfamiliar with SCSI chain theory, one terminator must be located before the first device and one must be located directly before or directly after the last device. Most Macintosh manuals provide illustrations of SCSI chains, although at least one person on Usenet was unable to find a helpful discussion of SCSI termination for the IIfx in his manuals. There are an unknown number of exceptions to the rules (possibly caused by an incomplete SCSI standard or by manufacturers' lack of compliance), so be prepared to experiment when putting together a long SCSI chain. A good stiff drink probably wouldn't hurt either.
The terminator for the first end of the IIfx chain is inside the Mac. The terminator (50-pin) for the end of the chain comes loose in the IIfx box (and must be used at the cost of an incredible speed drain or possibly damaging your computer. IIfx owners having SCSI devices with internal terminators must remove the them. Easy so far, eh? But what happens when you already have an Ehman Syquest drive that sports a 25-pin SCSI connector similar to the ones on the back of the Mac? We have heard two suggestions from different sources. Shane at Ehman suggested sandwiching the terminator between 25-to-50-pin cables to place the termination directly before the last device. Alternately, a LaCie rep suggested attaching a 25-to-50-pin cable to the open port of the last SCSI device and letting the terminator hang off the cable at the 50-pin end. Evidently Ehman will be switching to standard 50-pin connectors soon.
And then there's the question about moving drives between IIfx's and older Macs. I asked the folks on AppleLink about this and was assured that IIfx terminators can be used on older Macs. Good thing. We're looking forward to SCSI-2 because it will be a more rigorous standard and should help to eliminate these troubles.
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MacUser -- Aug-90, pg. 243