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SCA-SCSI adapter - max speed?

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I have one of these great adapters and it's brilliant, it converts the SCA connector on a modern SCSI hard disk to the 50-pin one found inside most Macs with an internal SCSI bus. 


My question is, if I use this adapter to connect my SCA drive to the 50-pin connector on my Adaptec 2940 Fast & Wide SCSI card, will I get the benefit of faster speeds that this drive is capable of providing?


I connected the drive (an HP 10K rpm 72GB SCA drive) to my beige G3 using the SCA adapter and the internal SCSI bus, it benchmarks at only 5MB/sec. I'm hoping to get much faster performance than that with my SCSI PCI cards. 

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Posted (edited)

I would expect 10MB/sec with the drive connected to the 50 pin port on the model of 2940 you describe.  10MB/sec is the limit for 8-bit (50 pin) Fast/Narrow SCSI.  If you had an Ultra SCSI capable card like the 2940UW, you *might* get up to 20MB/sec if the cabling is adequately screened.  I believe that your 2940 card supports 20MB/sec with high voltage differential (HVD) signalling on its wide bus.  Your more modern SCA hard drive certainly uses low voltage differential (LVD) signalling, so it's not a great idea to connect your drive to the 68 pin connector on your card as malfunctions and damage can occur.  Narrow SCSI uses single ended (SE) signalling, so there's no risk of damage using the 50 pin connector.  If 10MB/sec isn't fast enough, I'd look at a card like a Mac version of the AHA-2940U2W which is Ultra2 and LVD.  Connecting your SCA drive to its 68 pin Ultra2/Wide SCSI internal connector would offer a theoretical data rate of 80MB/sec.


Here's an eBay link to one example of the 2940U2W


Edited by rsolberg
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It goes at the speed of electricity in metal ;)


An SCA adapter is just a physical adapter. There's no electronics at all, with the possible exception of SCA adapters with termination. 8 bit width can run at up to 160 MB/sec, and wide at up to 320 MB/sec, if the controller, the drive and the cabling all support it. Only if the termination is SE (single ended) will the speed will be limited.


An Adaptec 2940 should get 20 MB/sec (ultra SCSI) with a narrow cable or 40 MB/sec (wide ultra SCSI) with a fast drive.

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