From: "Brandon Davis" <[EMAIL PROTECTED]> Date: Fri, 26 Dec 2003 19:55:27 -0800
SCSI 50-Pin M-F Internal Passive Terminator #3449322827 http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=3449322827
Darn, $10. I have some old 2 GB IBM drives with no built-in termination that could use a pass-through terminator like that, but not at a price higher than the drives' worth. :-)
You don't place SCSI terminatation in the "middle" of your device cabling, ever ...for a compact Mac internal drive cabling, you would have end of the cable plugged into the logic board, the next plug into the drive, and the final plug would be the terminator. If you have external drives, you will still have to have [external] termination at the end of your SCSI chain.
During this discussion, I've been wondering if one might need a terminator at the motherboard end of the cable.
As I understand it, on these old machines the motherboard does not supply any termination. Apple relied on the internal SCSI cable being very short to allow termination of only the internal drive to provide sufficient SCSI termination. So, in fact, only one end of the SCSI chain is terminated, absent external drives.
Could the problem be that with newer drives, the newer drive won't tolerate the unterminated MB end of the cable, even with the very short cable? This argument falls apart, if folks have tried this with a properly terminated external SCSI chain connected and it still didn't work.
So, what I'm thinking is that when one does not have any external SCSI devices connected (with the final one terminated) perhaps one should try plugging a DB25 terminator into the SCSI port on the back of the Mac and see if that solves the problem. Naturally, one would also set termination on the internal drive to 'On'.
Some illustrations:
T: and :T are SCSI termination D is a SCSI device T:D and D:T are a device with termination installed or activated. MB is the motherboard ----- is internal SCSI cabling ===== is external SCSI cabling.
Original Mac config. with no external chain:
T:D----MB Only one end termination, but short cable.
Brandon's suggestion:
T:---D---MB Not relying on drive termination, still only one end term.
Original Mac configuration with external chain:
T:D---MB====D:T Both ends terminated
My proposal:
T:D---MB:T Both ends terminated.
If the internal drives under discussion truly have faulty on-board termination, then this wouldn't help. But if the problem is that the MB end of the SCSI chain is left flapping in the breeze, then a DB25 terminator on the outside of the Mac would solve the problem. And it has the advantage that you must remove it if you install an external SCSI cable, so there's no danger of leaving SCSI termination in the middle of the SCSI cable.
I find this scheme useful with the NuBus Jackhammer card because it has actual termination resistors on the card. It's a pain to open the machine to move the resistors if one is often removing and installing an external segment to the SCSI chain, so I just leave a 68 pin terminator on the external port of the JackHammer and remove it when I install an external device.
Jeff Walther
Jeff Walther
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