I got one of the MacResq refurbished Seagate's SCA 80's to work (finally) in my s-900 by juggling cables through the regular SCSI chain but I was unable to get the adaptor which is supposed to work as well with the Adaptec Ultra Wide LVD card, to communicate with the sixty eight pin connector on the adapter, though I bought the more expensive Ultra 3 SCSI TPO cable and an internal terminator from Cyber-guys, but I didn't get the single ended terminator, not being quite sure what the latter term meant and assured by the salesman that it didn't matter. At Macresq the tech specialist warned me that termination was tricky on the Seagates but I was unable to understand what single ended meant and what pins or jumpers on the Seagate turned it on or off as a single ended drive. Can anyone explain this to me.
First, I will reiterate once more that SCA drives (80 pin drives) are more trouble than they're worth. But there are about a billion of them out there at low prices.
Now on to our regularly scheduled pedantry... Single ended simply means that it is not differential. It refers to the form of the electrical signals sent over the cables and is not descriptive of the wires or cabling.
In the beginning there was single ended SCSI and differential SCSI and only very high end installations used differential so all you needed to know was to avoid differential stuff.
Now days, we have the wonderful LVD which is Low Voltage Differential. U2W SCSI and the later versions (U160, U320) are LVD. Most, if not all, of the U2W drives are capable of operating in Single Ended mode, which means that they adjust their form of electrical signaling to the old style if they find themselves on a SCSI bus where that is what is being used.
SCSI termination and cabling for LVD is quite different from single ended though they appear similar. The single ended 68 pin cable is a just a smooth ribbon cable. The 68 pin LVD cable is that weird looking twisty turny stuff where the pairs are broken out of the ribbon and twisted around each other.
The terminators for single ended (SE) and LVD are also different. However, there's a detail. Most of the LVD terminators will also automagically do SE termination. But the reverse is not true. So, if you get an LVD terminator, you're probably okay, no matter which regime you're operating in, but with an SE terminator, you can only use it on an SE bus. Usually the LVD terminators will have "LVD/SE" embossed on their plastic indicating their dual nature.
Similarly, the LVD cabling seems to work fine on SE busses, but don't try the reverse.
If you're running an SE bus (UW or earlier) then it doesn't matter much which form of cabling you choose. But these days, if you're buying new, you may as well pick up the LVD cabling and termination.
Also note that most U2W and later drives do not have an option to enable on-board termination. They depend on there being a separate terminator installed on the SCSI cable. UW drives and earlier have a jumper which will enable termination on the drive, so that the drive can go at the end of the SCSI cable instead of a termination module. This means that you generally need one more position (plug) on an LVD SCSI cable than you would have needed on an SE SCSI cable.
Jeff Walther
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