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SCSI question

S

Spleeze

Limp Gawd
Joined
Oct 15, 2003
Messages
433
Can anyone explain to me explicitly when and where I would need a SCSI terminator? I have a mobo with built-in adaptec U160 SCSI and I just bought an old 18 gig quantum atlas III to use with it, what sort of configuration would i need to ensure that I won't get read/write errors? (that's what the terminators prevent, right?)

If i have one drive and one cable can i place the drive at the end of the cable and then I'll be fine? I dont need terminators on the connectors between the drive and the cable do I?

If there is a nice SCSI faq or information resource out there I'd appreciate that too, I'm kinda new to using SCSI...
 
depending on the controller and the length of the SCSI cable, a terminator may not be required.
For example, on one of my setups, I had 2 drives and a short cable on an Adaptec 2940UW.
I changed cases and needed a longer cable and then I had issues with the controller not seeing the drives, found an active terminator, plugged it to the end of the cable and all was fine.
 
On any SCSI bus, the terminator must be at the end of the cable. Sometimes you can get away with otherwise if you're running, say, one drive on a really short cable, but that's still out of spec.
For low voltage differential (Ultra2/Ultra80, Ultra 160, and Ultra 320), there is normally a terminator built into the cable. Only single ended drives have built in terminators, and the fastest single ended SCSI is 40MB/s wide ultra scsi. If it doesn't have a terminator on it, it may not be an LVD cable. Also, LVD cables generally are made of twisted pairs rather than a totally flat ribbon cable.
If your cable has a terminator on it, just use that. If not and your drive has a terminator you can set it to on and put the drive at the end of the cable. Then if you've got an LVD drive w/ no terminator & the cable doesn't have one either you'll need to pick up an LVD terminator. Personally I'd just get the whole cable. Unless you get a long one the cable isn't much more than just a terminator. Stand alone internal ribbon cable terminators are sort of an odd item and are priced accordingly.
Another thing to remember is that if you have one device that isn't LVD on the bus, all devices on the same bus will be forced to run in single ended mode at a maximum speed of 40MB/s.
 
Indeed, I myself have run a single MAP3734NP without termination on a very short cable, but only in the interim while I was waiting on a new cable. It *might* work, but for the peace of mind there's no reason not to terminate after even a single drive. Weird SCSI voodoo detection issues and corruption are no fun :(
 
Vertigo Acid said:
Indeed, I myself have run a single MAP3734NP without termination on a very short cable, but only in the interim while I was waiting on a new cable. It *might* work, but for the peace of mind there's no reason not to terminate after even a single drive. Weird SCSI voodoo detection issues and corruption are no fun :(
Click to expand...

My sentiments exactly. Theres really no reason to take the chance, because if it doesn't work, major headaches may ensue.
 
so i can assume that the drives themselves have terminators built in, so if i have a cable with 2 connectors and i place 2 drives on the cable, i dont need a terminator, right?

what about a situation like this:

untitled0pv.jpg


would i need terminators on those unused connectors? it seems kinda silly to me if i do, and it seems kinda silly to not put a single drive on the end of the cable anyhow... (eliminating the need for terminators, if i'm correct)...
 
Spleeze said:
so i can assume that the drives themselves have terminators built in, so if i have a cable with 2 connectors and i place 2 drives on the cable, i dont need a terminator, right?

what about a situation like this:

http://img324.imageshack.us/img324/3155/untitled0pv.jpg

would i need terminators on those unused connectors? it seems kinda silly to me if i do, and it seems kinda silly to not put a single drive on the end of the cable anyhow... (eliminating the need for terminators, if i'm correct)...
Click to expand...


No the terminator goes on the end of the cable not the unused connectors.


The terminator is a series a resistors that prevents the signals in the wires from bouncing off the end of the cable. It is important that you use a terminator or you can have problems that are often not reproducable and hard to pin point.
 
thanks for the links i'll check them out

so i'm right in assuming that a drive at the end of a cable is sufficient for termination?

why not always make sure that you put drives on a cable from the end toward the mobo if that eliminates the need for terminators?
 
Spleeze said:
thanks for the links i'll check them out

so i'm right in assuming that a drive at the end of a cable is sufficient for termination?

why not always make sure that you put drives on a cable from the end toward the mobo if that eliminates the need for terminators?
Click to expand...


Because that may not be enough especially when you have multiple devices on a single cable.

I would really recomend getting a decent cable that has a terminator built in. They are really not that expensive.
 
Spleeze said:
thanks for the links i'll check them out

so i'm right in assuming that a drive at the end of a cable is sufficient for termination?

why not always make sure that you put drives on a cable from the end toward the mobo if that eliminates the need for terminators?
Click to expand...

Also, not all drives can self-terminate.

 
SVC.com has good deals on rounded SCSI cables; I've used them @ U160 with setups ranging from 1 to 4 drives with good results
 
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