External SCSI termination

[Syntho]

I read that on a 9600, if nothing is connected to the external SCSI bus that it will terminate itself. Well what if you have a cable connected by itself or a drive connected that's powered off?

More importantly, my external HD enclosure has worked perfectly even though I have no SCSI terminator plug on the 2nd 50pin Centronics connector. Hmm...

I'm about to switch the drive in my enclosure and I think I'm gonna run into trouble because there are three or four potential termination points:

1) HD itself

2) The enclosure (maybe it's built to terminate without a term plug?)

3) The optional term plug on the enclosure, plus

4) I'm using an SCA to 50pin ribbon adapter which has term. jumpers on it

I'm not sure how to tell where/if termination is on and which point to supply it at.

 

[Elfen]

1) What ever internal drive you have MUST BE TERMINATED. It might work fine when it's the single device without termination but when you add other devices, the need for that first drive to be terminated becomes important.

2) When in doubt, have a Pass-Through Terminator on each external device. Best to have a single terminator on the last device but that is not 100% perfect.

3) Always use Thick SCSI External cables. Thin cables causes issues that look like parallel printer cables. At best they are good for hanging yourself with when everything goes wrong.

4) Set the SCA to 50 Adapter as the last chain and set the termination on the jumper.

 

[johnklos]

External motherboard SCSI on a 9600 is a little forgiving because it's configured to run at no more than 5 MB/sec (non-synchronous mode). This means it'll sometimes work with two terminators or none.

The best thing to do is to check the drive inside of the enclosure. If the drive is an SCA drive with an adapter, then you already answered one question - the drive has no termination. Next, does the SCA adapter have resistor packs and jumpers? If not, then it doesn't have termination. If so, is the termination jumper on?

Sometimes there are extra jumpers with a few possibilities. One is to provide device power to terminators. Another is to provide bus power to terminators. Another, or a combination of the two is to connect bus and device power both together and to terminators.

If the SCA adapter doesn't have termination, then get a termination plug.

 

[Syntho]

SCA drives don't have jumpers? I thought so but maybe I'm wrong. I'm going to inspect it and see.

You raise a good point about termination power. The internal SCSI bus of the 9600 has termination power supplied by the Zip drive by default. I don't think the external bus has some kind of automatic termination power so that means I probably have to supply it on the SCA adapter, if it indeed does have it.

Terminating on the SCA adapter seems ideal but now that I think of it, someday I may want to daisy chain SCSI devices. If I set it to terminate on the adapter I probably couldn't use the extra Centronics connector on the enclosure. I'd also have to rip the HD out of the enclosure to change the settings on the adapter. That makes me think using a term. plug on the back of the enclosure is a better idea. Just hope I can get term. power supplied somehow.

 

[Syntho]

Does the external SCSI bus of a PM9600 have termination power supplied from the computer itself already? Or will I have to supply that via a device? I'm pretty sure the internal one doesn't since the stock zip drives have term. power enabled.

 

[trag]

Does the external SCSI bus of a PM9600 have termination power supplied from the computer itself already? Or will I have to supply that via a device? I'm pretty sure the internal one doesn't since the stock zip drives have term. power enabled.

All Macs except the Plus and, IIRC, one of the PowerBooks supply termination power. So termination power should be disabled on your devices. I once had a set of Seagate Barracudas, back when Barracudas were new and spiffy, which just would not work right until a nice fellow at Seagate tech support told me to disable term power.