Date: Thu, 19 May 2005 10:24:29 +1100 From: David Elmo <[EMAIL PROTECTED]>
From: [EMAIL PROTECTED]
SCSI is easy.
If it was so easy how come it gives us all such headaches? It is not so much the basics that is in sensible dispute (each device must have a unique ID, the ends of the chain must be terminated) but in the implementation on particular chains and devices and the peculiarities of the different types of SCSI and the different types of termination. BTW, let me renounce any faith in the possibility of a program to answer how to go about anything to do with SCSI. After the basics, all we can do is collect strategic ploys and facts and muddle along... like most chess players, even quite good ones...
Anyway, I noticed that on one of my CD SCSIs there was a pin pair marked "term power" and another was "termination" Most of my other CD SCSIs had only "Term power" marked on one pin pair, no mention of "termination" on any other pin pair. What gives here?
My experience with SCSI is that if one configures it properly (i.e., according to the "rules" for SCSI) it works properly. To give that some context, I've been using SCSI systems since about '92 starting with a Mac Plus. At one point I was administering about 50 machines (PCs) at work, all with simple SCSI cards so that they could interface with external tape and CDROM drives (back when CDROM drives were several hundred dollars and sharing them made economical sense). I have not gone beyond LVD SCSI (80 MB/s) but I've configured RAIDs and the like. At one point I had a four drive RAID in an 8100 clone spread across the two built-in SCSI busses and a JackHammer card (it really was faster that way).
All of the "SCSI Voodoo" that I have seen has been from folks who had an improperly configured chain, which, by some miracle, worked, and then stopped working at some point. The fact that it worked at first convinces them that it was configured properly, so they then conclude that their "properly" configured chain stopped working for no good reason. And they declare "SCSI Voodoo".
However, I would not agree that SCSI is easy. It is a bit complex. By complex, I mean that it has several rules and the applications of some of the basic rules are not always as obvious as one might wish.
For example, the term power that you mention. In order for SCSI termination to work properly, termination power must be present. In general, the SCSI host (controller) supplies proper termination power. But, in some cases inferior cables fail to pass that termination power on to the next device and so it never reaches the end of the chain where it is needed. Also, if more than one device is supplying termination power to the chain, it can cause problems, perhaps contention resulting from differences in the level settings of the voltage regulators involved.
And IIRC, the Mac Plus is not terminated internally and does not supply term power. So the first SCSI equipped Mac from Apple already required screwy configuration, needing the first SCSI device attached to have a short cable and a pass-through terminator and term power enabled.
And then there's the fact that folks often enable termination power thinking that it is termination, and so have too much of the one and not enough of the other. Your CDROM drives are likely Apple models which have no provision to supply termination. On machines shipped with those drives there was a termination module on the end of the SCSI cable itself. I did once get an actual telephone help person on the line who was familiar with those drives (4X or 8X IIRC) but it was more than six years ago and I can't remember if he told me that there was no termination available on those drives, or if there was an unlabeled jumper on the jumper bank that was actually termination.
Another tangle of yarn is adapted drives. Adapting between 50 pin, 68 pin and 80 pin drives causes termination issues because it is possible to leave the top 18 wires swinging in the breeze with no termination, or terminate them twice where only once was expected. Choosing the correct adapter and making the proper termination choice is not a simple matter with adapted drives and the requirements change depending where in the SCSI chain you place the adapted drive. My rule of thumb with adapted drives is that if you have to ask, then you don't understand SCSI well enough to be happy with adapted drives. They're a crap shoot (leading to SCSI voodoo) for the less experienced.
Finally, SCSI chain length is confusing. The length of the chain supported varies with the SCSI implementation, e.g. original SCSI, SCSI-2, Ultra SCSI (AKA SCSI-3), LVD, etc. The first problem is identifying which flavor of SCSI you're actually using. The second problem is remembering which chain length goes with the flavor that you are using. The third problem is having cables of sufficient quality that that maximum length will actually work.
So, while my experience has shown me that SCSI works as expected and according to its rules, I would not say that SCSI is easy. None of the rules are difficult to understand, but there are a number of them beyond the basics and some sublety to the ones that are well known.
Jeff Walther
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