Yo compacters everywhere, To an earlier post, Eagle <[EMAIL PROTECTED]> askz:
>Do these SCSI ID switches have any sort of standard? Yeah. But... After a lot of reading and more than a little practice with this area of SCSI voodoo I share the following guess at what is happenin'. These SCSI ID switches on the back of SCSI boxes come in a variety of newer forms that have a clicking counter with tiny buttons where you click up or down to select a SCSI ID of 0 thru 6. A few of these counters go all the way from 0 thru 9 but only work from 0 thru 6. Ignore the 7 to 9 numbers, 0 thru 6 is all that works. Often, these click switches are located right next to the SCSI cable where, when you flip back the wire retainer of the SCSI plug, it hits an ID select button giving you an unintended ID change. Then you wonder why you have the symptoms of a SCSI ID conflict. That wire retainer changed your external SCSI device from ID 1 or ID 6 to ID 0 in conflict with your internal hard drive. These click switches are okay. Just some box builders put them in a poor place. There are older ID switches out there that are a tiny rotary dial needing a small screwdriver to rotate the switch selecting an ID of 0 thru 6. Sometimes called a "star" switch, these seem to work fine but may be difficult to read and are often damaged by hamfisted screwdriver holders. A few rare SCSI boxes will have you set jumpers as you do on the drive itself to select the SCSI ID. The difficulty here is that these jumper pins are seldom labeled (A0, A1, A2) as are the ID pins of most drives. I have seen but never used SCSI devices that used DIP switches to set the SCSI ID. They appeared to be ancient including a 1x Denon caddy style CD-ROM drive. And, yes, many a hard drive gives you no hint as to which six pins in a bank of many are the six pins used to select the SCSI ID. This is where you hope to find information about SCSI ID pins of your specific drive at the drive manufacturer's website. This is where you will get into a lot of reading. I've been able to sort this one out through tedious trial and error with no harm resulting from plugging the SCSI ID switch into a sequence of these pins. This SCSI ID selector switch connects via ribbon cable to your drive. Now, there are a variety of plugs, usually of six holes, that plug onto six pins on the drive. Some variation of Murphy's Law says that your switch plug and the six pins of your drive are likely not the same. Or, you will find the location of the pins on the drive or the placement of the drive in the box prohibit the plug being attached. This is where you decide to have a fixed ID by just putting a jumper on the pins of the drive. The convention of assigning ID addresses is: 0 is usually the internal hard drive; 3 is usually the internal CD-ROM drive; and 7 is always the motherboard. You cannot select 7 on most ID selector switches so is moot. If you can, DON'T! And, yes, often you can plug this connector on backwards. If it doesn't seem to work, or the SCSI ID you actually get is different that what the switch reads, turn it over. You will read of conventions for the remaining ID addresses 1,2,4,5, & 6. But you do not read of them often enough to remember them or where you read it. That is okay as, except for ID 7, these are conventions rather than some immutable law of physics. You can assign any ID 0-6 to any SCSI devise on your SCSI chain with due respect for the RULES which do tie to immutable laws of physics. Wherever SCSI rules are written be it in ROM, firmware, software, wherever, some of these ID addresses are more privileged than others. Know these rules and know how to use your thumb and you can hitch a ride on SCSI anywhere you want. In the days of old (Macs), especially our old Macs, the startup logic of the Mac first examines the floppy drive for a system folder. If none is there, it next examines the SCSI chain beginning with SCSI ID 0. Is no system folder is there, it next tries SCSI ID 6, then 5, then 4, then 3, then 2, then 1, then it goes back to look at the floppy drive again repeating the whole sequence a few times until it blinks a flashing ? inside a floppy icon at you. So, a boot or "crisis" floppy will be seen and used before startup even gets to your SCSI hard drive be it internal or external. If it finds a floppy with no system folder, as this is confusing and your Mac has the intelligence of a brilliant two year old, it spits out the floppy so it doesn't see it again. After the floppy stop, your Mac looks to find a system folder at SCSI ID 0. Do you still ask why the internal hard drive is "usually" given SCSI ID 0? It's a control thing. If, for some reason, you have no system folder on a mounted internal hard drive at SCSI ID 0, your Mac will seek the remaining SCSI ID in reverse order from 6 to 1. Why reverse order? I never read that one although I remember learning long ago that it is easier to count backwards in raw binary than forwards. Suspecting this code is in ROM, they made it the minimum possible code-backwards. Should you have no or defective system software in your floppy drive or internal hard drive, your Mac, counting backwards with nothing else hooked onto your Mac's external SCSI port will encounter your CD-ROM drive at SCSI ID 3 where it may find your system software or Norton's CD laying in wait for it. Unless you are Jeff Garrison or Stuart Bell stuffing a CD-ROM drive inside your compact, this will not happen. This gives you six reasonable SCSI ID addresses to assign to external devices daisy chained onto your SCSI port on the back of your compact Mac. Yes, unless the dreaded SCSI Voodoo haunts your setup, you can get six SCSI devices plugged in and all going at once, sort of, sometimes. Clean software, good devices, and short fat SCSI cables help here. So much for the rules and conventions of SCSI ID. What do we really do? Well, my real crisis external hard drive, I usually give SCSI ID 6 so it will be seen first after the internal drive. My real crisis external drive has my fully configured standard load of software on it as well as a suite of big name heavy hitter utilities. This drive is partitioned so that the diagnostics are in the first (user assignable) partition and the fully configured software load is on a separate partition. Both partitions are large enough to accommodate a lot of change. My external hard drive with lots of applications, or data, or for scavenging software is given SCSI ID 1. It has no system folder. If I really need SCSI ID 1 for a system folder, I wouldn't be using this drive anyway. This seems to go along with a sort of convention I read from time to time that scanners and other non-system folder devices should be connected using SCSI ID 1 or 2 so as not to deprive SCSI ID 4, 5, and 6 from startup devices. On a compact, we, as in we of this compact forum, seem to have a convention that our external CD-ROM drives use SCSI ID 3. And this is what I do. Although I have had two external CD-ROM drives connected at the same time using SCSI ID 3 and 4. Removable media drives such as SyQuest, ZIP, or my ORB get SCSI ID 4. External hard drives which have a system folder get SCSI ID 5 or 6 with ID 6 being for my main crisis drive. There are few conflicts on my bench short of the occasional aberrant SCSI connector wire retainer or my finger coming into contact with an unguarded SCSI ID click switch. Where my SCSI ID number scheme overlaps, I have to make a temporary change. I have found every SCSI device should have its assigned SCSI ID written in pencil on the front and back of the drive. Even a brief temporary ID should be written. When you are deep into the night diagnosing you Mac, you are not usually remembering SCSI ID addresses every time you might. So write the ID on the box. I write the SCSI ID assigned to the internal hard drive on the drive where I can see it without removing the drive. I also note it on the back of the Mac. There is many an external SCSI device, especially external drives you have produced from salvage, where the drive has a jumper-fixed SCSI ID yet the device still has an unused SCSI ID switch on the back of the box. In this not unusual case, clearly mark the actual fixed SCSI ID as well as a note that the switch isn't connected. Put a piece of tape over it or remove and tape it inside the box is better. Without touching the mighty mysteries of SCSI VooDoo better discussed elsewhere, this is my best understanding of the rules, conventions, and practice of SCSI ID. Bill -- Compact Macs is sponsored by <http://lowendmac.com/>. Support Low End Mac <http://lowendmac.com/lists/support.html> Compact Macs list info: <http://lowendmac.com/lists/compact.shtml> The FAQ: <http://macfaq.org/> --> AOL users, remove "mailto:"; Send list messages to: <mailto:[EMAIL PROTECTED]> To unsubscribe, email: <mailto:[EMAIL PROTECTED]> For digest mode, email: <mailto:[EMAIL PROTECTED]> Subscription questions: <mailto:[EMAIL PROTECTED]> Archive:<http://www.mail-archive.com/compact.macs%40mail.maclaunch.com/> Using a Mac? Free email & more at Applelinks! http://www.applelinks.com
