At 11:18 -0500 03/07/2003, [EMAIL PROTECTED] wrote: >My big question is where on the scsi chain should the boot drive >reside? Right now CD ROM is where it always was with the first plug on >the cable and the scsi id is "3". the system drive is at the end of the >chain and has an id of "5". Does all this mean anything. I had the >system running where it is now, but since I opened the computer up, >nothing works.
You probably have your SCSI chain(s) misconfigured. Here is a basic primer on SCSI which I posted to PCI PowerMacs list a while back: ======================================= SCSI Voodoo exists because SCSI works when it shouldn't, not because it doesn't work when it should. So folks misconfigure their SCSI chains, the machine works anyway, they think everything is fine, then one day it stops working and they declare SCSI Voodoo. If your cables are short and your SCSI bus is slow it will still work without termination, sometimes. The things which termination deal with are into the realm of analog electronics as opposed to digital and so it's not an all or nothing deal. Sometimes poor or no termination is still good enough. A great site: http://www.scsifaq.org . Basic SCSI: 1) I will use the terms chain and bus pretty much interchangably. 2) Every device on the SCSI bus must have a unique SCSI ID. It doesn't matter what order the IDs are in, just as long as they are each unique. 3) A "device" is anything on the bus, including the host computer, a SCSI card, disk drives, scanners, etc. 4) No branches are allowed in a SCSI chain, i.e. no Y formations. 5) Every SCSI bus/chain has two ends and only two ends. It's a line because there are no Ys. 6) Each end of the SCSI bus must be terminated. 7) You, the user, may only need to provide termination at one end, because some device (host computer, or SCSI card) may be providing termination automatically for the other end of the chain. 8) Here are some example SCSI chains: Key ===== internal SCSI ribbon cable :T or T: A SCSI terminator or termination on a SCSI Device Dn A disk device of some type, such as a disk drive, n is an integer which I'm just using as a label for easy communication. It has nothing to do with SCSI IDs. C A connector on a SCSI cable with no device attached MB The host motherboard, or a SCSI card -------- external SCSI cable Example 1, Internal SCSI only; Good Termination T:MB===D1====C====D2:T The motherboard provides termination at one end and the device at the end of the chain provides termination at the other end. Notice that the device providing termination is on the last connector on the cable. ------------------------------------------------------------------------------------------------------------ Example 2, Internal SCSI only; Bad Termination T:MB===D1====D2:T====C The motherboard is still good, but now the terminating device at the other end of the chain is no longer at the end of the cable. Unless the distance from D:T to C is only a few inches this is a SCSI no-no. ------------------------------------------------------------------------------------------------------------ Example 3, Internal SCSI only; Bad Termination T:MB===D1:T====C====D2 The motherboard is still good, but the second terminating device is not at the end of the cable where it belongs. D1 should have termination disabled and D2 should have it enabled, or their positions on the cable could be reversed. ------------------------------------------------------------------------------------------------------------ Example 3a, Internal SCSI only; Bad Termination T:MB===D1====C====D2 The motherboard is still good, but there is simply no termination for the other end of the chain. This is bad as both ends of the bus must be terminated. ------------------------------------------------------------------------------------------------------------ Example 4, External SCSI only; Good Termination T:D2----D1------D3------MB:T Note that this looks pretty much exactly like good internal termination. The biggest difference to note is that with external devices one usually leaves the on-board (built-in) termination turned off and uses an external terminator module. The module plugs into the unused cable connector at the end of the SCSI chain. Every external SCSI device should have two SCSI connectors. One for each of the two external cables that connects it to the neighboring devices. When you reach the end of the chain, there's only the one incoming cable, so the other cable connector is available for a terminator module to plug into. Also note that if you use an external module, it is very difficult to incorrectly terminate your external SCSI chain. You can only plug the terminator into the last device on the chain, becuase it's the only one with an empty cable connector. However, you can still screw up by enabling termination on one of the intermediate devices on the chain. Some external devices have only one SCSI connector and have termination permanently enabled. Such devices must go at the end of the SCSI cable. ------------------------------------------------------------------------------------------------------------ Example 5, External SCSI only; Bad Termination T:D2----D1:T------D3------MB:T In this case SCSI device D1 somehow has termination enabled. It is not at the end of the SCSI chain so this is wrong. There are two primary ways this could happen. D1 could be a disk drive of some sort and the user left the "Termination Enable" jumper installed so that D1's built-in termination is still enabled. Or the case in which D1 is installed may have a built-in termination feature and it may be switched on. Some of the APS brand cases came with this termination feature. So, when you put SCSI devices on an external SCSI chain be sure to disable the built-in termination (remove the SCSI Term Enable jumper), unless you will not be using an external terminator, and the device will be at the end of the chain. But using the built-in termination is a bad practice because you cannot see at a glance whether termination is enabled on a device. And you would have to pull the device out of its case to check the jumpers. ------------------------------------------------------------------------------------------------------------ Example 6, External SCSI only; Bad Termination T:D2:T----D1------D3------MB:T This example builds on the information in example 5. The last device on the chain is terminated but it is double terminated. This happens when the user leaves the "Termination Enable" jumper installed on the SCSI device and installs an external termination module on the unused cable connector on the last device. This is also a big no-no. ------------------------------------------------------------------------------------------------------------ Example 7, External SCSI only; Bad Termination D2----D1------D3------MB:T This one is trivial. There is no termination at the left end of the SCSI chain. Termination needs to be provided in some manner. ------------------------------------------------------------------------------------------------------------ Example 8, Internal & External SCSI; Good Termination T:D2----D1------D3------MB===D5====C====D4:T Most Macs which shipped with built-in SCSI have both an internal and an external SCSI connector and these connectors are both on the same SCSI bus. In a PCI Mac such as the x500 and x600 series which have two SCSI busses, this internal/external SCSI bus shows up as Bus 1 in utilities such as SCSIProbe. So, in this case we have devices on both the internal connector and the external connector. This involves two different types of cabling: a ribbon cable on teh inside and external cable in a chain of cables and boxes on the outside. However, it is still a straight line. The rules still apply. There must be a terminator at each end of the chain. So the last device on the ribbon cable must be at the end of the ribbon cable and it must have termination enabled (usually the "Termination Enable" jumper installed). The last device on the external chain must have a terminator installed. The motherboard detects that it is no longer at the end of the SCSI chain and automatically disables its built-in termination. This is documented in the Apple Developer Notes for these machines. ------------------------------------------------------------------------------------------------------------ I'm going to skip the "Bad Termination" examples for internal & external because they can be drawn from teh examples for internal only and external only. Mac models such as the x500 and x600 machines and the 8100 have two built-in SCSI busses. Each bus must be properly terminated. So an example would be: SCSI Bus 0 T:MB===D====C====D:T SCSI Bus 1 T:D----D------D------MB===D====C====D:T The machines with two built-in SCSI busses have two internal SCSI connectors. One of those connectors is the sole connector for SCSI Bus 0. The other connector is the internal connector for SCSI Bus 1 and is on the same SCSI bus as the external SCSI connector. I think that's enough. There's a bunch I could write on SCSI IDs and SCSI cards, but there's a length limit on messages to the list. Jeff Walther -- SuperMacs is sponsored by <http://lowendmac.com/> and... Small Dog Electronics http://www.smalldog.com | Refurbished Drives | Service & Replacement Parts [EMAIL PROTECTED] | & CDRWs on Sale! | Support Low End Mac <http://lowendmac.com/lists/support.html> SuperMacs list info: <http://lowendmac.com/supermacs/list.shtml> --> 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/supermacs%40mail.maclaunch.com/> --------------------------------------------------------------- >The Think Different Store http://www.ThinkDifferentStore.com ---------------------------------------------------------------
