Information 2.

External SCSI drives on AV systems.

The good thing about standards is....

There's wide and there's narrow and there's fast and there's slow and there's ultra-wide and fast and low-Voltage differential and single-ended and about six different types of connector. AAAARGH!

The golden rules are: get your SCSI I.D.s sorted out and put your best drive first in the chain.

SCSI I.D.s (identities) are usually set with a small switch on the drive. It can be set from 0 to 7 on older drives and it's best to stick with this range if you're uncertain. You cannot have two things in the same chain with the same ID or the controller doesn't know which one it's talking to. And another thing, you have to avoid ID7, or is it 0 you have to avoid? Oh dear.

The basic SCSI chain can cope with 8 devices (IDs 0, 1, 2, 3, 4, 5, 6 and 7) but that doesn't mean you can run 8 drives on one chain... The SCSI controller card itself takes up one ID, usually 7, but it can be 0 on some old systems.

A further complication comes when the system only has one bus (SCSI chain) which ends up on a socket on the back of the machine. Drives connected here cannot have the same ID as any of the internal drives on this bus. Some Macintosh PCs in the 9000 series had an internal bus which was available on the back of the machine on which you had to avoid ID3 because the internal CD rom drive used it.

The key here is to know your machine. Read the book, find out if the bus you are about to connect to is separate from the internal one or not. Usually it is.

A SCSI bus must be terminated. If you know video, you'll know that a video line "terminates" at the end with a 75 Ohm load, either in the equipment or at a separate "terminator" (no Arnie jokes please..). This prevents the signal being reflected back from the far end of an unterminated line, causing reflections on the picture.

These reflections, should they happen on a SCSI bus would cause data corruption, lost files, tearing out of hair etc. So we have to fit a terminator the end of our SCSI bus, ie at the last drive on the chain. The terminator is a special SCSI plug which sometimes has an indicator light on it (but not always). Beware of devices claiming to "auto terminate" don't trust 'em, use a plug-in terminator.

If you have a mixture of "wide" (68 pin connector) drives and "narrow" (50 pin connector) drives, always put the wide drives first in the chain, ie nearest the controller. You should use a special adaptor to convert from wide to narrow, this adaptor should terminate the unused part of the wide bus that doesn't get carried on into the narrow bus. Many types of adaptor are available and they don't always terminate, check with your supplier! Use the shortest cables that you can, total length of cable on a bus should be well under 5 metres.

OK, so it's all plugged up with the right adaptors and cables and it has a terminator at the end of the chain. The IDs on the drives are set to something safe like 3, 4, 5 and 6 but it still doesn't work. What to do?

First switch off, check all the cables, especially the "wide" 68 pin ones. Take out the plugs and look carefully to see if any of the pins are bent, it's all too easy to do. If they aren't too flattened you should be able to lever them straight with a mini screwdriver. Let's hope you don't have any of the horrible micro-miniatue "ultra-SCSI" connectors in your chain, they break very easily and can even be inserted the wrong way round without much effort, check them carefully! If you are happy that there are no connector problems and it still doesn't work, you'll have to break it down.

Try drives and cables one by one, cables are chief suspect even if the connectors are OK. You should be able to narrow it down to one component. Don't rush it, think logically.

By the way, if you change the SCSI ID of a drive, you must power the drive off and back on again to get it to assume the new identity.

There are so many types of SCSI bus and drives that it is possible to arrive at a combination of drives that just won't work on the same bus. They are OK individually but not together, it's rare but it can happen.

Good luck.

A "don't quote me on this" production. (c) Dave Pick 2001.

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