How Many SCSI Drives?

[mboconnell]

How many SCSI drives can the Mac SCSI bus support? Is it actually all 7 devices, or is it 6 plus the host adapter?

I found a source for some really cheap swapable drive trays and 9G drives but I don't want to buy too many

I was thinking of riviting together a form-fitting aluminium case because I found some plate stock in my basement.

edited for spelling

 

[register]

A Macintosh SCSI device uses 3 Bits to address ID 0...7, a total of eight IDs to choose from. That is the reason, why you have three jumpers to set the ID. ID 7 is used by the host controller, leaving seven IDs free for other devices. You may attach up to seven devices with different IDs from ID 0...6 to one host controller. Internal and external devices sharing the same host controller also need to use different IDs 0...6, each. You may attach more than seven SCSI devices to one computer, if you have several SCSI host adaptors in the same computer.

 

[Charlieman]

Creating a tower with several drives is the best solution for multiple external drives because it reduces the length of the SCSI bus. However, I wouldn't want to have six or seven drives in a single unit. It is a bit inflexible if you want to share the drive unit between multiple Macs. You may also want to connect an additional SCSI device to the chain at some time (a scanner, tape backup etc) so full occupancy will create pain, unless you have another SCSI adapter.

Do you have a SCSI adapter for this unit or are you planning on running it off the external SCSI II port on a Mac? For performance reasons, the latter is not recommended.

 

[Aoresteen]

Don't forget that the internal drive takes up SCSI ID 0 by default, leaving 6 external devices that can be connected.

Some Macs have two SCSI controllers. My PM9500 has two; internal and external.

 

[mboconnell]

What I have now, and don't like very much, is an HP SCSI SX-W hard drive array. I have used it with PC's in the past, but it weights ~80 pounds and sounds like a NASCAR race when it is operating. I wanted to replace it with a smaller, sleeker unit. It uses HD-68 pin interface, and I have a cable that adapts it to DB-25, but I think the disks are somewhat unhappy with the communication rate to my Mac (I was planning on hooking it up to an LCIII as networked-storage for all my other mac stuff). I asked about the number of drives because that monster is a 7-disk array, but I have never had more than 6 disks installed. (4x4.5Gb and 2x18Gb). If the bus could handle seven, I'll put them in my case, if not, then I get to build a smaller, more eye-pleasing case.

 

[Charlieman]

You have a very nice piece of kit and it would be a shame to use it on a Mac of limited performance. The Wide SCSI in the box should nicely degrade to Narrow SCSI II (as you have on your LCIII), but performance will be limited (ie I would not expect random errors, just slowness).

Why not save up for a nice PCI PowerMac, a 10/100 PCI ethernet card and a SCSI card with a Wide external interface? You'll have a monster server with ridiculous storage and the ability to deliver any file in the bat of an eyelid.

 

[tomlee59]

I asked about the number of drives because that monster is a 7-disk array, but I have never had more than 6 disks installed. (4x4.5Gb and 2x18Gb). If the bus could handle seven, I'll put them in my case, if not, then I get to build a smaller, more eye-pleasing case.

Just to make sure that you took note of Aoresteen's math, the *total* number of drives on the scsi bus can be as high as 7. Included in that total are any internal drives. So, if you already have an internal drive, then you may have up to 6 external drives. If you have, say, an internal HD and a CDROM drive as well, then you can have only 5 external drives, etc. Each SCSI device on a bus, whether internal or external (it's all the same bus), must have a unique ID. Since only 3 bits are allocated for the ID (in the original implementation), and one ID is reserved for the host (e.g., computer), you're left with that maximum of 7 other scsi devices.

Sorry if this is already obvious. Redundancy never hurts (it's just dull).

 

[equill]

To the excellent encapsulations of register and tomlee59 I should add only that termination power may become insufficient in a full SCSI daisy-chain. If any of the devices in the middle of the chain is capable of it, setting supply of 'termination power to the bus' on those devices may augment the available power. (Many Seagate drives, for example, allow this by jumpering the last pin pair, TP, on the underside header block J2. Do not, however, also enable the 'termination enable' pair TE at the other end of the block on such drives in the middle of the chain.)

de

 

[mboconnell]

Thanks very much for the help guys. What I've decided to do is build 2 enclosures, the first with 1 9Gb HDD and 1 CD-RW, and the second with 4 9Gb drives, this would allow me to alter the configuration later should the need arise.