External Internal SCSI on an 8500

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External Internal SCSI on an 8500

So I read an article on making your interal SCSI bus external and then mounting drives in an external enclosure years ago and just perused it again and I thought it might be a fun hack. I have a stack of old 500 MB internal apple HD's and whatnot and thought I would use those and some other stuff. The thing I'm curious about is, if I remeber correctly, SCSI is a total of 27 devices on the bus? but on the back of the drives the jumpers only offer a limited number of jumper combos. How do I make sure that there is no conflict? Ideas?

Jon
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Whoa, that's *7* devices for

Whoa, that's *7* devices for plain SCSI, and 15 for Wide. There are really 8 and 16 but the controller takes up one ID for itself.

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Not 27

Narrow SCSI, meaning all the motherboard SCSIs in Macs, is limited to seven devices. Which means a limit of 14 if you use both the internal and external bus on an 8500.

Wide SCSI, the 68 pin 16 bit connector supports 15 devices on one chain, although not all devices support the extra selection bit.

--Peace

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Woops

Alright....that settles that. Some things to chew on. Think I might make the internal routed to another box and make the external routed into the box.

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I used to run two honkin' big 5 1/4" full-height SCSI drives on my 6100 - ran the SCSI cable out the Nubus slot door and into the (functioning) PC tower next to it on the desk. The PC provided power and space to mount the drives, but didn't access them.

Worked fine till some a**hole stole the PC.

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External case

I have an Apple brand external SCSI case w/ internal power supply and a 2GB HD if you'd be interested in that, I can send it to you for fairly cheap. If nothing else, the external power supply and adapter that converts the internal drive's scsi port to a large external scsi could be helpful. I can provide pictures if you'd like.

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Or you can scour ebay for a l

Or you can scour ebay for a longish internal SCSI cable with the two external sockets and multiple internal connectors, then find a suitable case and any old power supply. A trashed PC minitower would give you the power and casing. You might need a few drive power double adapters if there aren't enough drive power plugs on the PS.

Or find an electronics shop that stocks the IDC crimp external adapters and add them to a suitable internal cable. I crimped mine with my teeth, but a bench vice would probably work better Smile Pliers didn't give enough leverage.

Old PCI SCSI cards are cheap too, and each of them will give you another seven devices.

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Performance of old SCSI drives

My recollection of the old Apple 500MB drives is that a single drive was nowhere near fast enough to consume the entire bandwidth of the lousy Narrow SCSI speed on a NuBus PowerMac 6100. The external SCSI bus on the 8500 will provide similar performance to the 6100. Even when you copy between two drives on the same chain, the SCSI bus limits performance less than the actual drives.

I'm pretty confident that if you shove your old SCSI drives in a tower, using the external 8500 SCSI bus, performance will be adequate. If you have a couple of really fast drives that you need to mount externally, you should be able to find a decent PCI SCSI card relatively cheaply.

Running ribbon cables outside the Mac for external SCSI devices is something that many of us have done, but it is a bad thing for data integrity. Even the cheapest, nastiest external SCSI cables have better shielding than a ribbon cable.

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Re: Performance of old SCSI drives

charlieman wrote:
Running ribbon cables outside the Mac for external SCSI devices is something that many of us have done, but it is a bad thing for data integrity. Even the cheapest, nastiest external SCSI cables have better shielding than a ribbon cable.

Oh yeah I agree. I wasn't recommending anyone copy my old cheap hackage. The cable I was describing is for mounting the drives -inside- the new external enclosure, and connecting to a proper external SCSI cable on the outside. So the connectors on each end would be the female, panel mount SCSI sockets, not the male plug. Sorry if that wasn't clear.

And if the old drives leave you some bus speed headroom, maybe you could RAID stripe them.

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