Connecting Two Macs to One SCSI hard drive

[Dog Cow]

Anyone tried this?

Connecting two MacPlusses to one SCSI drive!

Here is a nice question.
How reliable is it, to connect two Mac's to the same SCSI hard disk?

I just tried it, and is *seems* to work, that is, both Mac's boot

from the drive, and they can read from and write to it.

...

 

[Dennis Nedry]

NOT a good idea! Very interesting that it sort-of works though.

 

[PowerPup]

I remember reading in Mac's Secrets 5th Edition that it's not good to connect two macs to the same scsi chain. (Unless one is a PowerBook.) This is because both the macs motherboard's scsi bus are set to ID 7 or 8 (Don't remember the exact number.) It's risky and can damage one or both the macs.

 

[Bunsen]

The replies in the thread you linked pretty much cover it. Highly not recommended.

both the macs motherboard's scsi bus are set to ID 7 or 8

7

 

[Dog Cow]

It's risky and can damage one or both the macs.

I would still like to try it. Need to get two Macs that I don't care about, though. The Mac LC, and maybe one other...
Too bad the original poster never mentioned whether the two Mac Plus computers were still working afterward. They probably were, though.

 

[Bunsen]

Despite everyone here, and the original thread you linked, telling you it's a really bad idea, and why ... why?

 

[Dennis Nedry]

Pin 25 is a supply rail for termination power. Both Macs should have identical voltages to prevent power supply problems.

Look at this pinout:

http://pinouts.ru/HD/ScsiExternalAmigaMac_pinout.shtml

You must consider data directions. Any item with an arrow pointing to the right is an OUTPUT from the Mac. These signals change very quickly, so hooking two Macs together will result in many of these signals conflicting simultaneously. One Mac attempts to output low, the other attempting to output high.

You will VERY LIKELY BURN your SCSI chips if you do this for very long. Maybe instantaneously. The Macs will still work, but will be limited to booting from floppies with SCSI blown.

Maybe a modified idea could satisfy your curiosity. Take an external SCSI drive. Boot Mac A off of it. When it's done booting, pull the SCSI cable and plug it into Mac B and boot off of it. Pull the SCSI cable and switch back to Mac A. etc. You will screw things up this way and have to reformat the drive after a while, but you will not damage any hardware.

 

[Bunsen]

That hardly seems advisable either. SCSI-1 is *not* designed for hot-plugging.

The *only* way I could countenance having two Macs connected to one drive is if one of the Macs is OFF.

 

[Dennis Nedry]

I don't believe that you would blow any hardware by hot plugging. I have been known to hot-plug SCSI successfully countless times. Never had a problem. Connect drive, turn on, scan/mount with SCSIProbe. Then drag to trash or Command+Y before pulling it out. That actually works although it isn't technically supported.

What I suggested would not use SCSIProbe or eject properly. This is where you could lose data.

 

[Bunsen]

Connect drive, turn on, scan/mount with SCSIProbe. Then drag to trash or Command+Y before pulling it out.

Yes, I've used that routine too, but only in emergencies.

Dog Cow: quick question - is the object of the exercise to simply find out whether it works or not, or to have a usable setup?

 

[Mac128]

A more practical application would be to use the Plus as a head unit and then connect an external SCSI drive connected to it, and another Mac with a hard drive, like an SE. Boot up the SE, and interrupt it. Then access bothe hard drives from the Mac Plus. Then when the Mac Plus was turned off, both HDs would be accessible from the SE. But you'd still need active termination between the external drive and the Plus, for which there were switchable terminators, some of which were alleged to allow hot swapping, though I don't think they were particularly reliable.

If the idea is to use a SCSI drive as a shared network drive, then attach it to one Plus and access it via an EN/SC from the Other Plus. Youcould also access it via AppleTalk, but that sort of defeats the purpose of having the speed of SCSI.

 

[Dog Cow]

Dog Cow: quick question - is the object of the exercise to simply find out whether it works or not, or to have a usable setup?

Just to find out what happens. Satisfy curiosity.

 

[jsarchibald]

Can't you just blow up a couple of PCs to keep the Mac God happy?

 

[Gorgonops]

Pin 25 is a supply rail for termination power. Both Macs should have identical voltages to prevent power supply problems.

I don't *believe* old Macintoshes provide termination power, and I seem to recall it being necessary to ensure that one and only one drive/device on a SCSI chain provided it. It's been long enough I may be mistaken, however.

Now, I'm a little rusty when it comes to parallel SCSI, but... so far as I recall, and as much as it pains me to say it, I sort of doubt you'll *damage* anything (by that I mean "electrically") if were to try this, at least with Mac Pluses. The arrows on that pinout diagram are somewhat misleading, as SCSI is essentially a peer-to-peer network. The thing is that as implemented by Apple the computer is *always* the initiator and the peripheral is *always* the target. (IE, so far as I know no on-motherboard 68k Mac SCSI controller actually allows device "bus mastering".) Which *basically* means the drive should never send a command which would "wake up" both Macs at once*. All transfers, whether read or write, will be "initiated" by one of the Macs, and probably the worst thing that could happen would be if both of them tried to do it at once. However, even then, the fact that the "global" /BSY signal should be asserted as long as the previous transaction is occupying the bus *should* prevent the other Mac from starting its transaction until the bus is clear. Thus... it is my "fairly certain" belief that it's unlikely you'll actually seriously "short anything out" by doing it.

(* The one exception I can think of is if you have a "slow" SCSI target like a scanner or tape drive attached, and the software driver supports using the "Reselection" protocol phase. A device sending a reselect ping to the host *might* wake up both Macs at once. I don't know if the Mac SCSI driver and hardware supports reselection... I'd be sort of surprised if the Mac Plus does, honestly, given the way it basically bit-bangs the SCSI controller. But again, I don't know.)

But yes, if you run both off the same file system you *will* make hash out of it. The fact that the bus arbitration circuitry will *probably* prevent you from shorting out your SCSI controller's data pins will do nothing to prevent both Macs from making mutually-incompatible changes to vital filesystem structures in sequential transfer events.