Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!uunet!lll-winken!ames!coherent!dplatt From: dplatt@coherent.com (Dave Platt) Newsgroups: comp.sys.mac.hardware Subject: Re: SCSI question Message-ID: <41881@improper.coherent.com> Date: 13 Dec 89 18:50:56 GMT References: <1204@mit-amt.MEDIA.MIT.EDU> Reply-To: dplatt@coherent.com (Dave Platt) Organization: Coherent Thought Inc., Palo Alto CA Lines: 101 In article <1204@mit-amt.MEDIA.MIT.EDUmjkobb@media-lab.media.mit.edu (Michael J Kobb) writes: > Ready for a stupid question? There are very few stupid questions. Yours isn't one. Getting multi- vendor SCSI-bus configurations to work is a bit of a black art sometimes. > I just received my new LaCie hard disk, > and I plugged it into my Mac Plus. Since the LaCie has the small SCSI > connectors, I put it first in the chain, and chained my GCC HyperDrive > FX/20 off of it. The HyperDrive's SCSI address is 6; the LaCie's is 1, > so the Mac booted from the HyperDrive. Then, I wanted to test the > system on the LaCie, so I shut down and turned off the HyperDrive. > With the HyperDrive off, the system wouldn't boot. I can only surmise > that this is the result of improper termination. Yup. > The LaCie has > internal, and (I suppose) automatic termination. "Internal" I'll believe. I'm not sure I believe "automatic" (a terminator which senses whether it's needed, and connects or disconnects itself on that basis). I could be wrong, of course ;-). > The FX/20 requires > one of the blocks. The question: does the block go in the unused SCSI > connection on the last device in the chain, or between the cable and > the drive? Most manuals seem to say that it should be placed between the cable and the last device on the chain. In practice, it works equally well when plugged into the unused SCSI port on the last device. In at least one case (my "stripped Bering" box containing a Teac tape-drive), it works *better* when plugged into the spare port... the terminator's female connectors (into which the cable would plug) don't seem to make good contact with the cable. > The manual for the FX/20 had conflicting info, and the > LaCie didn't come with an installation manual at all! Sigh. Here's the situation you're facing: 1) All terminators require electrical power to function properly. If you have an unpowered terminator on your bus, you probably won't be able to access any device on the bus. 2) All external (plug-in block) terminators receive their power from the SCSI bus. At least one device on the bus must provide this power... +5 volts to pin 25 on the 50-pin connector, I believe. 3) Internal terminators are normally designed to accept power from the device to which they are connected. Some of them (in better- designed devices such as the Teac tape mechanism) can also accept power from the SCSI bus, if the device to which they are attached is powered down. 4) The Mac Plus does not provide terminator power, due to power-supply limitations. The II, SE, and all machines in those families do provide terminator power. I'm not sure about the portable. 5) Your FX/20 does provide terminator power. Your LaCie apparently does not. So... if your FX/20 is powered on, its external terminator-block receives power, and everything works find. If you power the FX/20 off, its external terminator doesn't get any power, its external terminator jams the SCSI bus, and you can't access the LaCie. Possible solutions: 1) Leave the FX/20 powered on whenever you use your Mac. To boot from the LaCie rather than the FX/20, use the "Startup Device" cdev in the Control Panel, and select the LaCie. This will override the usual SCSI-bus search preference (0-6-5-4-3-2-1). If the LaCie is powered off, or doesn't have a bootable System on it, then the Mac will revert to the standard device-search and will boot from the FX/20 (usually). If you want to switch back to the "boot from the FX/20" mode, use the Startup Device cdev again. 2) Modify the LaCie chassis to provide terminator power... it's usually done by wiring the +5 power supply to the appropriate pin on the SCSI connector through a blocking diode and a 1-amp fuse. This will void your warranty, most likely. 3) Build yourself a dummy SCSI-bus device whose sole purpose is to provide terminator power. Hook it up at the end of the chain, after the FX-20. Plug the external terminator block into this device rather than into the FX-20. 4) Trade in your Plus for a more recent Mac which does provide terminator power. I suggest approach (1) as being the simplest and most hassle-free. -- Dave Platt VOICE: (415) 493-8805 UUCP: ...!{ames,apple,uunet}!coherent!dplatt DOMAIN: dplatt@coherent.com INTERNET: coherent!dplatt@ames.arpa, ...@uunet.uu.net USNAIL: Coherent Thought Inc. 3350 West Bayshore #205 Palo Alto CA 94303