Path: utzoo!yunexus!ists!jarvis.csri.toronto.edu!rutgers!ucsd!ogccse!blake!milton!uw-beaver!uw-june!gordon From: gordon@cs.washington.edu (Gordon Davisson) Newsgroups: comp.sys.mac.hardware Subject: Re: 5.25" Hard Disk in a IIci? Keywords: IIci, hard disk Message-ID: <9848@june.cs.washington.edu> Date: 17 Nov 89 09:39:30 GMT Article-I.D.: june.9848 References: <429@tci.bell-atl.com> <1989Nov14.152256.303@agate.berkeley.edu> Reply-To: gordon@june.cs.washington.edu.cs.washington.edu (Gordon Davisson) Distribution: comp Organization: University of Washington, Computer Science, Seattle Lines: 31 In article <1989Nov14.152256.303@agate.berkeley.edu> bmug@garnet.berkeley.edu (BMUG) writes: >(Strangely, I can get the IIci to work with the internal 5.25" >drive and Radius card, but only when an external hard disk is >attached and powered up; without that, I get the sweet arpeggio >which denotes a hardware fault. Removing all Nubus cards works >too, but you can't do too much without a monitor :-(.) I can think of a couple of explanations for this, both having to do with the way SCSI terminators are powered. First: when you hooked up the external drive, did you disconnect any of the internal terminating resistors networks? The naive (i.e. standard :-() way of setting up the terminators draws about 15mA when idle. The second, more sinister possibility is that either the IIci or your internal HD may connect the SCSI bus's "terminator power" line directly to +5, allowing the external HD's power supply to help power things inside the IIci. This sort of setup is rather unstable, though it may work if the two power supplies are tuned right. (You're supposed to run terminator power through a diode to avoid this, but Apple hasn't always done this in the past. I don't know what they do on the IIci.) Actually, I can think of a third possibility, namely that either the IIci or the internal HD may have its terminators powered through the SCSI bus's "terminator power" line. If is was done right, this would allow the external HD to power the internal terminators without also connecting to the +5 supply. This is very unlikely; I don't know of anyone who goes to this sort of trouble over terminator power. -- Gordon Davisson Westwind Computing (206) 632-8141 4518 University Way NE, Suite 313, Seattle WA 98105