Xref: utzoo comp.sys.mac.hardware:3819 comp.periphs.scsi:616 Path: utzoo!utgpu!news-server.csri.toronto.edu!cs.utexas.edu!samsung!usc!apple!coherent!dplatt From: dplatt@coherent.com (Dave Platt) Newsgroups: comp.sys.mac.hardware,comp.periphs.scsi Subject: Re: Why can't I turn off my tape drive? Message-ID: <59953@coherent.coherent.com> Date: 8 Jun 90 17:42:07 GMT References: <10567@spool.cs.wisc.edu> Reply-To: dplatt@coherent.com (Dave Platt) Followup-To: comp.sys.mac.hardware Organization: Coherent Thought Inc., Palo Alto CA Lines: 114 In article <10567@spool.cs.wisc.edu> tonyrich@titanic.cs.wisc.edu (Anthony Rich) writes: > I recently put together a Teac 150MB SCSI tape backup unit by mounting a > raw Teac N50 drive in a 5.25" enclosure/power supply... > > But after doing a backup, if I turn the Teac off while the Mac is still on, > the Mac crashes on the first mouse double-click and won't reboot from the > hard drive; the floppy with the flashing question mark comes up. Then if > I boot from a floppy, either the hard drive isn't mounted at all or a > dialog comes up with a picture of the hard disk that says "This disk is > damaged; do you want to erase it?" I'd be interested to see if the disk would be recognized properly if you were to power down the Mac II (with the pushbutton switch in back), wait a minute or so, and then power it back up. Cycling the power will reset the SCSI-bus hardware in a way which a crash-and-reboot will not; it'll also reset the internal hard disk. > But if I turn the Teac tape unit back back on and reboot, the hard disk > WILL then be recognized (even if I haven't repaired it with SUM II yet) and > the Mac will reboot from it, although it takes a while because apparently > some built-in filesystem check-and-repair is done to correct the hard disk's > "damage" before the "Welcome to Macintosh" message comes up. This checkout process is performed if the Mac "mounts" a volume, and sees that the "I'm in use and have not been dismounted cleanly" flag is set. HFS checks the directory structures, and makes certain that all extents mentioned by any accessible file are marked "in use" in the extent allocation bitmap. This checking process won't perform any extensive repairs, but it does ensure that you won't end up allocating the same disk block to two different files. > 1. Is this normal behavior for a Teac 150MB tape backup unit connected to > a Mac II? Shouldn't I be able to turn the tape drive off while the > Mac is on without crashing the Mac? This is not normal behavior. I'm using a very similar setup... Mac II, with a Teac N50 in an external drive case (there's also a Toshiba CD-ROM mechanism in the same box); internal termination packs on the Teac controller board. I can run a backup or restoration using ExpressTape (nee Nuvo Labs FastTape), and then turn off the power to the external box; the Mac does not crash. The Mac _will_ hang if I've used the CD-ROM drive during the current session... the Toshiba/OMI CD-ROM driver periodically polls the drive to see if a new disc has been inserted, and these SCSI polls will hang up and time out if the drive is turned off. This is not a problem with the tape drive, however, since it isn't polled unless ExpressTape is running. > 3. Is there a switch or some other hardware on the tape drive that I can > or should have modified that will fix the problem? I don't believe it's a hardware-configuration problem. Certainly, I've never seen such problems in my setup, and I haven't done any reconfiguration on my drive. Here's my guess: I seem to remember hearing that the new version of Retrospect comes with an INIT that patches the SCSI Manager, and gives Retrospect the ability to use the disconnect/reconnect capability of the tape drive (that is, send a block of data to the drive controller, instruct the drive to disconnect from the SCSI bus, and then reconnect when the controller buffers begin to get empty). This allows Retrospect to overlap disk accesses with tape motion, and speeds up the backup process. I have a hunch that Retrospect's use of this facility is somehow leaving the SCSI bus in a strange state. Perhaps the modified SCSI Manager is occasionally polling the tape drive, and is becoming sadly confused if the drive vanishes from the bus. Perhaps the Mac's SCSI controller chip is being left in an unstable condition. Perhaps it's the phase of the moon ;-}. If Retrospect does come with an overlapped-I/O INIT or Control Panel device, I suggest that you remove it; if the use of overlapped I/O is a Retrospect configuration option, try turning it off. Then reboot (even better, shut down and then power-up again), do a backup, eject the tape, turn the drive off, and see if the problem still remains. I think there's a fair chance it will have gone away. If this fails, try using a different backup utility... ExpressTape or FastTape, rather than Retrospect. > I don't think it's a SCSI termination problem, but correct me if I'm wrong. > It's my understanding that the Teac N50 has its own internal termination, > and presumably so does the (internal) Wren IV hard drive, so I haven't added > any SCSI terminators, and the tape drive itself functions perfectly. You are correct... the N50 has internal terminators. The Teac is fairly sensitive to proper termination. I tried using an external terminator with my external drive box (for flexibility, so that I could remove termination if I wanted to), and the terminator or cable wasn't making good electrical contact... the Toshiba CD-ROM drive would show up in a SCSIProbe inquiry, but the Teac wouldn't respond properly... SCSIProbe could tell that there was something at that address, but said "No data available" or "Device busy". > When I put a Microtek 300Z scanner in the SCSI chain between the tape drive > and the Mac, again with no extra termination, there is no change in behavior; > turning off the tape drive crashes the Mac. However, I can turn the scanner > (and its special SCSI adapter box) on and off while the Mac is on with no > problems whatsoever. Why can't I do that with the tape drive? You should be able to. I suppose it's possible that turning off the external drive is somehow putting a power-surge on the SCSI bus and is scrambling the bus... but I haven't seen that sort of problem with my Teac. -- 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