chai@hawk.cs.ukans.edu (Ian Chai) (03/22/91)
(By the way, does anyone know how to utilize the "Keywords" and "Summary"
fields?)
Anyway, I recently found a solution to my SCSI connection woes and so
I thought I'd share it on the net in case anyone else had this problem.
I have a IIx with the Apple's standard 40meg internal, and a Mac!Direct!
135 meg external, a Ruby Systems TX150 TEAC Tape backup drive, and an
old, defunct, Ehman Engineering 30meg.
When the Ehman drive died, I bought the Mac!Direct! to replace it, and
then found I had a problem: The Ehman had used Mac-like SCSI connectors,
while the MD uses standard shielded SCSI. "So, what?" you might ask,
"Didn't they ship the corresponding cable with it?" Well, yes, but
remember my Tape drive? Right: it has standard SCSI, so I now needed
a standard - standard, rather than another mac-mac.
Well, as a temporary fix, I hooked up the old, dead Ehman in the middle:
Mac :==| Tape |==: Ehman :==| MD ]
: Mac-type SCSI
| Std-type SCSI
] Terminator.
That worked fine. The MD is externally terminated, while the Ehman and
Tape are both "automatically terminating". (How *do* they do that,
anyway?)
But when I got the proper std-std SCSI connector last night, and took
out the Ehman, thus:
Mac :==| Tape |==| MD ]
It didn't work! I tried it the other way and:
Mac :==| MD |==| Tape
And it still didn't work. (Note: didn't attach termination, as the Tape
was "automatically terminating" and, in fact, had worked fine with the
Ehman in that configuration, before Ehman died.)
What worked was
Mac :==| MD |==| Tape ]
!!! So why did
Mac :==| Tape |==: Ehman :==| MD ]
work? And, in fact,
Mac :==| MD |==: Ehman :==| Tape
Mac :==: Ehman :==| Tape
and
Mac :==| Tape |==: Ehman
have all worked in the past... I think it has something to do with
Ehman's "internal termination" somehow doing the termination in the
Mac :==| MD |==: Ehman :==| Tape
case despite being in the middle.
Ian Chai
chai@cs.ukans.edu
2fntnougat@ukanvax.bitnet
--
Ian Chai Internet: chai@cs.ukans.edu
Bitnet: 2fntnougat@ukanvaxltepper@csn.org (Larry Tepper) (03/24/91)
In article <1991Mar22.014907.15725@hawk.cs.ukans.edu> chai@hawk.cs.ukans.edu (Ian Chai) writes: >... But when I got the proper std-std SCSI connector last night, and took >out the Ehman, thus: >Mac :==| Tape |==| MD ] >It didn't work! I tried it the other way and: >Mac :==| MD |==| Tape >And it still didn't work. (Note: didn't attach termination, as the Tape >was "automatically terminating" and, in fact, had worked fine with the >Ehman in that configuration, before Ehman died.) >What worked was >Mac :==| MD |==| Tape ] >!!! So why did >Mac :==| Tape |==: Ehman :==| MD ] >work? And, in fact, >Mac :==| MD |==: Ehman :==| Tape >Mac :==: Ehman :==| Tape >and >Mac :==| Tape |==: Ehman >have all worked in the past... I think it has something to do with >Ehman's "internal termination" somehow doing the termination in the >Mac :==| MD |==: Ehman :==| Tape >case despite being in the middle. I work for Compatible Systems. We make Ether+, a SCSI<->Ethernet for the Macintosh. We also made QuickShare, a SCSI interface for the PC bundled with software to let it talk to a Mac. I have had lots of fun over the last several years wasting time rearranging chains of SCSI peripherals. I would be suspicious of the "automatically termination" in your tape drive, especially considering that when you put a terminator on the tape drive, things started working. Perhaps only one of the two SCSI connectors on the tape drive provides automatic termination, using a mechanical widget to detect whether or not a cable is plugged in. Either the widget is broken or you plugged your cable into the wrong one. Basically, here's what I've learned about SCSCI chains over the years (or at least I think I've learned this :-): Sometimes, SCSI chains with > 2 terminators will work. Often, short SCSI chains with 1 terminator will work. Occasionally, long SCSI chains with 1 terminator will work. The faster the Macintosh, the lower the odds that the above three will work. Almost always, SCSI chains with terminators at each end of the chain will work. Note that any Macintosh without an internal drive is not terminated. They are almost ALWAYS at one end of the bus. Sometimes, apparently valid SCSI chains won't work in a particular arrangement, but rearranging devices fixes the problem. Right now, for instance, my SCSI chain looks like this: [Internal disk==Mac IIsi==External disk==CD ROM==Ether+] If I switch the order of the external disk and the CD ROM, I can't access the external disk anymore. SCSI Probe doesn't see it. We've cooked up a three-way 25-pin connector for use on Mac's w/o internal drives. It plugs into the back of the Macintosh, and provides two 25-pin female connectors. You then plug two terminated halves of a SCSI chain into it, and voila, the SCSI chain "almost always works". -- Larry Tepper, Compatible Systems Corp. 303-444-9532, FAX 303-444-9595
dplatt@ntg.uucp (Dave Platt) (03/25/91)
In article <1991Mar23.190342.523@csn.org> ltepper@csn.org (Larry Tepper) writes: > Almost always, SCSI chains with terminators at each end of the chain > will work. Note that any Macintosh without an internal drive is not > terminated. Except for the IIfx, which is handled properly... it comes with an internal terminator that's used if there's no internal hard disk. >We've cooked up a three-way 25-pin connector for use on Mac's w/o internal >drives. It plugs into the back of the Macintosh, and provides two 25-pin >female connectors. You then plug two terminated halves of a SCSI chain >into it, and voila, the SCSI chain "almost always works". Slick! Another approach to the same problem is to install an internal terminator in a Mac that has no internal hard disk. I seem to recall reading that someone (MicroNet?) makes such a beast. -- Dave Platt VOICE: (415) 813-8917 UUCP: ...apple!ntg!dplatt USNAIL: New Technologies Group Inc. 2468 Embarcardero Way, Palo Alto CA 94303