lenny@icus.UUCP (Lenny Tropiano) (07/20/88)
With the recent announcement by IDT Systems to support SCSI devices on
the AT&T 3B1, some questions have come up in my mind. I haven't had
too much experience with SCSI devices. The only experience that I had
was hooking up some SCSI tape drives to 3B2/400 units. [For those
who are wondering what SCSI stands for it is: [S]mall [C]omputer
[S]tandard [I]nterface]
The questions that I have are:
1. Do I have to buy a special interface when I purchase SCSI devices,
or if the device says it is SCSI compatible will it include
the necessary hardware to just hook up the 50-pin Centronics SCSI
cable directly?
2. How do you hook a multiple of devices (up to 7) to the SCSI host
adapter? Do you just need to daisy-chain them along and give
each a particular identification in the device driver?
3. Do SCSI devices cost MORE or LESS than other devices? In
particular, tape drives and hard drives.
Please E-Mail if possible, I will summarize for the net-at-large...
Thanks,
Lenny
--
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Telex-net: 154232428 ICUS | Another-net: attmail!icus!lennyrusty@hodge.UUCP (Rusty Hodge) (07/22/88)
In article <433@icus.UUCP>, lenny@icus.UUCP (Lenny Tropiano) writes: > With the recent announcement by IDT Systems to support SCSI devices on > the AT&T 3B1, some questions have come up in my mind. > The questions that I have are: > > 1. Do I have to buy a special interface when I purchase SCSI devices, > or if the device says it is SCSI compatible will it include > the necessary hardware to just hook up the 50-pin Centronics SCSI > cable directly? A raw SCSI drive usually has a 50-pin header connection. The standard SCSI connection is a 50-pin Centronics. The Macintosh folks use a DB-25. Don't ask me to explain, but it works. I think that IDT is going to use a DB-25 a la Macintosh. You could just buy any Mac hard drive system and plug it in. Or make a cable and use a raw drive. > 2. How do you hook a multiple of devices (up to 7) to the SCSI host > adapter? Do you just need to daisy-chain them along and give > each a particular identification in the device driver? You just daisy-chain them. The drive has a Device ID jumper on it. There are 8 ids, the host interface is one of them. > 3. Do SCSI devices cost MORE or LESS than other devices? In > particular, tape drives and hard drives. Embedded-SCSI drives cost about 10% more than ther drives sometimes. Some drives (Seagate ST-277N, 60mb 40ms) are very competitive: $450 or less. A SCSI CDC Wren 300mb 16.5ms drive should be able to be found for just over $2000. A 150mb version for under $1300. You can buy a complete CMS Mac External 140mb system for under $1500. This includes a case, power supply and 1-year warranty. There are plenty of other suppliers to choose from (Like Micah, right Larry? :->). Well, this is exciting. I've finally answered a question from Lenny. It feels good to answer rather than ask for a change! :-> -- Rusty Hodge, HCR Inc, 1588 N. Batavia St. Orange, CA 92667 (714) 974-6300 rusty@hodge.cts.com [uunet vdelta crash]!hodge!rusty FAX (714) 921-8038
vixie@palo-alto.DEC.COM (Paul Vixie) (07/23/88)
In article <704@hodge.UUCP> rusty@hodge.UUCP (Rusty Hodge) writes:
# A raw SCSI drive usually has a 50-pin header connection. The standard SCSI
# connection is a 50-pin Centronics. The Macintosh folks use a DB-25. Don't
# ask me to explain, but it works.
The 50-pin ribbon cables have ground on every other pin, which is common. So
there are only 25 pins worth keeping if you don't care about horizontal
shielding between each pair of pins.
The Apple DB25 setup has a shield around all 25 pins, which is connected, at
the Centronics-to-DB25 connection, to the bus-bar of 25 grounds found in the
50-pin ribbon cable.
It sounds flinky but it works.
--
Paul Vixie
Digital Equipment Corporation Work: vixie@dec.com Play: paul@vixie.UUCP
Western Research Laboratory uunet!decwrl!vixie uunet!vixie!paul
Palo Alto, California, USA +1 415 853 6600 +1 415 864 7013brianm@sco.COM (Brian Moffet) (07/25/88)
In article <704@hodge.UUCP> rusty@hodge.UUCP (Rusty Hodge) writes: > >A raw SCSI drive usually has a 50-pin header connection. The standard SCSI >connection is a 50-pin Centronics. The Macintosh folks use a DB-25. Don't Simplest reason is the SCSI has about 1/2 of its lines as ground. This allows them to be strung out great distances from the machine. With the Mac Interface (Most Amiga drives also) you have to keep the drive really close to the computer. -- Brian Moffet brianm@sco.com {uunet,decvax!microsof}!sco!brianm The opinions expressed are not quite clear and have no relation to my employer. 'Evil Geniuses for a Better Tommorrow!'
ems@Apple.COM (Mike Smith) (07/29/88)
In article <698@viscous> brianm@sco.COM (Brian Moffet) writes: >In article <704@hodge.UUCP> rusty@hodge.UUCP (Rusty Hodge) writes: >> >>A raw SCSI drive usually has a 50-pin header connection. The standard SCSI >>connection is a 50-pin Centronics. The Macintosh folks use a DB-25. Don't > >Simplest reason is the SCSI has about 1/2 of its lines as ground. >This allows them to be strung out great distances from the machine. >With the Mac Interface (Most Amiga drives also) you have to keep the >drive really close to the computer. You must have a different definition of 'really close' than I do. I've seen Mac drives on cables about 10 feet long. I *THINK* it will go the full 20 ft. in the spec., but haven't personally seen it. E. Michael Smith ...!sun!apple!ems 'If you can dream it, you can do it' Walt Disney This is the obligatory disclaimer of everything. (Including but not limited to: typos, spelling, diction, logic, and nuclear war)
jacobsen@daniel.its.rpi.edu (Dana Jacobsen) (07/29/88)
Dana Jacobsen TGitM@rpitsmts.bitnet "The Truth Shall Make You Free." ghost@paraguay.acm.rpi.edu -RPI-ACM South American Thinktank ..!seismo!itsgw!brazil!ghost Troy, NY Dana Jacobsen TGitM@rpitsmts.bitnet "The Truth Shall Make You Free." ghost@paraguay.acm.rpi.edu -RPI-ACM South American Thinktank ..!seismo!itsgw!brazil!ghost Troy, NY
henry@utzoo.uucp (Henry Spencer) (08/02/88)
In article <14901@apple.Apple.COM> ems@apple.apple.com.UUCP (Mike Smith) writes: >>With the Mac Interface (Most Amiga drives also) you have to keep the >>drive really close to the computer. > >You must have a different definition of 'really close' than I do. I've >seen Mac drives on cables about 10 feet long... In SCSI cabling, as in Ethernet cabling and RS232 cabling, there is a big and important difference between what you can sometimes get away with and what is guaranteed to work consistently and reliably. -- MSDOS is not dead, it just | Henry Spencer at U of Toronto Zoology smells that way. | uunet!mnetor!utzoo!henry henry@zoo.toronto.edu
farren@gethen.UUCP (Michael J. Farren) (08/04/88)
In article <14901@apple.Apple.COM> ems@apple.apple.com.UUCP (Mike Smith) writes: > >I've seen Mac drives on cables about 10 feet long. I *THINK* it will go the >full 20 ft. in the spec., but haven't personally seen it. And *I*, personally, wouldn't try it. My data been bery, bery, good to me - I'd like to be just as good to it. Long cables at high data rates are bad news. Especially with an interface that ignores grounding conventions (like Apple's). -- Michael J. Farren | "INVESTIGATE your point of view, don't just {ucbvax, uunet, hoptoad}! | dogmatize it! Reflect on it and re-evaluate unisoft!gethen!farren | it. You may want to change your mind someday." gethen!farren@lll-winken.llnl.gov ----- Tom Reingold, from alt.flame
thad@cup.portal.com (08/29/88)
Many systems supporting SCSI (Amiga, Mac, etc.) use a DB25 connector at
the host-adapter end to save real-estate; the SCSI devices themselves
use the "correct" 50-pin connectors (either Centronics 50-pin or the
dual-25 row kind).
The so-called "Mac System Cable" has a DB-25 on one end and a Centronics
50-pin connector on the other. SCSI interconnects are 50-wire.
At the end of this posting are the pinouts of the DB25 and the 50-wire.
One thing I've noticed (after adding over 1GB HD and a Fujitsu 190MB tape
drive to my Amiga's SCSI bus) is that some device manufacturers take liberties
with SCSI wires 20,22,24,28,30, and 34; some tie these to ground, some tie
them to +5VDC, and some leave them (properly) unconnected.
Another caveat with multiple devices on a given bus is to be SURE that only
one is supplying +5VDC on SCSI pin 26 for terminator power. Some mfrs provide
jumpers to enable/disable, some feed thru a diode, some feed the power out
"raw", and some do nothing.
What I do on my system is supply terminator power from the host adapter, and
use an external terminator at the END of the bus.
From my experience, what I suggest is that when you're making the cabling,
pull pins 20,22,24,26,28,30,34 from the connector that attaches to the embedded
SCSI drive and/or SCSI translator. I've noticed that Apple's SCSI interconnect
cables do NOT pass pins 20,22,24,28,30,34 (for the reason I noted above).
Another observation I'd like to make is that MANY of the 3rd party consumer-
grade mfrs of SCSI HDs wire their boxes INCORRECTLY. SCSI specs mandate no
more than a 10cm "stub" off the bus, yet many Mac-type drives I've opened up
(just gotta see what's in there! :-) have a 2" 50-wire connection joining the
two Centronics connectors continuing with a 15" stub to the 50-wire connector
on either an embedded SCSI drive or an Adapter 4000A or 4070 translator. This
mis-wiring would account for the problems people have running a SCSI bus out
to 20 feet (I have no problems since I wired my systems correctly, and the
last device is 19' from the host adapter). Remember: SCSI wiring is
supposed to be DAISY-CHAINED, per:
WRONG: CORRECT:
____________________ __________________
/ \ / \
[] | [] |
\ \_DISK \_DISK
/ /
[] [] |
\__________________/
The "[]" is a 50-pin Centronics connector
SCSI Connector (DB-25)
Pin Name
-------------
1 REQ
2 MSG
3 I/O
4 RST
5 ACK
6 BSY
7 GND
8 DB0
9 GND
10 DB3
11 DB5
12 DB6
13 DB7
14 GND
15 C/D
16 GND
17 ATN
18 GND
19 SEL
20 DBP
21 DB1
22 DB2
23 DB4
24 GND
25 N.C. (or) Terminator Power
SCSI Connector (50 pin header)
Pin Name Pin Name
-----------------------------------
1 GND 2 DB0
3 GND 4 DB1
5 GND 6 DB2
7 GND 8 DB3
9 GND 10 DB4
11 GND 12 DB5
13 GND 14 DB6
15 GND 16 DB7
17 GND 18 DBP
19 GND 20 GND <- caution: not always
21 GND 22 GND <- caution: not always
23 GND 24 GND <- caution: not always
25 N.C. 26 Terminator Power
27 GND 28 GND <- caution: not always
29 GND 30 GND <- caution: not always
31 GND 32 ATN
33 GND 34 GND <- caution: not always
35 GND 36 BSY
37 GND 38 ACK
39 GND 40 RST
41 GND 42 MSG
43 GND 44 SEL
45 GND 46 C/D
47 GND 48 REQ
49 GND 50 I/O
Thad Floryan [thad@cup.portal.com (or) ..!sun!portal!cup.portal.com!thad]