Questions on SCSI device interfacing...

[thad at cup.portal.com]

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 at cup.portal.com  (or)  ..!sun!portal!cup.portal.com!thad]