From: garyf@wang.com (Gary Field)
Newsgroups: comp.sys.amiga.hardware
Subject: Re: Ami SCSI DB-25 pinouts & tape funniness
Date: 14 Apr 92 20:18:58 GMT
Organization: Wang Labs, Lowell MA, USA

koren@hpfcdc.fc.hp.com (Steve Koren) writes:


>A while back, I posted a note about problems I was having with a
>tape drive I had purchased.  I still don't have this working, and
>this is my second attempt at finding out why.  First a disclaimer:
>I am a software guy who only rarely and in times of dire need
>ever works with hardware.  If some of these questions seems
>nieve, that is why.

>I have been poking around trying to find out how the SCSI cable
>I have is connected.  Since this drive has a 50 pin ribbon
>connector on it, and my Amiga has a DB-25, they won't talk unless
>I use the following:

>  1. DB-25 to DB-50 (?) cable I bought from a local Mac place
>  2. A DB-50 to 50 pin ribbon adapter

>I at first, perhaps nievely, though that SCSI is SCSI.  Not so,
>apparently.  I have the pinouts for the DAT drive, and they
>look like this (50 pin ribbon):

>  All odd pins except 25 are connected to ground
>  25 should float
>  pins 2,4,6,8,10,12,14, and 16 are -DB(0) thoruhg -DB(7)
>  pin 18 = -DB(P) (I assume this is a parity bit - ??)
>  20, 22, 24 are ground
>  26 is terminator power (I'm guessing +5, but it doesn't say)
>  28 and 30 are ground
>  32 is -ATN
>  34 is ground
>  36 is -BSY
>  38 is -ACK
>  40 is -RST
>  42 is -MSG
>  44 is -SEL
>  46 is -C/D
>  48 is -REQ
>  50 is -I/O

>I don't have the pinouts on the Amiga DB-25 connector, so I can't
>compare.  Does anyone have this they could send to me?

>Now, I had originally assumed that this 25 to 50 pin cable I got
>from the Mac place would do the job.  Perhaps this is my problem.
>I have not attempted to determine exactly what is connected to
>what, but I have done a little poking around with a multimeter, and
>have determined a few strange things.  For example:

>   DB-25 end      DB-50 end
>   -----------------------------
>   pin 1      ->  1,2,3,49
>   pin 2      ->  46,48,50
>   pin 3      ->  6

>This smells funny.  If pin 1,2, and 3 are all tied together on the
>DB-50 end, then its trying to force -DB(0) to be ground in all cases,
>according to the pinouts above.  Something is obviously amiss.

>The Mac place where I bought this assures me that it is indeed a
>25 pin to 50 pin SCSI cable, and the package says that too.  Is
>there something radically difference about Mac SCSI?  I always thought
>SCSI pinouts were pretty much a standard across platforms.

>(I would have bought the cable from an Amiga dealer but there are no
>Amiga dealers here - the closest one is a 120 mile drive, and I don't
>trust that one any farther than I could throw them).

>What's the deal?  Am I just colossally ignorant of hardware matters,
>or is this really not the right cable?  Where can I get one that will
>work (including the DAT drive requirements that all odd pins except
>25 be tied to ground, and that 25 must be left open)?

>I'd be greatful to anyone who can tell me what is going on here.
>Right now I have a very expensive DAT drive sitting here useless,
>and that is somewhat frustrating.

>  - steve

The cables you are using should be correct. I hesitate to call the
DB-25 kludge that Apple perpetrated a standard, but it is widely
accepted as a method of connecting to SCSI busses.

I won't type in the entire list but here are a couple to verify
your connections:

   	Mac DB-25	D shell 50 (alternative 2)	50 pin ribbon

REQ	1			49			48
RST	4			45			40
DB0	8			26			2
DB7	13			33			16
GND	7,9,14,16,18,24		1-12 & 14-25		all odd except 25

I think you will find it all makes sense now!
Gary

-- 

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