From: Hans Ridder <ridder@zso.dec.com>
Subject: Re: Ami SCSI DB-25 pinouts & tape funniness
Originator: ridder@zowie.zso.dec.com
Organization: Digital Equipment Corporation - DECwest Engineering
Date: Thu, 9 Apr 1992 20:59:15 GMT

In article <24360029@hpfcdc.fc.hp.com> koren@hpfcdc.fc.hp.com (Steve Koren) writes:
>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

It's probably not a "DB-50," but actually a 50-pin "micro ribbon"
connector, sometimes refered to as a 50-pin "Centronics style," or
"External SCSI" connector.  This should not be confused with the 50 pin
IDC ribbon cable connector on the drive.  And by "they won't talk," I
assume you mean "to get them physically connected," right?

>I at first, perhaps nievely, though that SCSI is SCSI.  Not so,
>apparently.

These days, for the most part, SCSI is SCSI.  With old (3-5 years)
surplus drives you might have some troublesome differences.

>I have the pinouts for the DAT drive, and they look like this (50 pin
>ribbon):

[SCSI pinout deleted]

Without checking too closely, this looks like a normal SCSI ribbon (IDC
type) connector pinout.

>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?

I found the pinout in the A2091 manual.  I can't put my fingers on it
right now.  If you really need it, I can send it via email, or someone
else will post it.

>...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....

You're probably confused by the pin number differences on the two types
of 50 pin SCSI cables.  On the micro-ribbon ("External") connector they
count like so (connector held horizontally):

    1  ... 25
    26 ... 50

and on the ribbon cable on the drive (your pinout) they count like so:

    1 3 ... 49
    2 4 ... 50

(You can just imagine the lower set of pin number super-imposed on the
External connector to use your pinout on the External connector.)

>Is there something radically difference about Mac SCSI?  I always
>thought SCSI pinouts were pretty much a standard across platforms.

The Mac cable should be just fine for your Amiga.  The Mac people call
it a System Cable.  Note that the DB-25 cable is *not* standard SCSI.
Apple invented it, and Commodore followed suit.

>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.

Other than software/switch setting problems, the only thing I can think
of is that your External SCSI to drive cable is wired backwards, or has
an open/short.  Check with your meter from the DB-25 all the way to the
drive connector.  (Oh, you need the DB-25 pinout....)

If you can get ahold of another drive (even a disk drive,) you could
plug it in the place of the tape drive just to test the cable.  (you
could actually remove your intenal drive and test with it!)

>  - steve
-- 
  Hans-Gabriel Ridder				Digital DECwest Engineering
  ridder@rust.zso.dec.com			Bellevue, Washington, USA
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