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From: mueller@hatteras.cs.unc.edu (Carl Mueller)
Newsgroups: comp.sys.amiga.hardware,comp.periphs.scsi
Subject: Re: cabling
Summary: Whoa!  Check again.
Message-ID: <700@borg.cs.unc.edu>
Date: 22 Jan 91 21:30:43 GMT
References: <2331.AA2331@caleb>
Sender: news@cs.unc.edu
Followup-To: comp.sys.amiga.hardware
Organization: University of North Carolina, Chapel Hill
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In article <2331.AA2331@caleb> Jim writes:
>...
>      2.  Buy a Mac SCSI cable and then direct connect it to 50 conductor
>          ribbon.  (WRONG!)
>...
>
>It should be obvious that suggestion 2 won't work since the Mac 50 pin
>connector is obviously wired differently than the standard SCSI 50 pin
>connector.  I wonder why Apple decided to make life difficult for
>themselves and the rest of the world by being different?
>...
>Thanks to all who tried to help.
>                                                Jim Pritchett
>
>UUCP:  texsun.central.sun.com!letni!rwsys!caleb!jdp
> or    spudge.lonestar.org!caleb!jdp
> or    letni.lonestar.org!dms3b1!caleb!jdp

At first I was going to say that I don't believe the cable you
received is a proper Mac SCSI cable.

However, I looked carefully at your pin-outs again, and realized
that you may either 1) have the proper cable, but have mis-numbered
your pins in making your pin-out, or 2) the cable company mis-numbered
the pins on the pin-out.

Think about it for a bit.  The standard DB-type connector is numbered
something like this:

1  2  3  4  5  6  7  8  9  10 11 12 13
 14 15 16 17 18 19 20 21 22 23 24 25

The standard header connector is numbered like this:

1  3  5  7  9  11 13 15 17 19 21 23 25 27 29 31 33 35 37 39 41 43 45 47 49
2  4  6  8  10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50

The question now is, how was the Centronics-type AMP connector numbered in
your diagram?

Regardless of the numbering, all that matters is that the right pins
on the DB-25 go to the right pins on the header (geometrically
speaking).  Check your cable again.  I also predict that pins 10, 12,
14, and 15 on the "Mac" end do indeed go to one of the ground pins on
the "Ext" end.

Something to remember: The only reason Apple started with the
non-standard DB-25 SCSI was to save connector space on the rear of the
Mac Plus.  It became a pseudo-standard which Commodore (among others)
picked up.

-Carl (mueller@cs.unc.edu)