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From: jjg@walden.UUCP (John Grana)
Newsgroups: comp.sys.mac
Subject: Re: Making *own* SCSI Hard Drive
Message-ID: <88@walden.UUCP>
Date: Mon, 27-Jul-87 17:32:58 EDT
Article-I.D.: walden.88
Posted: Mon Jul 27 17:32:58 1987
Date-Received: Wed, 29-Jul-87 02:53:58 EDT
References: <380@umbc3.UMD.EDU> <458@osupyr.UUCP> <19683@ucbvax.BERKELEY.EDU> <472@osupyr.UUCP>
Reply-To: jjg@walden.UUCP (John Grana)
Organization: Awcs Inc.   Henrietta, New York
Lines: 46
Keywords: NRC5380 SCSI WD33C9[23]

In article <472@osupyr.UUCP> czei@osupyr.UUCP (Michael S Czeiszperger) writes:
>
>    What chip are you refering to?   I'm writing code for a custom SCSI
>    controller built from a NCR5380 interfaced to a HD64180 microprocessor.
>    I was aware there was a more sophisticated chip in the works, but
>    didn't know it was being sold at the moment.  (You can't be refering
>    to the 5380 because it most definately doesn't decode command descriptor
>    blocks.)
>
Take a look at the Western Digital WD33C92/93 SCSI Interface Controller. I have
written the code for a SCSI<->VMEbus host adapter using both the NCR5385 (a
little more sophisticated then the NCR5380) and the WD33C92. I would term the
NCR538[05] as first generation chips and the WD33C92 as a 2nd generation.
For example, using the "Select and Transfer" command (assuming you don't handle
multi-threading), you load the command desc. block into the chip, load the
destination target id, then issue the Select and Transfer command. At that
point the WD33C92 will handle ALL the SCSI bus activity (Selection, Command
Out, optional Data In or Out, Message In and finaly Status In). At completion,
the chip interrupts with the SCSI status byte. The 33C92 is used for 
differential interfacing, the 33C93 has the 48MA drivers built in (for single
ended , i.e. Mac).
>>
>
>It seems kind of strange that one protocol could support two different
>physical cables.  According to the ANSI description, a SCSI cable can
>be 50-conductor flat cable, or a 25-signal twisted pair cable.  This
>implies that there must be 50 pin connections.  I wonder why the mac
>has a 25 pin connector?  Are the cable and connector requirements
>of SCSI just suggestions?  (from American National Standard X3.131-
>1986, pgs. 16,19) 
>
>
Reading those pages... "The nonshielded SCSI device connector shall be a
50-conductor connector" etc.,etc. (it also says SHALL be 50 when talking
about the shielded).
It seems Apple took some liberty in cable specs. for the sake of space
limitations. It just means your getting less grounding in the cable (i.e.
don't try running a 25 meter cable between the Mac and drive/controller).

						- jjg

--
John Grana
..!seismo!rochester!cci632!walden!jjg
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