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From: bmw@aesat.UUCP (Bruce Walker)
Newsgroups: net.micro
Subject: Re: \"?Availability of ST500 interface cards for Apple?\"
Message-ID: <381@aesat.UUCP>
Date: Sat, 13-Apr-85 11:48:28 EST
Article-I.D.: aesat.381
Posted: Sat Apr 13 11:48:28 1985
Date-Received: Sat, 13-Apr-85 15:16:57 EST
References: <9861@brl-tgr.ARPA>
Reply-To: bmw@aesat.UUCP (Bruce Walker)
Organization: AES Data Inc., Mississauga Ont., Canada
Lines: 40
Summary:  SASI != ST506


The question was asked: "Is the ST506 bus some kind of standard, and is it
the SASI bus I've heard about".

Well, yes and no.  Yes, the ST506 "bus" is an industry standard for a means
of connecting a hard disk *controller* to a winchester disk drive.  And no,
it is NOT the SASI (now SCSI) interface bus standard (which is a means of
connecting a host computer to a controller; thus a system may contain
*both* busses: HOST <--[SASI]--> CONTROLLER <--[ST506]--> WINCHESTER ).

The ST506 interface is a sort of "souped-up" floppy interface.  The data
format looks a lot like a standard soft-sectored floppy except that the rate
is 5Mbits per second versus the floppy's 125 or 250Kbits/sec.  There are two
cables, a 34 pin cable with slow signals like "head select" and "step" and
a twenty pin cable with the read and write signals transferred using
differential drivers and receivers (for noise immunity and minimal signal
distortion).

The data signals also contain the clock and are encoded in an MFM format.
This means that it is up to the controller to separate the data/clock
stream when reading and precompensate the write data when writing.

Virtually all of the smaller Winchesters (typically 5Mb to 50Mb) adhere to 
the ST506 standard which was set by Seagate with their (you guessed it)
ST506 5Mb mini winchester.  It is sometimes known as the "ST506/412"
intfc as well.

The SASI/SCSI bus is a parallel interconnect scheme where one or more host
systems can talk to one or more peripherals in a generic high level
"language" like: read a/some block(s), write a/some block(s), perform
self-test, etc.  The interface from local processor bus to SASI/SCSI is
called a "host adapter".  The bus itself is a 50 pin ribbon cable which
contains an 8 bit (with 1 parity bit) data bus along with stuff like
REQ(uest) ACK(nowledge), OUT/IN, C(ontrol)/D(ata) etc.

Hope some of this helps!


Bruce Walker     {allegra,ihnp4,linus,decvax}!utzoo!aesat!bmw