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From: daveh@cbmvax.UUCP (Dave Haynie)
Newsgroups: comp.sys.amiga
Subject: Re: mount vs. 3.5" drives & diskchange
Message-ID: <2165@cbmvax.UUCP>
Date: Wed, 29-Jul-87 15:27:19 EDT
Article-I.D.: cbmvax.2165
Posted: Wed Jul 29 15:27:19 1987
Date-Received: Fri, 31-Jul-87 05:45:51 EDT
References: <641@neoucom.UUCP>
Organization: Commodore Technology, West Chester, PA
Lines: 38

in article <641@neoucom.UUCP>, wtm@neoucom.UUCP (Bill Mayhew) says:
> Summary: Drives have a PAL chip
> 
> There is a PAL chip on the drive interface board.  It does at least
> three things I know of.

Actually, it's usually an open-collector NAND gate and a D-type flip
flop.  The original external 3.5" drive uses a 74LS38 and a 74LS74 
for this function; the 3.5" drive has the simplest ID code, which is
produced on the RDY line by the flip flop.  The 5.25" drive has an
additional chip, the 74LS132, which is another 2 input x 4 device NAND,
but with schmitt inputs.  This drive, being self-powered, also has a
voltage sensor that gates the Read/Write line to the floppy disk, so
that if the host Amiga is turned off, no accidental writes will take
place.  All the functional descriptions listed look OK, and I guess they
could even be implemented with a PAL, though it would cost much more than
a pair of TTL parts.

> Note that the little 1040 (or whatever the # is) 3.5 inch
> drives block DC power to subsquent drives in the string.  The
> purpose being to prevent some idiot from plugging in 18 drives and
> sending his/her Amiga's power supply into a core meltdown.  

The 1010 drives actually pass +5V, but not +12V.  The +12V supply is used
on most of the 3.5" and 5.25" drives as a motor supply.  All of the logic
on both drives can be drive only with +5V.  That's why, if you hook up a
second external 3.5" drive with no +12V supply, the Amiga OS will recoignze
the drive, but list it as BAD.  Also, the passing of +5V is important to
self powered drives like the 1020 (5.25").  If the power isn't passed, the
drive will think that the host Amiga system is shut off, and it will not
let WRITE be gated through.n

> Bill

-- 
Dave Haynie     Commodore-Amiga    Usenet: {ihnp4|caip|rutgers}!cbmvax!daveh
"The A2000 Guy"                    PLINK : D-DAVE H             BIX   : hazy
     "Catch a wave and you're sittin' on top of the world" -Beach Boys