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From: FSTKKVPG@UIAMVS.BITNET
Newsgroups: comp.sys.apple
Subject: (none)
Message-ID: <8710171413.aa13271@SMOKE.BRL.ARPA>
Date: Sun, 18-Oct-87 20:24:45 EDT
Article-I.D.: SMOKE.8710171413.aa13271
Posted: Sun Oct 18 20:24:45 1987
Date-Received: Tue, 20-Oct-87 01:07:02 EDT
Sender: daemon@ucbvax.BERKELEY.EDU
Organization: The ARPA Internet
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    The reason why 5.25" drives can only be daisy-chained onto the back
of 3.5" drives on the //c is due to hardware differences.  On the //c, the
external drive connector is a dual-purpose port, for both a generic (dumb)
5.25" drive, and the intelligent 3.5" UniDisk.
--
    On coldstart, the SmartPort controller goes down the chain of intelligent
devices assigning a device number to each one it finds.  These device numbers,
obviously, are unique.
--
    When information is passed down by the SmartPort, the device number to
which the information belongs is included in with the data packet, and any
device preceding it just passes it thru, instead of keeping it.  This is what
happens when the P6 Prom for the 5.25" drive takes control of the port for
use by the 5.25" - no device number is passed, so none of the 3.5" drives
attempt to act upon the information.
--
    On a //e, the primary difference is that the P6 Prom is not included
in the chain, but instead is on the 5.25" controller.
--
---------------
--
    Not the best explanation, but hopefully it explains well enough...
--
                                            --Kevin Trojanowski
                                              [FSTKKVPG@UIAMVS]