Date   : Tue, 19 Feb 1991 00:16:24 GMT
From   : uhccux!uhheph.phys.hawaii.edu!ralph@ames.arc.nasa.gov (Ralph Becker-Szendy)
Subject: Using 3.5" floppies instead of 8" floppies

In article <1991Feb11.092925.46847@cc.usu.edu> slsw2@cc.usu.edu writes:
... stuff deleted ...
>3.5" drives use CMOS drivers for the signals that they drive, and therefore
>cannot drive the terminators generally used for 5.25" and 8" drives. The
>data sheets for the Intel 82077A therefore recommend that if you are going to
>mix 3.5" drives with 5.25" drives, you include a buffer between the 3.5" and
>5.25" drives so that the drive doesn't have to try to drive the 5.25"
>terminator.
... stuff deleted ...

Maybe sometimes true. But not always. I was worrying about the termination 
problems when I first added my new 3.5" drive to the existing system. But it 
turns out to work just fine with a Sony 3.5" drive. Currently my setup is: 
Controller in the middle of two cables. On one side about 10' twisted-pair 
cable to an 8" drive, properly terminated at the drive. On the other side 
maybe 2' of regular flat cable to a 3.5" drive and two 5.25" drives, again 
properly terminated at the last 5.25" drive. I know it shouldn't work, it is 
doubly terminated, and NONE of the drive outputs should handle the load of
two terminators, least of which the 3.5" drive. On the other hand, CMOS output
drivers (see 74HCxxx) are very good at driving resistive loads, sometimes I get
the feeling the are more reliable at that than LS drivers. So trial and error
is in order.

-- 
Ralph Becker-Szendy                          UHHEPG=24742::RALPH (HEPNet,SPAN)
University of Hawaii                              RALPH@UHHEPG.PHYS.HAWAII.EDU
High Energy Physics Group                                  RALPH@UHHEPG.BITNET
Watanabe Hall #203, 2505 Correa Road, Honolulu, HI 96822         (808)956-2931


End of INFO-CPM Digest V91 Issue #39
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