Date   : Sun, 07 Oct 1984 00:13:01 PDT (Sun)
From   : Alastair Milne <milne@Uci-750a.ARPA>
Subject: Re: Rainbow 100 reads IBM diskettes

    It is true that the Rainbow's drives can be configured in software to read
discs of a variety of formats, including IBM's 8 sector-per-track format,
which they seem to read quite reliably (once correctly set).

    ** HOWEVER .... **

    I *STRONGLY* suggest *NOT* writing to IBM PC formatted discs with 
the Rainbow 100 or the Rainbow 100+.  Our group regularly uses both machines, 
and we need to move text files between them.  We soon found that letting the 
Rainbow write to IBM discs results in areas on the disc which are unreadable 
to the IBM ("bad blocks" under the UCSD p-System).  Massive writes, such as 
moving files around on the disc, "seduce" the disc: the IBM will never read it 
again (except to format it): only the Rainbow will be able to use it.

    I know of two possible reasons for this:

    - number of tracks per inch: the IBM uses 48 TPI, the Rainbow 96 TPI
       (lets them get 800 UCSD blocks on one side of the disc).  The tracks
       that the Rainbow writes are too thin for the IBM.
    - number of parameters that software can set vs. all the parameters 
      that the drives actually use:  I do not believe that enough parameters
        are provided to let the software give an accurate description of the
       drive to be simulated.  One can, for instance, get close enough to the
       IBM's format to permit (reasonably) reliable reading; but not reliable
       writing.

    I am no expert in disc drive technology, so I may be quite wrong about
both of these.  I would be interested to hear from people who know better.

                               A. Milne

                               Educational Technology Center, UC Irvine

PS.  Users of the p-System on the Sage, take note: SageUtil can be used to 
     configure the drives so that they write to Rainbow discs very reliably.
     I find this very useful when doing work for the Rainbow because of the 
     much greater speed of the Sage.
     Does that fact that the Sage drives are also 96 tracks per inch have
     anything to do with this?

                                       agmm