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Path: utzoo!utgpu!jarvis.csri.toronto.edu!cs.utexas.edu!usc!zaphod.mps.ohio-state.edu!mips!apple!netcom!allanh
From: allanh@netcom.UUCP (Allan N. Hessenflow)
Newsgroups: comp.misc,comp.sys.mac.programmer,comp.graphics
Subject: crc functions
Keywords: ccitt
Message-ID: <8548@netcom.UUCP>
Date: 7 Mar 90 00:31:05 GMT
Followup-To: comp.sys.mac.programmer
Organization: NetCom- The Bay Area's Public Access Unix System {408 249-0290 guest}
Lines: 16


I've been using a crc routine from a document by Stephen Satchell which
is supposed to be the same one used in xmodem.  I seem to remember reading
about the properties of this crc a long time ago that it would catch all
single bit errors and all double bit errors for messages < n bytes long,
where n is significantly larger than 6.  However, I've recently discovered
that this is not the case.  Specifically, 
$41 76 03 01 00 09 has a crc of 37fb, while
$41 76 03 01 00 08 has a crc of 37fa.
Is this normal for the CCITT CRC, or is the function I'm using faulty?  I'm
fairly certain it's not a coding/translation error, as I've tried two
different functions from the same document with the same results.


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