SUBROUTINE CF.CALC.LRC(t.message,lrc)
*
*    Application:
*    Subroutine to calculate longtitudinal redundany check for a string
*    representing an APACS credit card transaction X/Y/Z message (the
*    final result of XORing the 7-bit characters comprising the
*    transaction, the same algorithm working for both transaction messages
*    sent and acknowledgements received.
*
*    Limitations:
*    Will only work on platforms which carry raw control characters in 
*    data without substitution or conversion.  Users may encounter spurious 
*    errors in instances where the transaction checksum evaluates to exactly 
*    255 (hex FF) - traditionally the segment mark/item mark character and 
*    used as the internal string terminator on many platforms.  Could also 
*    be vastly more efficient if using the native XOR function available on 
*    some platforms.
*
*    Used in conjunction with credit card terminal automation programs.
*
*    t.message: transaction 
*    lrc      : resultant LRC check character
*               -1 = STX missing
*               -2 = ETX missing
*
*    ========================================================================
*
*    Conditions of use:
*    This source code is the property and copyright of Gulraj Rijhwani and
*    Courtfields Limited.  Specific licence is granted to copy and use this
*    program source, in accordance with the terms and conditions laid out
*    following.  For the purposes of these terms and conditions the term
*    "developer" is defined to be the individual or company employing the
*    author incorporating use of this source.  Any use of this source code is
*    deemed to be full and unreserved acceptance of these stated terms.
*
*    1)  All instances of this source code must carry all attached
*        annotations and comments including, but not limited to, these
*        these conditions of use in their entirety.
*    2)  The developer may not alter the program source in any way, without
*        first obtaining the express written consent of duly authorised
*        company officers of Courtfields Limited, or its successors.
*    3)  This source code may not be included in whole or in part in any
*        proprietary program suite except as a separately compiled module
*        in wholly unmodified form.  Any installed object code must be
*        accompanied by the original source code at all times.
*    4)  In the event of any breach of these terms and conditions the
*        developer shall be liable for an annual usage charge of 200 pounds
*        sterling for every end user terminal or session using or having
*        access to use the resulting executable code.
*    5)  Gulraj Rijhwani and Courtfields Limited make no warranties for
*        the fitness of this source code, and may not be held liable
*        for any failure to fulfil the expectations or requirements of
*        the developer.
*
*    ========================================================================
*
     lrc = -1; * Transaction incomplete - no STX
*
     IF t.message[1,1] = CHAR(2) THEN
*
      lrc = -2; * Transaction incomplete - no ETX
*
      IF t.message[LEN(t.message),1] = CHAR(3) THEN
*
* Curiously the LRC check skips the initial STX character (so so can we),
* but includes the terminating ETX.  Since we know the last character
* will always be ETX and the LRC algortihm is order independent we can
* pre-initialise the final result and skip the final character XOR
* iteration.
*
       no.of.chars = LEN(t.message) - 1
       lrc = 3
*
       FOR c.ptr=2 TO no.of.chars
*
        t.char = SEQ(t.message[c.ptr,1])
        b=1
        old.lrc = lrc
        lrc = 0
*
* This loop performs the actual XOR
*
        FOR bit.count=1 TO 7
         b2 = b + b
         lrc = lrc + REM(old.lrc-REM(old.lrc,b) + t.char-REM(t.char,b),b2)
         b = b2
        NEXT bit.count
*
       NEXT c.ptr
*
       lrc = CHAR(lrc) ;* And convert to ASCII
*
      END
*
     END
*
     RETURN