Check digits are digits added to the end of a value that contains
only numbers (0-9) or letters of the alphabet (A-Z). A FORMSPEC
option can be selected that causes a modulus check to be made on
a value entered at the terminal.
Whenever a value is entered for which check-digit verification
has been specified, a modulus calculation is performed. The result
of a modulus calculation is a single digit that must match the last
digit (the "check digit") of the entered value. If the match is
not successful, an error is diagnosed. Note that for modulus checks
to be meaningful, the last digit verified by such a check must have
been calculated by the same method used for the check.
REFSPEC has an option that lets you add a check digit to an
entered value. The check digit is added when REFORMAT is run. Thus,
the check digit is added after the value is entered at the terminal
but before it is input to the application program. In this case,
the modulus calculation is made programmatically.
The modulus calculations used by VPLUS/V are either modulus
10 or modulus 11. A value is checked by one or the other of these
calculations, depending on which was used to add the check digit.
Thus, a value whose check digit was added using modulus 10 can be checked
only by modulus 10; and a value with a modulus 11 check digit can
be checked only with modulus 11 calculations.
