Bit Addressing - Allen-Bradley Logix5000 Reference Manual

Chapter 2 Bit Instructions (XIC, XIO, OTE, OTL, OTU, ONS, OSR, OSF, OSRI, OSFI)

Bit Addressing

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Bits in a Logix controller exist in one of several forms:
• BOOL tag
• BOOL array
• Bits within an integer (SINT, INT, or DINT) tag
• Bits within an integer array
Any of the above constructs may also exist as members of a structure, whether it's
a system defined data type (SDT) or user-defined data type (UDT).
Bits defined as BOOLs are referenced simply by name. For example, XIC
myBool.
Bits within BOOL arrays are referenced by a subscript appended to the array
name. Subscripts can be a literal value (for example, XIC boolArray[31]), a tag
(for example, XIC boolArray[index]), or an expression (for example, XIC
boolArray[index+1]).
Bits within an integer tag or integer array may be addressed similarly. This is what
Logix often refers to as bit addressing mode for integers. IEC 61131-3 might
refer to it as 'partial access of ANY_BIT variables' . Bit addressing for integers is
programmed by adding a dot (or period) to the integer indicating subelement
processing followed by a subscript (for example, myInteger[3].11 addresses bit 11,
subelement [3]of the myInteger array). There is a shorthand form for literal
subscripts in this case, which is simply a dot followed by the literal value (for
example, myInteger.3, meaning bit 3 of myInteger). Similar to subscripts for
BOOL arrays above, one can specify a tag or expression for the subscript.
Integer arrays are dealt with similarly except that you must specify a member of
the array from which indexing begins. For example, XIC
myIntArray[4].[index+1] says to start at the 5th element of myIntArray and
access bit 'index+1.' Bounds checking is performed on all subscripts to make sure
the reference does not go beyond the boundary of the data object specified.
Literals are checked for conformance during verification.
Rockwell Automation Publication 1756-RM003N-EN-P - October 2011