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Index Registers Section 9-17
Indirect Addressing When an Index Register is used as an operand with a “,” prefix, the instruction
will operate on the word indicated by the PLC memory address in the Index
Register, not the Index Register itself. Basically, the Index Registers are I/O
memory pointers.
• All addresses in I/O memory (except Index Registers, Data Registers, and
Condition Flags) can be specified seamlessly with PLC memory
addresses. It isn’t necessary to specify the data area.
• In addition to basic indirect addressing, the PLC memory address in an
Index Register can be offset with a constant or Data Register, auto-incre-
mented, or auto-decremented. These functions can be used in loops to
read or write data while incrementing or decrementing the address by one
each time that the instruction is executed.
With the offset and increment/decrement variations, the Index Registers can
be set to base values with MOVR(560) or MOVRW(561) and then modified as
pointers in each instruction.
Note It is possible to specify regions outside of I/O memory and generate an Illegal
Access Error when indirectly addressing memory with Index Registers. Refer
to
Appendix D Memory Map of PLC Memory Addresses for details on the lim-
its of PLC memory addresses.
I/O Memory
Pointer
Set to a base value
with MOVR(560) or
MOVRW(561).
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