Index Registers; What Are Index Registers; Using Index Registers - Omron CJ - PROGRAMMING MANUAL 08-2008 Programming Manual

Index Registers

6-2

Index Registers

6-2-1

What Are Index Registers?

6-2-2

Using Index Registers

1,2,3...
Index Registers function as pointers to specify PLC memory addresses,
which are absolute memory addresses in I/O memory. After storing a PLC
memory address in an Index Register with MOVR(560) or MOVRW(561),
input the Index Register as an operand in other instructions to indirectly
address the stored PLC memory address.
The advantage of Index Registers is that they can specify any bit or word in
I/O memory, including timer and counter PVs.
MOVR(560)
Index Registers can be a powerful tool when combined with loops such as
FOR-NEXT loops. The contents of Index Registers can be incremented, dec-
remented, and offset very easily, so a few instructions in a loop can process
tables of consecutive data very efficiently.
Indirect
addressing
Basic Operation
Basically, Index Registers are used with the following steps:
1. Use MOVR(560) to store the PLC memory address of the desired bit or
word in an Index Register.
2. Specify the Index Register as the operand in almost any instruction to in-
directly address the desired bit or word.
3. Offset or increment the original PLC memory address (see below) to redi-
rect the pointer to another address.
4. Continue steps 2 and 3 to execute the instruction on any number of ad-
dresses.
Offsetting, Incrementing, and Decrementing Addresses
The following table shows the variations available for indirect addressing.
Variation
Indirect addressing
Indirect addressing with constant offset
Pointer
Index Register
Increment IR0 and repeat
instruction execution
,IR@
Constant ,IR@
(Include a + or – in the constant.)
Section 6-2
All areas of
I/O Memory
Table data
Syntax
281