Index Register [V And Z]; Numbers Of Index Registers; Functions And Structures - Mitsubishi Electric MELSEC FX3U Programming Manual

FX /FX /FX Series Programmable Controllers
3G 3U 3UC
Programming Manual - Basic & Applied Instruction Edition
4.11

Index Register [V and Z]

Index registers can be used in the same way as of data registers. But they are special registers since they
can change the contents of device numbers and numeric values by program when combined with another
device number or numeric value in operands of applied instructions.

4.11.1 Numbers of index registers

The table below shows numbers of index registers (V and Z). (Numbers are assigned in decimal.)
When only "V" or "Z" is specified, it is handled as "V0" or "Z0" respectively.
Index type
V0 (V) to V7, Z0 (Z) to Z7
16 points
*1. The characteristics related to protection against power failure cannot be changed by parameters.

4.11.2 Functions and structures

1. 16-bit type
Index registers have the same structures as data registers.
16 bits
V
V0 to V7:
8 points
2. 32-bit type
Make sure to use Z0 to Z7 when indexing a device in a 32-bit applied instruction or handling a numeric value
outside the 16-bit range.
V0 (high-order side)
V1 (high-order side)
V2 (high-order side)
V3 (high-order side)
V4 (high-order side)
V5 (high-order side)
V6 (high-order side)
V7 (high-order side)
150
*1
16 bits
Z
Z0 to Z7:
8 points
32 bits
Z0 (low-order side)
Z1 (low-order side)
Z2 (low-order side)
Z3 (low-order side)
Z4 (low-order side)
Z5 (low-order side)
Z6 (low-order side)
Z7 (low-order side)
This is because FX PLCs handle Z as the low-order side of a 32-bit
register as shown in combinations of V and Z in the figure on the left.
Even if V0 to V7 on the high-order side is specified, indexing is not
executed.
When index registers are specified as a 32-bit device, both V (high-
order side) and Z (low-order side) are referred to at the same time.
If a numeric value for another purpose remains in V (high-order side),
consequently the numeric value here becomes extremely large, thus
an operation error occurs.
Example of writing to 32-bit index registers
FNC 12
K300
DMOV
K300→(V2,Z2)
Even if an index value in a 32-bit applied instruction does not exceed
the 16-bit numeric range, use a 32-bit operation instruction such as
DMOV for writing a numeric value to Z as shown in the above figure
so that both V (high-order side) and Z (low-order side) are overwritten
at the same time.
4 Devices in Detail
4.11 Index Register [V and Z]
Z2