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Chapter 4. Ladder Diagram (LD) Programming
Do I/O for Inputs
When DO_IO receives power flow and input references are specified, the PLC scans input points
from the starting reference (ST) to the ending reference (END). If a reference is specified for ALT, a
copy of the new input values is placed in memory beginning at that reference, and the real input
values are not updated. ALT must be the same size as the reference type scanned. If a discrete
reference is used for ST and END, ALT must also be discrete.
If no reference is specified for ALT, the real input values are updated. This allows inputs to be
scanned one or more times during the program execution portion of the CPU scan.
Do I/O for Outputs
When DO_IO receives power flow and output references are specified, the PLC writes to the output
points. If no value is specified in ALT, the range of outputs written to the output modules is specified
by the starting reference (ST) and the ending reference (END). If outputs should be written to the
output points from internal memory other than %Q or %AQ, the beginning reference is specified for
ALT and the end reference is automatically calculated from the length of the END—ST range.
Operands
Parameter Description
ST
The starting address of the set of input or output points or words
to be serviced. ST and END must be in the same memory area.
If ST and END are placed in BOOL memory, ST must be byte-
aligned. That is, its reference address must start at (8n+1),
for example, %I01, %Q09, %Q49.
If ST and END are mapped to analog memory, they can have
the same reference address.
If ST is mapped to an I/O variable, the same I/O variable must
also be assigned to the END parameter, and the entire
module is scanned.
END
The address of the end bit of input or output points or words to
be serviced. Must be in the same memory area as ST.
If ST and END are placed in BOOL memory, END's reference
address must be 8n, for example, %I08, %Q16.
If ST and END are mapped to analog memory, they can have
the same reference address.
If ST is mapped to an I/O variable, the same I/O variable must
also be assigned to the END parameter, and the entire
module is scanned.
ALT
For an input scan, ALT specifies the address to store scanned
input point/word values. For an output scan, ALT specifies the
address to get output point/word values from, to send to the I/O
modules.
Note:
ALT can be a WORD only if ST and END are in analog
memory.
104
PACSystems* RX7i, RX3i and RSTi-EP CPU Programmer's Reference Manual
Allowed Operands Optional
I, Q, AI, AQ, I/O
No
Variable
I, Q, AI, AQ, I/O
No
Variable
I, Q, M, T, G, R, AI, AQ Yes
GFK-2950C
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