Chapter 4. Ladder Diagram (LD) Programming
GFK-2950C February 2018 105
Example - Do I/O for Inputs
When DO_IO receives power flow, the PLC scans references
%I0001—64 and %Q0001 is turned on. A copy of the scanned
inputs is placed in internal memory from %M0001-64. Because a
reference is specified for ALT, the real inputs are not updated.
This allows the current values of inputs to be compared with
their values at the beginning of the scan. This form of DO_IO
allows input points to be scanned one or more times during the
program execution portion of the CPU scan.
Because a reference is entered for ALT, the values at %AQ001—
004 are not written to output modules. When DO_IO receives
power flow, the PLC writes the values from references %R0001-
0004 to the analog output modules and %Q0001 is turned on.
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