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Chapter 4. Ladder Diagram (LD) Programming
Examples Comparing PTCON and POSCON
PTCON
The logic in the following example starts execution with all variables set to 0. Before the second
sweep begins, the Xsition variable used on the PTCON instruction is set to 1. It retains that value for
sweeps 2, 3, and 4. Then it is reset back to 0 before sweep 5 begins and retains its 0 value for sweeps
5, 6, and 7. This pattern repeats. The PTCON instruction in rung two passes power flow on the 2nd
sweep, the 8
th
sweep, the 14
th
sweep, and so on. These are sweeps where the Xsition variable's value
becomes a 1, after having been a 0 on the previous sweep. On all other sweeps, the PTCON
instruction does not pass power flow.
POSCON
If a POSCON is used in place of the PTCON in the following example (keeping the rest of the logic
identical), the same alternation of the Xsition variable's value occurs. The POSCON instruction
passes power flow on sweeps 2, 3, and 4; then again on sweeps 8, 9, and 10; and so forth. The
POSCON's behavior is dependent on Xsition's transition bit. Since Xsition's value is written once and
then simply retained for three sweeps, its transition bit retains its same value for three sweeps. Thus
the POSCON will pass or not pass power flow for three sweeps in a row. Note that if Xsition's value is
actually written on each sweep, the POSCON and the PTCON behave identically.
Logic Example Using PTCON
th
On the 2nd sweep, turn Xsition ON for 3 sweeps; on the 5
sweep, turn it OFF for 3 sweeps, etc.
GFK-2950C
February 2018
101
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