Allen-Bradley MicroLogix 1400 Instruction Manual page 143

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While the PTO instruction is being executed, status bits and information
are updated as the main controller continues to operate. Because the PTO
instruction is actually being executed by a parallel system, status bits and
other information are updated each time the PTO instruction is scanned
while it is running. This provides the control program access to PTO
status while it is running.
TIP
The charts in the following examples illustrate the typical timing
sequence/behavior of a PTO instruction. The stages listed in each chart
have nothing to do with controller scan time. They simply illustrate a
sequence of events. In actuality, the controller may have hundreds or
thousands of scans within each of the stages illustrated in the examples.
Conditions Required to Start the PTO
The following conditions must exist to start the PTO:
• The PTO instruction must be in an idle state.
• For idle state behavior, all of the following conditions must be met:
– Jog Pulse (JP) bit must be off
– Jog Continuous (JC) bit must be off
– Enable Hard Stop (EH) bit must be off
– Normal Operation (NS) bit must be off
– The output cannot be forced
• The rung it is on must transition from a False state (0) to a True state
(1).
Momentary Logic Enable Example
In this example, the rung state is a momentary or transitional type of
input. This means that the false-to-true rung transition enables the PTO
instruction and then returns to a false state prior to the PTO instruction
completing its operation.
If a transitional input to the PTO instruction is used, the Done (DN) bit
turns on when the instruction completes, but only remains on until the
next time the PTO instruction is scanned in the user program. The
PTO status is only as fresh as the scan time of the controller. Worst case
latency is the same as the maximum scan of the controller. This condition
can be minimized by placing a PTO instruction in the STI (selectable timed
interrupt) file, or by adding PTO instructions to your program to increase
how often a PTO instruction is scanned.
Using High-Speed Outputs
Publication 1766-RM001A-EN-P - October 2008
141