PTO [Pulse Train Output] Instruction in Allen Bradley PLC

[Pulse Train Output] Instruction in Allen Bradley PLC

MicroLogix 1200 and MicroLogix 1500        
of Instruction

Use this instruction to control and monitor the pulse train output
function, which in turn controls the physical PTO output. The operation of the
PTO function is controlled by a state machine. The rung state when the instruction
is executed is only one of several inputs used to control this state machine,
the other being the state of the enable bit. The frequency that appears at the
physical output only changes at a pulse boundary. This means that the ramp at
the physical output during the Acceleration and Deceleration phases are
dependent on the processor scan time. For this reason we recommend operating
the PTO from the STI user interrupt.


executed on a true rung, the PTO instruction sets (1) the EN bit and executes
the state machine.
executed on a false rung, the PTO instruction resets (0) the EN bit and
executes the state machine.
The EN, CO, PO, NO, ER, ID, AS, RS, DS and DN Status/Control bits
are reset.
The Error Code is set to zero.
The Current Frequency and Current Position are set to zero.
The state machine are initialized so that the Pre-Idle state is
entered the first time a PTO instruction is executed.
Zone Execution
an active MCR zone, the PTO operates as if the rung state were false. 
 Entering Parameters
 Enter a 0 or a 1 for the PTO number. The PTO status and control
information is contained in a Pulse Output (PO) control file or (PTO) function
file. A list of equirements regarding the use of the PTO follows:
You can assign outputs O:0/2 and O:0/3 to any of the PTO or PWM
functions but, the same output cannot be assigned to more than one function.
The PTO number also designates which PTO control file is used by
the instruction.
There is no restriction on the number of PTO instructions that can
be in your program. Each time one of the instructions is executed, it updates
the PTO function as described by the PTO state machine.
The output image for the output controlled by the PTO function is
updated to reflect the state of the physical output every time that the PTO
instruction is evaluated.
If the PTO function is being used, the output image for the output
controlled by the PTO function is not transferred to the physical output. This
constraint also applies to immediate outputs. The PTO function is being used if
and only if a transition out of the Pre-Idle state is made. For example, a PTO
instruction may be contained in a subroutine or interrupt file that is never
executed, in this case the PTO function is not used.
Immediate outputs update the output image (but not the physical
If the output controlled by the PTO function is forced, the force
overrides the PTO and the PTO errors out. Some of the errors generate a
recoverable major fault (code 003B). Click here for a list of faults which may
be generated.
When the PTO is in the Idle or Error state, the output controlled
by the PTO is always off (unless a force overrides the physical output.)
If the PTO instruction is being used when a power fail occurs, the
corresponding output image are set to zero.
Note:  A maximum of two elements can be configured
for the PTO and PWM files combined.
Source:Rockwell Softwar Inc.

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