Siemens SIMATIC S7 System Manual page 291

The following conditions set the bits of the status byte (SMB66, SMB76, and SMB566):
● If an "Add Error" occurs in the pulse generator that results in an invalid frequency value,
the PTO function terminates and the Delta Calculation Error bit (SM66.4, SM76.4, or
SM566.4) is set to 1. The output reverts to image register control. To correct this issue,
try adjusting the PTO profile parameters.
● Manually disabling a PTO profile in progress sets the PTO Profile Disabled bit (SM66.5,
SM76.5, or SM566.5) to 1.
● The PTO/PWM overflow/underflow bit (SM66.6, SM76.6, or SM566.6) is set to 1 if either
of these situations occur:
– An attempt is made to load the pipeline while it is full; this is an overflow condition.
– A PTO profile segment is too short to allow the CPU to calculate the next segment,
● You must clear the PTO/PWM overflow/underflow bit manually after it is set to detect
subsequent overflows. The transition to RUN mode initializes this bit to 0.
Note
• Ensure that you understand the definition of the PTO/PWM mode select bit (SM67.6,
SM77.6, and SM567.6). The bit definition may not be the same as some legacy products
that support a Pulse instruction. In the S7-200 SMART, the user selects PTO or PWM
mode with the following definition: 0 = PWM, 1 = PTO.
• When you load a cycle time/frequency (SMW68, SMW78, or SMW568), pulse width
(SMW70, SMW80, or SMW570), or pulse count (SMD72, SMD82, or SMD572), also set
the appropriate update bits in the control register before you execute the PLS instruction.
• For a multiple segment pulse train operation, you must also load the starting offset
(SMW168, SMW178, or SMW578) of the profile table and the profile table values before
you execute the PLS instruction.
• If you attempt to change the time base of a PWM output while the PWM is executing, the
request is ignored and a non-fatal error (0x001B - ILLEGAL PWM TIMEBASE CHG) is
created.
S7-200 SMART
System Manual, V2.3, 07/2017, A5E03822230-AF
and an empty pipeline is transferred; this is an underflow condition, and the output
reverts to image register control.
Program instructions
7.7 Pulse output
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