Siemens Simatic S7 Series System Manual

Program instructions
7.7 Pulse output
S7-200 SMART
System Manual, 09/2015, A5E03822230-AC
257
The PTO generator performs repeated additions to the working frequency to create a linear
change in frequency over time. The constant value added to the frequency has a limited
resolution. This limited resolution can introduce some truncation error into the resulting
frequency. Thus, the PTO generator does not guarantee that the pulse train frequency can
reach the ending frequency that was specified for that segment. In the figure below, you can
see that the truncation error affects the accelerating PTO frequency. The output should be
measured to verify that the frequency is within an acceptable frequency range.
①
Desired frequency plot
②
Actual frequency plot
If the frequency difference (Δf) between the end of a segment and the beginning of the next
is not acceptable, try adjusting the ending frequency to compensate for the difference. This
adjustment might be an iterative process to get the output within an acceptable frequency
range.
Note that changes in segment parameters affect the time it takes the PTO to complete. You
can use the equation for the time duration of the segment, found later in this section of the
manual, to see what effect this has on the timing. An accurate segment duration time can
require some flexibility in the value of the ending frequency or the number of pulses for a
given segment.
While the simplified example above is useful as an introduction, real applications can require
more complicated waveform profiles. Remember that you can only assign frequencies as an
integer number of Hz and perform the frequency modification at a constant rate. The
S7-200 SMART CPU selects that constant rate and that rate can be different for each
segment.
For legacy projects that were developed in terms of cycle time, instead of frequency, you can
use the following formulas to convert to frequency:
CT
Final
= CT
Initial
+ (ΔCT * PC)
F
Initial
= 1 / CT
Initial
F
Final
= 1 / CT
Final
where:
CT
Initial
Starting cycle time (s) for this segment
ΔCT
Delta cycle time (s) of this segment
PC
Quantity of pulses in this segment
CT
Final
Ending cycle time (s) for this segment
F
Initial
Starting frequency (Hz) for this segment
F
Final
Ending frequency (Hz) for this segment

Siemens Simatic S7 Series Specifications

General

CPU	Varies by model (e.g., S7-300, S7-400, S7-1200, S7-1500)
Communication Interfaces	PROFIBUS, PROFINET, Ethernet, MPI
Programming	TIA Portal, STEP 7
Programming Languages	LAD, FBD, STL, SCL, GRAPH
I/O Modules	Digital, Analog, Specialty modules available
Operating Temperature	0°C to 60°C (standard models)
Mounting	DIN rail mounting
Operating Voltage	24V DC or 120/230V AC (varies by model)
Protection Class	IP20 (standard models)
Memory	Varies depending on the S7 model. Includes work memory (RAM) for program execution, load memory (EEPROM or Flash) for program storage, and system memory for operating system functions. Memory sizes range from kilobytes to megabytes.