Siemens SIMATIC ET 200AL, System Manual

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What you should know about analog technology
2.18 Settling and response times of analog output modules
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
53
Application time
The CPU/IM (interface module) writes new output values to the internal memory of the
analog output module. The time required for this is not determined by the CPU/IM. These
output values are converted asynchronously to their entry in buffer memory. The worst-case
application time (t
X
) may be equal to the cycle time (t
Z
). This situation develops if the CPU
writes a new value to the internal memory of the module immediately after the transfer of the
value for conversion. This new value will not be processed until the next conversion.
The figure below shows acyclic access of the analog output module to output values from
the internal memory in non-isochronous mode. You can find more information on
isochronous mode in the paragraph on isochronous mode of the section High-Speed analog
modules (Page 113).
t
X
Application time
t
1
New digital output value is available in the internal memory of the module
t
2
Module activates the digital output value and starts conversion
t
3
Module ends conversion at the terminal of the analog output channel and outputs the analog signal
Figure 2-23 Application of the output values
Conversion time
The conversion time of an output channel starts with the transfer of the digital value from the
internal memory of the module (t
2
) and ends with the digital-to-analog conversion (t
3
).
Settling time
The settling time starts when the analog signal is output to the module (t
3
) terminals and
ends when the output value (t
4
) is reached. The output value is reached as soon as the
output signal has settled at its final value, taking into account a residual error of 1%.
The settling time depends on the type of output for the module, and on the connected load.
The load for the analog output consists of connecting line and connected actuator. When the
output is "Current", the settling time increases with high ohmic resistance. Inductive loads
cause subsequent fluctuation in the output value. When the output is "Voltage", the
capacitive load causes subsequent fluctuation.