Siemens SIMATIC ET 200AL, System Manual

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High-speed analog modules
8.2 ST modules vs HS modules
Analog value processing
Function Manual, 06/2014, A5E03461439-AC
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Effect of the cable length on the settling time
Long cables between signal source and module are a target for interferences. You should
therefore keep the cables as short as possible especially for HS modules. Cables have a
capacitive and inductive component and therefore have an effect on the settling behavior of
the signal.
The example below shows the jump of the output voltage of an HS output module with small,
medium and long cable length. The overshoot of the input signal increases with the length of
the cable and therefore the time until it reaches the setpoint.
t
E
Settling time
t
1
Module ends conversion at the terminal of the analog output channel and outputs the analog
signal
t
2
The signal running through a cable with short length has settled and the specified analog
output value has been reached (residual error 1 %)
t
3
The signal running through a 20 m cable has settled and the specified analog output value has
been reached (residual error 1 %)
t
4
The signal running through a 200 m cable has settled and the specified analog output value
has been reached (residual error 1 %)
Figure 8-8 Effect of different cable lengths on the settling time