Siemens LOGO! Series Manual page 50

LOGO! installation and wiring
2.3 Wiring LOGO!
Sensor connections
Connecting glow lamps and 2-wire proximity switches (Bero) to LOGO!
230RCE/230RC/230RCo or LOGO! DM8 230R (AC) and LOGO! DM16 230R (AC)
The figure below shows how you connect a switch with a glow lamp to LOGO! The current
that flows through the glow lamp allows LOGO! to detect a "1" signal even though the switch
contact is not closed. If, however, you use a switch the glow lamp of which is fitted with a
power supply, this response does not occur.
Take into account the quiescent current of any 2-wire proximity switches used. The level of
the quiescent current of some 2-wire proximity switches is high enough to trigger a logical "1"
signal at the LOGO! input. You should therefore compare the quiescent current of the
proximity switches with the technical data of inputs (Page 319).
Remedy
To suppress this response, use an X capacitor rated at 100 nF and 2.5 kV. In a destructive
situation, this type of capacitor safely disconnects. You must choose the voltage level for
which the capacitor is rated such that it is not destroyed in the case of overvoltage!
At 230 V AC, the voltage between N and an input I(n) must not be greater than 40 V to
guarantee a "0" signal. You can connect approximately ten glow lamps to the capacitor.
Restrictions
Signal status transitions 0 → 1 / 1 → 0:
After a 0 to 1 or 1 to 0 transition, the signal must remain constant at the input at least for the
duration of one program cycle, so that LOGO! can detect the new signal status.
The program execution time is determined by the size of the circuit program. The appendix
"Determining the cycle time (Page 349)" contains a benchmark test routine that you can use
to determine the current scan cycle time.
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Manual, 04/2011, A5E03556174-01
LOGO!