Toshiba T2N - Manual (Page 67)

3.Application Precautions for I/O Modules
52 PROSEC T2N
(5) When applying an AC input signal, if the external cables are long or if the
number of cores of a multi-core cable is large, a current induced from the
charged wires will flow in the open wire, depending on the mutual
capacitance between the cables. Sometimes a voltage may be
generated which reaches the ON level despite the contacts being open.
In this case, the general method is to decrease the input impedance and
lower the input ON level due to the induced current. Install a resistor or a
resistor + capacitor between input and common, or use a multi-core
shielded cable with a small cable capacitance.
Contacts open Current induced from charged wires flow through
cable capacitance
Current induced from charged wires flows through cable capacitance
For the maximum distance of external cable to an AC input module, it is
necessary either to take the above countermeasure within 100m length in
the condition that, out of 20 cores, 19 are charged wires and 1 is an open
wire, or to check whether the input voltage in this condition is less than the
OFF voltage. When handling large numbers of AC input signals,
precautions such as the above are required. Therefore, taking account of
cost of the system as a whole, one method is to study the interface by DC
signals.
(6) When connecting AC output type sensors, sometimes it is not possible to
detect the OFF state due to the leak current when the sensor is OFF.
In this case, counter by installing an external bleeder resistor as shown in
the drawing below.
AC output type
sensor Input
module
Bleeder resistor
Select bleeder resistors using the following points as a guide.
1) When the sensor is OFF, the voltage between the input
terminals must be less than the residual voltage in the OFF state
2) The current when the sensor is ON within the allowable value for
the sensor
3) Determine the wattage of the bleeder resistor by making an
allowance of approximately 3 times the current when the sensor is ON.