NPN Sensor and PLC Connection
We can connect a NPN sensor with PLC as in figure
The PNP, or sourcing output, has output logic levels that switch between the sensors
power supply voltage and an open circuit. In this case, as illustrated in figure 3-19, the PNP
output transistor has the emitter connected to Vcc and the collector uncommitted. When the
output is connected to a grounded load, the transistor will cause the load voltage to be either zero (when the transistor is off) or approximately Vcc (when the transistor is on).
PNP Sensor & PLC Connection
We can connect a PNP sensor with PLC as in figure 3-20.
Since discrete PLC inputs can be either sourcing or sinking, it is important to know
how to select the sensor output type that will properly interface with the PLC input, and how
to wire the PLC input so that it will interface properly to the sensor. Generally speaking,
sensors with sourcing (PNP) outputs should be connected to sinking PLC inputs, and sensor with sinking (NPN) outputs should be connected to sourcing PLC inputs.
For sourcing (PNP) sensor outputs, the PLC input circuit is wired with the common
terminal connected to the common of the sensor as shown in figure 3-21. When the PNP
transistor in the sensor is off, no current flows between the sensor and the PLC, and the PLC
input will be OFF. When the sensor circuitry switches the PNP transistor ON, current flows
from the Vcc power supply, through the PNP transistor, through the IN0 opto-isolator in the
PLC input, and out of the common terminal to return to the negative side of the power
supply. In this case, the PLC input will be ON. For this type of connection, the value of the
Vcc voltage must be at least high enough to satisfy the minimum input voltage requirement
for the PLC inputs. Notice the simplicity of the connection scheme. The sensor connects
directly to the PLC input. No other external signal conditioning circuitry is required
sink type(0V connected with common of input card) as shown in figure
We can also connect a sinking (NPN) sensor output to the same PLC input. However, since
the sensor has a sinking output, the PLC must be rewired as a sourcing input. This can be
done by disconnecting the common terminal of the PLC input from the negative side of Vcc
and connecting it to the positive side of Vcc as shown in fig 3-23. this connection scheme
converts all of the PLC inputs to sourcing; that is, in order to switch the PLC input ON, we
must draw current out of the input terminal. In operation, when the NPN transistor in the
sensor is OFF, no current flows between the sensor and PLC. However, when the NPN
transistor switches ON, current will flow from the positive side of the Vcc supply, into the
common terminal of the PLC, up through the opto-isolator, out of the PLC input terminal
IN0, and through the NPN transistor to ground. This will switch the PLC input ON. If it is
necessary to operate the PLC inputs and the sensor from separate power supplies, it is
permissible as long as the negative terminals of both power supplies are connected together.
Similarly other input devices like switches can connect with Digital Input Card when PLC is
Similarly other input devices like switches can connect with Digital Input Card when PLC is
source type (24V connected with common of input card) as shown in figure 3-24.
