Automating Manufacturing Systems - Process Control and ...

discrete sensors - 4.7
PLC Input Card for Sinking Sensors
Internal Card Electronics
PLC Data Bus
+V
00
current flow
+V
NPN
-V
NPN
sensor
+V
power
supply
-V
01 Note: When a PLC input card does not have a
common but it has a V+ instead, it can be
used for NPN sensors. In this case the current
will flow out of the card (sourcing) and
External Electrical we must switch it to ground.
ASIDE: This card is shown with 2 optocouplers (one for each output). Inside these
devices the is an LED and a phototransistor, but no electrical connection. These
devices are used to isolate two different electrical systems. In this case they protect
the 5V digital levels of the PLC computer from the various external voltages
and currents.
Figure 4.6
A PLC Input Card for Sinking Sensors
The dashed line in the figure represents the circuit, or current flow path when the
sensor is active. This path enters the PLC input card first at a V+ terminal (Note: there is
no common on this card) and flows through an optocoupler. This current will use light to
turn on a phototransistor to tell the computer in the PLC the input current is flowing. The
current then leaves the card at input 00 and passes through the sensor to V-. When the sensor
is inactive the current will not flow, and the light in the optocoupler will be off. The
optocoupler is used to help protect the PLC from electrical problems outside the PLC.
4.7.
The input cards for PNP sensors are similar to the NPN cards, as shown in Figure