Automating Manufacturing Systems - Process Control and ...

discrete sensors - 4.8
PLC Input Card for Sourcing Sensors
Internal Card Electronics
00
01
com
+V
PNP
-V
PNP
sensor
current flow
+V
power
supply
-V
Note: When we have a PLC input card that has
a common then we can use PNP sensors. In
this case the current will flow into the card
and then out the common to the power supply.
Figure 4.7
PLC Input Card for Sourcing Sensors
The current flow loop for an active sensor is shown with a dashed line. Following
the path of the current we see that it begins at the V+, passes through the sensor, in the
input 00, through the optocoupler, out the common and to the V-.
Wiring is a major concern with PLC applications, so to reduce the total number of
wires, two wire sensors have become popular. But, by integrating three wires worth of
function into two, we now couple the power supply and sensing functions into one. Two
wire sensors are shown in Figure 4.8.