How to Wire Discrete DC Sensors to PLC – Part 2


In this video, you will learn
how to wire a 3-wire DC sensor, like an inductive proximity
sensor, to a PLC input card. You will also learn what a
3-Wire Discrete sensor is and recognize some of the common
types of 3-Wire Discrete sensors. Before we start this video, I have a favor
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sent to you as quickly as it is published. Now let’s get into the video. DC sensors can be used to indicate the state
of a device or a process to the PLC program. Knowing these input states can allow
the PLC program to make decisions, such as, when to start or
stop a conveyor motor. In this diagram, the
Programmable Logic Controller or PLC, is running a ladder logic program. The inductive proximity switch is
connected to a PLC input card, and the logic program is written
to sense the state of the switch. For instance, when a box is placed on
a conveyor at the “loading” position, the proximity switch senses
the presence of the box and commands the conveyor motor to start to transport the box
to the next inspection station. So, let’s see how we can make this important discrete sensor input information
available to a PLC program. In order to do this, we need to understand
more about these discrete sensors and how they are wired to the PLC. Discrete DC Sensors, or discrete
Direct Current sensors, operate in a circuit with a
battery or other power source, commonly known as a power supply. A power supply is placed in an electrical circuit to provide power to the connected devices, like proximity switches, or a motor. DC power supplies are referred to
by their voltage and capacity, indicated by how many
amperes they can supply. The more amperes a power
supply can deliver, the larger and more expensive
they generally are. In DC PLC circuits, the power supply almost always supplies 24 Volts
DC to the connected devices. In this diagram, a DC circuit
is shown which turns on a motor when a proximity switch
senses the presence of a part or an object in a specific location. The common trait of all discrete DC sensors is that they have exactly two
operating states, “on” and “off”. A discrete DC switch, as in this diagram, is “on” when an object is in proximity to the
switch and its internal contact is “closed”, forming a complete circuit and
allowing the flow of electrons. When the object is moved away
from the proximity sensor, its contacts are “opened”
again, the circuit is broken, and the flow of electrons stops. The state of the switch is “off”. Other common examples
of discrete DC Sensors include many other types
of proximity sensors, level switches, pressure switches,
and photoelectric sensors. If we want to monitor the state
of a proximity sensor in the PLC, we would need to connect the three wires of the
proximity sensor to a digital input card in the PLC. When the proximity sensor
does not sense a part nearby, the PLC would see the input
state as “off”, or a 0. The circuit to/from the PLC input
is “open” or not energized. When a part moves in front
of the proximity sensor, the PLC input senses the change in state, and transmits this data through a communication
channel to the processor memory, or data table. When the proximity sensor
senses a part nearby, the circuit to/from the PLC
input is “closed” or energized. We could then write a PLC program, which resides in the memory of
the PLC, to take some action, like starting a conveyor motor, when
a part is near the proximity sensor. Wiring a discrete DC sensor to a PLC
input card is easy and straightforward. In this video, we will consider
only devices that have three wires that need to be connected to
the PLC digital input card. An example, an inductive
proximity switch, is shown here. In a separate video, we will consider
some simpler “two-wire” devices. For most PLC’s, the input cards
must match the type of circuit that will be employed to
interface the device to the PLC. For example, a discrete DC device must
be wired to a discrete DC input card. A discrete AC device would require
a discrete AC input card. In this video, we will
consider only DC devices. In order to connect a “three-wire”
discrete DC device to the PLC, we will need to identify whether the
device is an NPN or a PNP type. NPN and PNP are two types
of bi-polar transistors, which are simply solid-state switches. You should always check the
manufacturer’s datasheet for the device you are going
to connect to the PLC to make sure you know its configuration. For now, just remember the NPN
switches are referred to as “sinking”, and PNP switches are
referred to as “sourcing”. Note the each of these
sensors has three wires. Wire 3 (black wire) is always
connected to the load, which is the terminal for an input
on a PLC digital input card. In this simplified diagram
of an inductive switch, there are three wires: one brown wire,
one blue wire, and one black wire. The standard convention is that the brown wire is
connected to the positive side of the power supply (+24 Volts DC), and the blue wire is connected
to the DC common terminal of the power supply. This is the negative (-) terminal
that is present on the power supply. The black wire is the output,
or “load” wire, of the sensor. It gets connected to the
PLC input terminal. Earlier we said an NPN sensor was
referred to as a “sinking” switch. This means that the “load”,
which is the PLC input, is wired as shown in this diagram. Notice that +24 Volts DC is wired to
the PLC module’s common terminal. In other words, when the
switch changes state, it “sinks” the load to common, or ground. For many PLC manufacturers,
connection of NPN sensors requires a specific model number
for the digital input card. Because NPN sensors are “sinking”, they require a “sourcing” type PLC input card. A PNP sensor is referred
to as a “sourcing” switch. This means that the “load”,
which is the PLC input, is wired as shown in this diagram. Notice that the blue “common” wire is connected
to the PLC module’s common terminal. In other words, when the
switch changes state, it “sources” or supplies +24
Volts DC to the PLC input. For many PLC manufacturers,
connection of PNP sensors requires a specific model number
for the digital input card. Because PNP sensors are “sourcing”, they
require a “sinking” type PLC input card. Because NPN sensors sink voltage and PNP
sensors source voltage to the PLC input, NPN and PNP sensors should never
be mixed on a PLC input card. Doing so can lead to an unsafe condition. In this diagram an NPN and a PNP sensor are both connected to the
same PLC input card. Note that the common terminal has
a brown wire from the NPN sensor with +24 Volts DC connected to a
blue wire from the PNP sensor, which would short the
power supply to ground. Now, let’s see how 3-wire
discrete DC sensors are connected to actual PLC I/O cards. This is a schematic of one model of
Siemens SM321 digital input card, part number 6ES7321-7BH01-0AB0. It is a 16-channel 24VDC digital input card that is very popular in Siemens
PCS7 and S7-300 PLC systems. This digital input card wiring scheme is similar to most digital input cards
used in the process industries. The SM321-7BH01 has twenty screw terminals
to which the digital inputs are connected. As we saw previously, for
3-wire discrete DC sensors, each digital input channel requires
the connection of three wires. The SM321 card is constructed so as to accept
only PNP-type sensors in a 3-wire configuration. Use of NPN sensors would require
a different model of I/O card. The SM321 card requires 24VDC power
to perform several functions. First, the card itself requires
power to light the status LED’s, perform diagnostics, and to communicate the
card’s input statuses back to the PLC. Power to the card is supplied by a 24 Volt DC
power supply connected to terminals 1 and 20. Internally inside the SM321 card, this same 24
Volt DC power is connected to terminals 10 and 11. The power at terminal 10 supplies power for
the top group of eight digital inputs, and the power at terminal 11 supplies power
for the bottom group of eight digital inputs. Let’s just focus on the top eight inputs,
labelled as Input 0 through Input 7. In practice, the PLC input
card channel connections, terminals 2 – 9 and 12 – 19, will
be brought out to a terminal block. These marshalling connections, as they are called, allow the field wires to be
more easily terminated in the control panel. These terminals are represented by
the letters A – H in this diagram. The installer usually “pre-wires”
the marshalling terminals, terminal 2 on the SM321-7BH01
card to terminal A, terminal 3 to terminal B, and so on. Now, let’s connect a PNP inductive
proximity switch from the previous example to the SM321 card. The eight marshalling terminals
A-H, are wired to Channels 0 – 7, and are the “load” terminals
in our discrete circuits. +24 Volts DC will be
obtained from Terminal 10. Terminal 20 is connected to
the power supply DC common. For a PNP device, care must be taken to
connect the brown wire to the +24 Volt DC terminal and the black wire to the
PLC input channel terminal. If we connect the proximity
switch to PLC input channel 3, then the black wire will land on terminal A. Finally, the blue wire will be connected to
the power supply DC common, terminal 20. When the proximity switch
senses an object nearby, the state of the switch will change, and the input channel to which
the proximity switch is wired, will indicate an “ON” state. Both the electronics in the
switch and the PLC card are powered by the same 24 Volt DC power supply. To review, we have learned that there are
many types of discrete 3-wire DC sensors that can be wired to a PLC input. Some 3-wire devices are
NPN, or “sinking” sensors, and some are PNP, or “sourcing” sensors. The difference is in how the device
operates when the switch actuates. PNP switches “source” 24 Volts DC to the
black wire when the switch is activated. NPN switches “sink” DC common to the
black wire when the switch is activated. Remember, in both cases, the black wire is
connected to the PLC input channel terminal. For most 3-wire devices, there
is typically one brown wire, one blue wire, and one black wire. A schematic of your specific DC
digital PLC input card is required to determine how your specific
device should be wired. For a 3-wire PNP discrete input device, the
brown wire will be connected to +24 Volts DC, the blue wire will be connected to DC common, and the black wire will be connected to
the PLC digital input channel terminal. Make sure that you Head on over to realpars.com. To find even more training material for
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