In this video, you will learn
how to wire a 2-wire DC sensor, like a switch or a contact,
to a PLC input card. You will also learn what a
2-Wire Discrete sensor is and recognize some of the common
types of 2-Wire Discrete sensors. If you enjoy these videos be sure
to click "like" and subscribe, so you never miss a new one. 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 pump. In this diagram, the Programmable
Logic Controller, or PLC, is running a ladder logic program. The pushbutton is connected
to a PLC input card, and the logic program is written to
sense the state of the pushbutton. When the pushbutton is not pressed,
the logic does not turn the pump on. Now, if the pushbutton is depressed, the PLC ladder logic program will sense
that the pushbutton has been pressed, and will start the pump. So, let's see how we can make this important
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 a switch, or a lamp. 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 lamp when a pushbutton is pressed. The common trait of all discrete DC sensors is that they have exactly two
operating states, “on” and “off”. A discrete DC pushbutton,
as in this diagram, is “on” when it is pressed and
its internal contact is “closed”, forming a complete circuit and
allowing the flow of electrons. When the pushbutton is released,
its contacts are “opened” again, the circuit is broken,
and the flow of electrons stops. The state of the pushbutton is “off”. Other common examples of discrete DC Sensors include proximity switches, selector
switches, and photoelectric sensors. If we want to monitor the state
of the pushbutton in the PLC, we would need to connect the two wires of the
pushbutton to a digital input card in the PLC. When the pushbutton
is not pressed, the PLC would see the input
state as “off”, or as a 0. The circuit to/from the PLC
input is “open” or not energized. When the button is pressed, the PLC input
senses the change in pushbutton state, and transmits this data
through a communication channel to the processor memory, or data table. When the pushbutton is pressed, 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 turning on a lamp or a pump,
when the pushbutton is pressed. Wiring a discrete DC sensor to a PLC
input card is easy and straightforward. In this video, we will consider
only devices that have two wires that need to be connected to
the PLC digital input card. An example, a Hall-Effect proximity
switch, is shown here. In a separate video, we will consider
more complex “three-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 only consider DC devices. In order to connect a “two-wire”
discrete DC device to the PLC, we will need to identify whether the
device is “polarized” or “non-polarized”. For a “polarized” device,
the two wires will be labelled: one wire is labelled “positive” and
one wire is labelled “negative”. In this simplified diagram
of a photoelectric switch, the “positive” wire must be connected to
the “positive” terminal of the PLC input, and the “negative” wire must be connected to
the “negative” terminal of the PLC input. If this convention is not followed, the device will not indicate correctly in
the PLC, and the device may be damaged. For this photoelectric switch, reversing
the “positive” and “negative” wires will damage the phototransistor and there
will be no power to the transmitter. The switch will not
function as intended. For “non-polarized” devices, like simple
pushbuttons and selector switches, the device may be wired with either wire connected
to the positive terminal of the PLC card, and the other wire connected to the
negative terminal of the PLC card. In this example, the proximity
switch is a non-polarized device. It has two wires,
a white wire and a black wire. On the left, the white wire is
connected to the positive terminal of the digital input channel and the black
wire is connected to the negative terminal. Because the device is non-polarized, the wires could be connected
in the opposite configuration and still provide the correct
indication of its state in the PLC. This is depicted in the
drawing on the right. Now, let's see how 2-wire discrete DC sensors
are connected to actual PLC I/O cards. This is a schematic of a 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 has twenty screw terminals to
which the digital inputs are connected. As we saw previously,
for 2-wire discrete DC sensors, each digital input requires
the connection of two wires. 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 24VDC power
supply connected to terminals 1 and 20. Internally inside the SM321 card, this same 24VDC 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 card to terminal
A, terminal 3 to terminal B, and so on. Now, let's connect the proximity
switch and the photoelectric switch from the previous examples
to the SM321 card. The eight marshalling terminals
A-H, are wired to Channels 0 – 7, and are the negative terminals
in our discrete circuits. +24VDC will be obtained from Terminal 10 for
the top 8 inputs wired to the SM321 card and from terminal 11 for the bottom
8 inputs wired to the SM321 card. Since the pushbutton is a
non-polarized 2-wire discrete device, we can connect either its
white or black wire to +24VDC, and the opposite wire to terminal A. For a polarized device,
like the photoelectric sensor, care must be taken to connect the
positive wire (or the red wire) to +24VDC and the black wire to the negative terminal
(or the PLC input card connection). If we connect the photoelectric
switch to PLC input channel 3, the red wire will be connected to +24VDC, and
the black wire will be connected to terminal D. To review, we have learned that there are
many types of discrete 2-wire DC sensors that can be wired to a PLC input. Some devices are polarized,
and some are non-polarized. For polarized devices it is important to
connect the positive wire to the power, or positive terminal, of the PLC input
and to connect the negative wire, or common, to the negative
terminal of the PLC input. For non-polarized devices, either wire
can be connected to the positive terminal, with the opposite wire connected
to the negative terminal. A schematic of your specific
DC digital PLC input card is required to determine how your
specific device should be wired. The terminals connected to +24VDC
are the positive terminals. In this example, the PLC input card
terminals are the negative terminals. Make sure that you Head
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