15. PLC Connections

• Many configurations and packages are available. But essential components are,

power supply: Provides voltage/current to drive the electronics (often 5V, +/- 12V, +/- 24V)

CPU: Where ladder logic is stored and processed, the main control is executed here.

I/O (Input/Output): A number of input/output terminals to connect to the actual system

Indicator lights: Indicate mode/power and status. These are essential when diagnosing problems.

• Common Configurations

Rack: A rack, or number of racks are used to put PLC cards into, these are easy to change, and expand.

Shoebox: A compact, all-in-one unit (about the size of a shoebox) that has limited expansion capabilities. Lower cost, and compactness make these ideal for small applications.

15.1 Switched Inputs and Outputs

The Obvious:

A PLC is just a computer, we must get information in so that it may make decisions, and have outputs so that it can make things happen.


Switches: contact, deadman, etc. all allow a voltage to be applied or removed from an input.

Relays: Used to isolate high voltages from the PLC inputs, these act as switches

Encoder: Can keep track of positions


• Outputs

Motors: motors often have their own controllers, or relays because of the high current they require.

Lights: can often be powered directly from PLC output boards.


• PLCs, and other industrial controls often use methods called sourcing or sinking current for outputs of devices. In this method the output of a device does not supply any power. Instead, the device only switches current on or off.

Sinking: When active the output allows current to flow to a common ground. This is best selected when different voltages are supplied.

Sourcing: When active, current flows from a supply, through the output though the output device and to ground. This method is best used when all devices use a single supply voltage.

• When selecting sensors

NPN is sinking

PNP is sourcing

15.1.1 Input Modules

• Input modules typically accept various inputs, depending upon specified values.

• Typical input voltages are,

12-24 Vdc

48 Vdc

10-60 Vdc

100-120 Vac

5 Vdc (TTL)

200-240 Vac

12-24 Vac/dc

24 Vac

• An example of a PLC input card is shown below.


• DC voltages are usually lower, and therefore safer (i.e., 12-24V)

• DC inputs are very fast, AC inputs require a longer time (e.g., a 60Hz wave would require up to 1/60sec for reasonable recognition).

• DC voltages are flexible being able to connect to greater varieties of electrical systems.

• DC input cards typically have more inputs.

• AC signals are more immune to noise than DC, so they are suited to long distances, and noisy (magnetic) environments.

• AC signals are very common in many existing automation devices.


15.1.2 Output Modules


• Typical output voltages are,

12-48 Vac

120 Vac

230 Vac

120 Vdc

230 Vdc

5Vdc (TTL)

24 Vdc

12-48 Vdc


• Some output modules allow us to use a single common. We refer to this type of output as current sinking.


• Other output modules allow us to use a single voltage supply. We refer to this type of output as current sourcing.



• Typical outputs operate in one of two ways.

Dry contacts: a separate relay is dedicated to each output. This allows mixed voltages (AC or DC and voltage levels up to the maximum), as well as isolated outputs to protect other outputs and the PLC. Response times are often greater than 10ms. This method is the least sensitive to voltage variations and spikes.

Switched outputs: a voltage is supplied to the PLC card, and the card switches it to different outputs using solid state circuitry (transistors, triacs, etc.) Triacs are well suited to AC devices requiring less than an amp. Transistor outputs use NPN or PNP transistors up to 1A typically. Their response time is well under 1ms.

Problem 15.1 Layout the electrical wiring for the control of a hydraulic press including,

- a double actuated 24VDC solenoid controlled valve for the advance/retract

- a 220VAC, 20A single phase motor for the hydraulic pump

- bottom and top limit switches

- an emergency stop button

- a manual retract button

- an automatic cycle button

Answer 15.1

• Inductive loads: Inductance is caused by a coil building up a magnetic field. When a voltage is removed from the coil the field starts to collapse, and as it does this the magnetic field is changed back to current/voltage. If this change is too sudden a large voltage spike is created. One way to overcome this is by adding a surge suppressor. One type of design was suggested by Steel McCreery of Omron Canada Ltd. - Relays

• Contactor: special relays for switching of large loads.

• Motor Starter: Basically a contactor in series with an overload relay to cut off when too much current is drawn.

• Rated Voltage: suggested operation voltage. Lower levels can result in failure to operate, voltages above shorten life.

• Rated Current: The maximum current before contact damage occurs (welding or melting).

• DC relays require special arc suppression. AC relays have a zero crossing to reduce relay arc problems.

• AC relays require a shading pole to maintain contact. If a DC relay is used with AC power on the coil it clicks on-and-off at the frequency of the AC (also known as chattering).

15.2 Problems

Problem 15.2 TRUE / FALSE -- A PLC input can switch a relay coil to control a motor.

Answer 15.2 false: a PLC OUTPUT can switch a relay

Problem 15.3 In the figure below, properly connect the input devices to the contacts shown.

Answer 15.3

Problem 15.4 For the circuit shown in the figure below, list the input and output addresses for the PLC. If switch A controls the light, switch B the motor, and C the solenoid, write a simple ladder logic program.

Answer 15.4
Answer 15.4

Problem 15.5 The main PLC rack refers to an enclosure that contains:

a) several local racks

b) one local and one remote enclosure

c) the processor

d) input and output modules

Answer 15.5 c

Problem 15.6 5. The digital I/O cards provide the interface between:

a) The PLC and the power supply

b) The CPU and a drop

c) Input modules and CPU

d) External equipment and the PLC

Answer 15.6 d

Problem 15.7 We have a PLC (rack 1) with a 24 VDC input card (slot 3), and a 120VAC output card (slot 2). The inputs are to be connected to 4 push buttons. The outputs are to drive a 120VAC lightbulb, a 240VAC motor, and a 24VDC operated hydraulic valve.

a) Draw the electrical connections for the inputs and outputs. Show all other power supplies and other equipment/components required.

b) One of the switches will turn the motor on, another switch turns the motor off. When the motor is on, the hydraulic solenoid will turn on. The light is on when the motor is off. Draw a Petri net of the control program.

c) Develop ladder logic for one of the states and one of the transitions.

Answer 15.7

Problem 15.8 What is the difference between a motor starter and a contactor?

Answer 15.8 a motor starter typically has three phases

Problem 15.9 Is AC or DC easier to interrupt?

Answer 15.9 AC is easier, it has a zero crossing

Problem 15.10 What can happen if the rated voltage on a device is exceeded?

Answer 15.10 it will lead to premature failure

Problem 15.11 What are the benefits of input/output modules?

Problem 15.12 Explain the operation of AC input and output conditioning circuits.

Problem 15.13 What is the wiring difference between a sourcing and sourcing output?

Problem 15.14 What will happen if a DC output is switched by an AC output.

Problem 15.15 You are planning a project that will be controlled by a PLC. Before ordering parts you decide to plan the basic wiring and select appropriate input and output cards. The devices that we will use for inputs are 2 contact switches, a push button and a thermal switch. The output will be for a 24Vdc solenoid valve, a 110Vac light bulb, and a 220Vac 50HP motor. Sketch the basic wiring below including PLC cards.