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page 215

18. PLC CONNECTION

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.

18.1 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.

Inputs:

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

etc.

• 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. etc.

• 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

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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

18.1.1 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

24Vac

An example of a PLC input card is shown below.

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e.g.

normally open push-button

24 V AC

Power

Supply

normally open temperature switch (See appendix in textbook for more symbols)

I:013

Push Button

01

I:013

Temperature Sensor

03

PLC Input Card

24V AC

00

01

02

03

04

05

06

07

GND

it is in rack 1 I/O Group 3

Note: outputs are normally high impedance. This means that they will use very little current.

Note: Only outputs are NPN/PNP or sourcing/sinking

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.

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