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THE ANALOGINPUT/OUTPUT SYSTEM

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Conversion of an analog signal by a transmitter and transducer

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Single-ended input

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Differential inputs

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ANALOG OUTPUT SIGNALS

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Single-ended outputs

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Differential outputs

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Block diagram of bypass/controlbackup unit

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The signal chain within a PLC

analogvariable

(e.g. 4..20mA)

filtering&

scaling

analog-digital

converter

processing

digital-analog

converter

analogvariablee.g. -10V..10V

time

y

time

y(i)

sampling

binaryvariable(e.g. 0..24V)

filtering sampling

time

y

transistoror

relay

binaryvariable

amplifier011011001111

counter

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non-volatilememory

0001111

time

y(i)

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SPECIAL FUNCTION I/O AND SERIALCOMMUNICATION INTERFACING

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Special function I/O interfaces

They provide the link between programmable controllers and devices that require special types of signals.

These special signals are not very common (only 5–10% of PLC applications).

They are divided into two categories:

• direct action interfaces• intelligent interfaces

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Direct action I/O interface application

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Intelligent I/O interface application

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FAST-INPUT/PULSE STRETCHER MODULES

•They detect input pulses of very short duration.•They operate as pulse stretchers, enabling the input signal to remain valid for one scan.

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Wire input fault module diagram

They are special input interfaces designed to detect short-circuit or open-circuit connections between the module and input devices.

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Fast-response interfaces

•They are extensions of fast-input modules.•These interfaces detect fast inputs and respond with an output.•The speed at which this occurs can be as short as 1 msec from the sensing of the input to the output response.•The output response time is independent of the PLC processor and the scan time.

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Weight input modules

They are special types of analog interfaces designed to read data from load cells.

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THERMOCOUPLE INPUT MODULES

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RTD INPUT MODULE

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PID MODULES

Proportional-integral-derivative (PID) interfaces are used in process applications that require continuous closed-loop control employing the PID algorithm.

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Block diagram of the PID algorithm

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Connection Diagram forAB 1771-PID module

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POSITIONING INTERFACES

They are intelligent modules that provide position-related feedback and control output in machine axis control applications.

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Encoder/counter modules interface

They interface encoders and high-speed counter devices with programmable controllers.

There are two types of encoder/counter interfaces:•Absolute encoders provide an angular measurement of the shaft.•Incremental encoders measure shaft rotation over distance by outputting a fixed number of pulses per shaft rotation.

The communication between the interface and the processor is bidirectional. The module accepts the preset value and other control data from the processor and transmits values and status data to the PLC memory.

The interface also lets the PLC know when the marker and limit switch are both energized, indicating a home position.

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STEPPER MOTOR INTERFACES

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Stepper motor with a position/velocity feedback scheme

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SERVO MOTOR INTERFACES

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Servo motor interface connection diagram

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ASCII input/output interfaces

•They send and receive alphanumeric data between peripheral equipment and the controller.

•Typical peripheral devices include printers, video monitors, and displays.

•The information exchange in either type of interface generally takes place via an RS-232C, RS-422, RS-485, or a 20 mA current loop standard communications link.

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COMPUTER MODULES—BASIC modules

•They also referred to as data-processing modules, are intelligent I/O interfaces capable of performing computational tasks without burdening the PLC processor’s computing time.

•In contrast to other intelligent I/O interfaces, such as servo controls, a BASIC module does not actually command or control specific field devices.

•It complements the performance of the PLC system.

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Network interface modules

They allow a number of PLCs and other intelligent devices to communicate and pass PLC data over a high speed local area communication network.

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FUZZY LOGIC INTERFACES

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Data transfer between I/O modulesand fuzzy module

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SERIAL COMMUNICATION

(a) Unidirectional,

(b) half-duplex,

(c) full-duplex.