chapter 08 output modules

7/29/2019 Chapter 08 Output Modules

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Chapter 8

Output Modules


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Objectives(1 of 2)

Describe the available types of outputmodules.

Explain applications where analog outputmodules would be used.

Define output module specifications from

data sheets.


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Objectives(2 of 2)

List the advantages and disadvantages ofusing relay-contact versus solid-state output

switching. List the advantages and disadvantages of

using solid-state versus relay-contact output

switching.


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PLC Output Section The output section of a PLC system is the

physical connection between the processor

and the outside world.


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Output Classifications

Output modules fall into three classifications.

Discrete

Analog

Specialty


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Discrete Outputs

Motor starter coils

Pilot lights

Solenoids Alarms

Valves

Fans

Control relays

Start pushbuttons

Stop pushbuttons Horns

Start / stop signals tovariable speed drives


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Operating Characteristics (1 of 2)

Discrete output modules are simply switchingdevices that carry out commands from the

processor. They receive their operating power from the

PLCs power supply.

Power output point switches to control fielddevices are provided by the user.


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Operating Characteristics (2 of 2)

Each output point contains a switchingdevice, which is located inside the module.

The switching device is turned on or offaccording to the bit value residing in thatparticular output modules status table

address.


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Three Basic Types

of Output Modules Solid-state (triac) switching for AC loads

MOSFET for DC loads

Mechanical relay


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Basic Wiring for a 120 VAC


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Discrete Output Module

Output Classifications


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Signal Flow from the CPU


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Block Diagram of a

Typical Output Module


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Omron CQM1 Specifications


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Sourcing Output Module

Specifications


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Relay Output Modules

Relay output modules switch smallmechanical relays.

They are also known as contact or drycontact outputs.


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Relay Output Module Usage

Switching small currents at low voltages

Multiplex analog signals

Interface control signals to variable drives

Switching low voltage circuits where leakagecurrent is a problem


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Simplified Single-Point

Relay Output


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Relay Output Module Variations

Combination input and relay output module

Isolated relay outputs

8 or 16 outputs with shared commons


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Combination 120 VAC Input

and Relay Output Module


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8-Point Relay Output Module


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AC Isolated Output Module


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Interposing Relays

Most output modules switch between one-half and four amps.

To switch higher current loads, a mechanicalrelay called an interposing relay is placedbetween the output point and the load.

Output module switches the relay. Relay switches the load.


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Interposing Relay Switches Load

That Exceeds Module Capability


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Choosing the Proper

Output Module Solid-state versus mechanical output

modules?


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Advantages to

Solid-state Switching Fast switching speeds

High reliability and almost unlimited life

Low power to energize

No contact arcing

Little to no switching noise

Positive switching, no contact bounce

Can be sealed for hostile environments


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Disadvantages to

Solid-state Switching May be destroyed on overload

Tend to fail in the ON state

Heat dissipation

More expensive than mechanical relays

Possible false trips from electrical noise


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Advantages to

Relay Output Switching Contacts forgiving to temporary overload

Immune to false trips from electrical noise

Little voltage drop across contacts

Definite ON or OFF state

No leakage

Contacts generate little heat

Inexpensive


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Disadvantages to

Relay Output Switching Mechanical switching slow

Mechanical life limited

May require up to 50 milliamps to energize

Subject to contact arcing or welding

Contact bounce

Cannot be completely sealed


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Surge Suppression

Electrical noise and microprocessorequipment do not play well together.

Electrical noise is generated by manycommon electrical devices.

Electrical noise can cause microprocessor

devices to malfunction.


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Industrial Noise-generating Devices

Electrical noise generated by

Motor starters

Relays and contactors

Solenoids

Clutches and brakes

Variable frequency drives


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Noise Reduction

Surge suppression

MOVs on triac output devices

Diodes on DC output circuits

Proper grounding and wiring practices

Follow manufacturer recommendations

Communications cable proper installation Proper cable selection, wire routing, and

termination


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Analog Output Modules

They accept a 16-bit output status word,which the module converts to an analog

value through a digital-to-analog converter. Analog output modules send out a varying

current or voltage signal.


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Typical Analog Field Devices

Temperature

Pressure

Position

Valves

Variable frequency drives

Meters


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Typical Analog Outputs

Analog output signals

0 to 10 volts DC

-10 to +10 volts DC 0 to 5 or 1 to 5 volts DC

4 to 20 milliamps

0 to 20 milliamps -20 to +20 milliamps


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Analog Module Configurations

Analog output information is sent to the fielddevice by way of an output channel.

Analog modules are All inputs

All outputs

Combination of input and output channels


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Analog Output Signals

Represent Values A binary output signal from 0 to 32767 from

the output status table could be converted to

a 0 to 10 V DC output signal to control ananalog field device.

Analog data format is determined by module

and module configuration.


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Local Versus Remote I/O

Local analog modules in an SLC 500 systemautomatically receive the binary value from

the output status table. Remote I/O analog signals require block

transfers.


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Analog Input Modules and the

Output Status Table Each analog channel will be represented by

an entire word in the output status table.

A two-channel analog output module willhave two words in the output status table torepresent the binary representation of theanalog voltage or current.


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Analog Data Is Represented

as Whole Word Data Analog signals are not digital signals

represented as 1s and 0s.

Analog information is represented as a16-bit signed integer in the data table.

Each channel will require one word.

Data table will reserve one word for eachchannel.

This is part of the I/O configuration process.


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Output Word Assignment

1746-NO4V is a four-channel analog voltageoutput module.

Four words will be assigned to the outputstatus table as part of the I/O configuration.

Module in slot 4 will have words O:4.0

through O:4.3, one for each channel.


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1746-NO4V Input

Status Table Words

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