chapter 08 output modules
TRANSCRIPT
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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