1. Chapter 8
Output Modules

2. Objectives (1 of 2) Describe the available types of output modules.
Explain applications where analog output modules would be used.
Define output module specifications from data sheets.

3. Objectives (2 of 2)
List the advantages and disadvantages of using relay-contact versus solid-state output switching.
List the advantages and disadvantages of using solid-state versus relay-contact output switching.

4. PLC Output Section
The output section of a PLC system is the physical connection between the processor and the outside world.

5. Output Classifications
Output modules fall into three classifications.
Discrete
Analog
Specialty

6. Discrete Outputs Motor starter coils Pilot lights Solenoids Alarms
Valves
Fans
Control relays
Start pushbuttons
Stop pushbuttons
Horns
Start / stop signals to variable speed drives

7. Operating Characteristics (1 of 2)
Discrete output modules are simply switching devices that carry out commands from the processor.
They receive their operating power from the PLC’s power supply.
Power output point switches to control field devices are provided by the user.

8. Operating Characteristics (2 of 2)
Each output point contains a switching device, which is located inside the module.
The switching device is turned on or off according to the bit value residing in that particular output module’s status table address.

9. Three Basic Types of Output Modules
Solid-state (triac) switching for AC loads
MOSFET for DC loads
Mechanical relay

10. Basic Wiring for a 120 VAC

11. Discrete Output Module Output Classifications

12. Signal Flow from the CPU

13. Block Diagram of a Typical Output Module

14. Omron CQM1 Specifications

15. Sourcing Output Module Specifications

16. Relay Output Modules
Relay output modules switch small mechanical relays.
They are also known as contact or dry contact outputs.

17. Relay Output Module Usage
Switching small currents at low voltages
Multiplex analog signals
Interface control signals to variable drives
Switching low voltage circuits where leakage current is a problem

18. Simplified Single-Point Relay Output

19. Relay Output Module Variations
Combination input and relay output module
Isolated relay outputs
8 or 16 outputs with shared commons

20. Combination 120 VAC Input and Relay Output Module

21. 8-Point Relay Output Module

22. AC Isolated Output Module

23. Interposing Relays
Most output modules switch between one-half and four amps.
To switch higher current loads, a mechanical relay called an interposing relay is placed between the output point and the load.
Output module switches the relay.
Relay switches the load.

24. Interposing Relay Switches Load That Exceeds Module Capability

25. Choosing the Proper Output Module
Solid-state versus mechanical output modules?

26. 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

27. 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

28. 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

29. 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

30. Surge Suppression
Electrical noise and microprocessor equipment do not play well together.
Electrical noise is generated by many common electrical devices.
Electrical noise can cause microprocessor devices to malfunction.

31. Industrial Noise-generating Devices
Electrical noise generated by
Motor starters
Relays and contactors
Solenoids
Clutches and brakes
Variable frequency drives

32. 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

33. 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.

34. Typical Analog Field Devices
Temperature
Pressure
Position
Valves
Variable frequency drives
Meters

35. 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

36. Analog Module Configurations
Analog output information is sent to the field device by way of an output channel.
Analog modules are
All inputs
All outputs
Combination of input and output channels

37. Analog Output Signals Represent Values
A binary output signal from 0 to from the output status table could be converted to a 0 to 10 V DC output signal to control an analog field device.
Analog data format is determined by module and module configuration.

38. Local Versus Remote I/O
Local analog modules in an SLC 500 system automatically receive the binary value from the output status table.
Remote I/O analog signals require block transfers.

39. 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 will have two words in the output status table to represent the binary representation of the analog voltage or current.

40. Analog Data Is Represented as Whole Word Data
Analog signals are not digital signals represented as 1s and 0s.
Analog information is represented as a 16-bit signed integer in the data table.
Each channel will require one word.
Data table will reserve one word for each channel.
This is part of the I/O configuration process.

41. Output Word Assignment
1746-NO4V is a four-channel analog voltage output module.
Four words will be assigned to the output status 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.

42. 1746-NO4V Input Status Table Words