Terms in this set (43)
What is the function of a PLC interface module?
To accept signals from the machine or process devices and to convert them into signals that can be used by the controller
What is the function of a PLC output interface module?
To convert controller signals into external signals that are used to control the machine or process
With reference to a PLC:
What is a remote rack?
Why are remote racks used?
a) A rack that is located away from the processor module near the field devices
b) To minimize the amount of wiring required
How does the processor identify the location of a specific input or output device?
By its address
List the three basic elements of rack/slot-based addressing.
Type refers to an input or output
Slot refers to the physical location of the I/O module
Word and bit refers to the actual module terminal connection
Compare bit level and word level addressing.
Bit level addressing specifies a discrete device that is connected to a specific terminal. Word level addressing specifies an analog device connected to a module that uses a word of information.
In what way does tag-based addressing differ from rack/slot-based addressing?
In tag-based addressing you use a tag (alphanumeric name) to address data (variables). In rack/slot-based addressing a fixed numeric format is used to identify the address data.
What do PC-based control systems use to interface with field devices?
An input/output PC interface card
What type of I/O modules have both inputs and outputs connected to them?
Combination I/O modules
In addition to field devices what other connections are made to a PLC module?
Connections for the power supply
Most PLC modules use plug-in wiring terminal strips. Why?
For ease of wiring and replacing modules
With reference to PLC discrete input modules:
What types of field input devices are suitable for use with them?
List three examples of discrete input devices.
ON/OFF devices such as selector switches, pushbuttons and limit switches
With reference to PLC output modules:
What type of field output devices are suitable for use with them?
List three examples of discrete output devices.
ON/OFF devices such as lights, solenoids and motor starters
Explain the function of the backplane of a PLC rack.
The backplane supplies current and voltage for the modules from the power supply
What is the function of the optical isolator circuit?
An optical isolator is used to provide electrical isolation between the field wiring and the PLC backplane internal circuitry.
Name the two distinct sections of an I/O module.
Power and Logic sections
What electronic element can be used as the switching device for a 120 VAC discrete output interface module?
What electronic element can be used as the switching device for DC discrete output module?
A discrete relay type output module can be used to switch either AC or DC load devices. Why?
A discrete relay-type module uses an electromechanical relay element for switching which allow it to work for AC or DC applications.
Compare discrete and analog I/O modules with respect to the type of input or output devices which with they can be used.
Discrete I/O modules allow only ON/OFF devices to be connected. Analog I/O modules allow analog or varying voltage or current devices to be connected.
Explain the function of the analog-to-digital (A/D) converter circuit used in analog input modules.
Used to convert analog signals to an equivalent digital value
Explain the function of the digital-to-analog (D/A) converter circuit used in analog output modules.
Used to convert digital signals to an equivalent analog value
Name the two general sensing classifications for analog input modules.
Voltage sensing and current sensing
List five common physical quantities measured by a PLC analog input module.
Temperature, speed, level, flow, weight, pressure, and position
What type of cable is used when connecting a thermocouple to a voltage sensing analog input module. Why?
A twisted shielded pair cable is used in wiring the circuit to reduce unwanted electrical noise signals that can be induced in the conductors from other wiring
Explain the difference between a unipolar and a bipolar analog input module.
Unipolar modules can accept an input signal that varies in the positive direction only. Bipolar signals swing between a maximum negative value and a maximum positive value.
The resolution of an analog input channel is specified as 0.3 mV. What does this tell you?
The analog input channel is capable of sensing voltages down to 0.3V increments.
In what two ways can the loop power for current sensing input modules be supplied?
The loop power may be supplied by the sensor or may be provided by the analog output module
List three field devices that are commonly controlled by a PLC analog output module.
Control valves, chart recorder, electronic drives
Explain the purpose of a redundant PLC processor.
Allows transfer of control to the second processor in the event of a processor fault
Describe three typical modes of operation that can be selected by the keyswitch of a processor.
Run mode, program mode, and remote mode
State five other functions, in addition to simple logic processing, that PLC processors are capable of performing.
Timing, counting, latching, comparing, motion control and complex math functions
Why do PLCs execute memory-checking routines?
To be sure that the PLC memory has not been corrupted
Compare the memory storage characteristics of volatile and nonvolatile memory elements.
Volatile: Loses all its stored information if all operating power is lost or removed. Memory is easily altered and usually supported by a battery backup power supply
Nonvolatile: Retains stored information when the power is accidentally or intentionally removed. Memory is generally unalterable
What information is normally stored in the ROM memory of a PLC?
ROM is normally used to store the programs and data that define the capabilities of the PLC.
What information is normally stored in the RAM memory of a PLC?
RAM is used as a temporary storage area of data that may need to be quickly changed
What information is normally stored in an EEPROM memory module?
An EEPROM memory module is used to store, back up, or transfer PLC programs
What are the advantages of a processor that utilizes a flash memory card?
They are extremely fast at saving and retrieving files
List three functions of a PLC programming terminal device.
Allows the user to enter, change or monitor a PLC program
Give one advantage and one limitation to the use of a hand-held programming device.
Handheld programmers are compact, inexpensive, and easy to use but they have limited display capabilities.
What is required for a personal computer to be used as a PLC programming terminal?
Appropriate programming software
How many programs can a PLC have stored in memory at any one time?
Outline four functions that an HMI interface screen can be configured to perform.
Replace hard-wired pushbuttons and pilot lights with realistic-looking icons
Show operations in graphic format
Allow the operator to change timer and counter presets
Show alarms, complete with time of occurrence and locations
Display variables as they change over time
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