lesson 3

Question 1

Modern industrial control systems are ____________

Answer 1

microprocessor-based programmable
systems

Question 2

The central processing unit can be divided into the

Answer 2

processor, memory, and input and output units or modules

Question 3

Figure 14.2 shows the block diagram of the basic controller.The memory can be divided into _____ for system operation, ____, and_____ for storing set point information, and look-up tables.

Answer 3

RAM
ROM
EEPROM (nonvolatile memory)

Question 4

Figure 14.3 shows the block diagram of a processor controlling two analog loops. The analog
output from the monitors is converted to a digital signal in an ______. The digital signal is selected
in a ________ and put into memory by the process, awaiting evaluation and further action. the digital output signal is fed to the actuator through a ____________ and
converted to an analog signal by a _____.

Answer 4

ADC
multiplexer
demultiplexer
DAC

Question 5

The units are interconnected by a two-way 16-bit data bus, a __________, and a ___________. The enable and address
buses are controlled by the _______, which uses software instructions for its direction. When addressing an input module, the module is selected with its _______. ________ then can be used to select which external
input data is to be put onto the two-way data bus.

Answer 5

one-way address bus
one-way enable bus.
processor
enable bus code
address bus

Question 6

_________ in the PLC is made up of two separate modes; these are the I/O scan mode,
followed by the execution mode.

Answer 6

Operation cycle

Question 7

is the period when the processor updates the output control signals, based on the information
received from the previous I/O scan cycle after its evaluation of the signals. The processor then scans the inputs in a serial mode and updates its internal memory as to the status of the inputs.

Answer 7

I/O scan mode

Question 8

follows the I/O scan mode. In this mode, the processor evaluates the input data stored in
memory against the data programmed into the CPU. The programs usually are set up using ladder networks, where each rung of the ladder is an instruction for the action to be taken for each given input data level. The rung instructions are sequentially scanned, and the input data evaluated. The processor then can determine the actions to be taken by the output modules, and puts the data into memory for transfer to the output modules during the next I/O scan mode.

Answer 8

Execution mode

Question 9

_________ is the time required for the PLC to complete one I/O scan plus the execution cycle. This time depends on the number of input and output channels, the length of the ladder instruction sets, and the speed of the
processor.

Answer 9

Scan time

Question 10

A typical scan time is between ____and ____. As well as evaluating data, the PLC also can generate
accurate time delays, store and record data for future use, and produce data in chart or graph form.

Answer 10

5 and 20 ms

Question 11

act as the signal interface between the monitoring sensors and actuators,
and the controller._____ also provide electrical isolation, if necessary, to convert the input signals into an electronic format suitable for evaluation by the controller; provide memory
storage; and format the output signals for displays and control functions.

Answer 11

Input/output modules

Question 12

Modules fall into three
categories:

Answer 12

(1) those for use with discrete I/O levels,
(2) those with analog signal levels,
(3) those that have intelligence to evaluate and modify the input signals before they can be used by the controller.

Question 13

Some modules are configured for local signals up to 500 ft, and some are for remote signals from 500 to 10,000 ft.

Input/output modules will typically have ______ inputs or outputs, but can be as high as ____, or as low as ____. Modules that have both input and output ports are also available.

Answer 13

16
32
4

Question 14

serve as On/Off signal receivers for the processor. The basic function of the input module is to determine the presence or absence of a signal. The inputs from peripheral
devices to the input modules can be ac or dc signals. The voltage ratings for input modules can
vary from 24V to 240 V, ac or dc, as well as 5V and 12V TTL levels.

Answer 14

Discrete Input Modules

Question 15

Figure 14.6 shows the block diagram of a discrete input module. The front ends of both the dc and ac modules are shown. With a high dc input
voltage, the voltage is stepped down to a low voltage, which then goes through a de-bounce circuit with a noise filter, and threshold detector for “1” or “0” detection, followed by optical isolation, so that the signal can be referenced to the signal ground of the processor. The ac module input uses a bridge rectifier to convert the ac to dc, then uses the same circuit blocks as in the dc module. The LED is used to indicate the input logic
level of the input signal. The input level LED indicators are normally located above the tag strip.

Answer 15

Discrete Input Modules

Question 16

Figure 14.7 shows the block diagram of a differential analog receiver. The signal is converted into a _______. The resolution of the ADC is normally ______ when using voltage input, and ______ when using a current input. The ADCs are _________ in length. The inputs are floating, so that the output from the ADC uses an _________to reference the digital signal to the ground level of the controller.

Answer 16

digital word
305.176 V
1.2207 A
14 to 16 bits
optoisolator

Question 17

_______ applications are temperature, humidity, pressure, flow rate, load, thermocouple, RTD, magnetic, and
acceleration sensors. The typical input voltage levels are from 50 to 500 mV, up to ±10V, and current ranges from 20
to 50 mA.

Answer 17

Analog input

Question 18

are used to interface output information from the controller to peripheral units, to provide
electrical isolation, and to provide the data in a suitable format for use by the external units.

The output from the modules can be either ______ ac or dc outputs, or relay contacts. The output voltage can be from 12V to 230V, ac
and dc, and TTL levels with multiple or isolated contacts. Table 14.3 shows a list of discrete output applications.

Answer 18

Discrete output modules

Question 19

________ outputs from the PLC drive analog meters, chart recorders, proportional valves, and variable speed drive controllers. They also are used for current or voltage to pneumatic transducers. The voltage and current output
ranges are the same as the input ranges. The _____________ of the converter is amplified and fed to a voltage or current driver, which can
have a single-ended output or a differential output. The output signal also will meet the standard voltage or current
control ranges.

Answer 19

Analog outputs

Question 20

A number of specialized modules have been developed to interface to the controller.
They normally contain their own processor and memory, and can be programmed to perform operations independent of the central processor.

Answer 20

Smart Input/Output Modules

Question 21

is universally used as a symbolic and schematic way to represent the interconnections between
the circuit elements used in programming a PLC. The ladder network also is used as a tool for programming the
operation of the PLC. The elements are interconnected between the supply lines for each step in the control
process, giving the appearance of the rungs in a ladder.

Answer 21

Ladder diagram

Question 22

A number of programming languages are in common use for controllers, as follows:

Answer 22

• Ladder;
• Instruction list;
• Boolean flowcharts;
• Functional blocks;
• Sequential function charts;
• High-level languages (ANSI, C, structured text).

Question 23

Switch Symbols

Answer 23

push button switches
limit switches
pressure switches
temperature switches
low level switches
flow meter (guide output)

Question 24

Relay and Timing Symbols

Answer 24

Contact Relay and Contacts
on-delay
off-delay
timer delay contacts
counter and contacts

Question 25

Output Device Symbol

A circle with the letter M and an appropriate number represents a motor.

A circle with radiating arms and a letter to indicate its color (e.g., R = red, B = blue, O = Orange,
and G = green), plus its number represents an indicator.

A solenoid that is used to operate a flow valve or move an
object, or for a similar, unspecified, use, is designated SOL with number.

Other output elements without specific
symbols, such as alarm, heater, or fuse, are represented by boxes, as shown, with the name of the element and its
number in the box to distinguish between similar types of elements used in different places.

Answer 25

motor
lamp
solenoid
other (alarm, heater, fuse)

Question 26

these are the three basic passive elements
used in electrical circuits, either as individual devices or in combination.

Answer 26

Resistors, capacitors, and inductors.