Deck 17: Programmable Logic Controllers

Question 1

The most popular way to isolate the CPU from the external circuit is through .

A) optical isolation
B) small relays
C) rheostats
D) bridge rectifiers

Answer 1

A

Question 2

Explain the mechanism by which the differential amplifier works.

Answer 2

An electronic device that is often used to help overcome the problem of induced noise is the differential amplifier. This device detects the voltage difference between the pair of signal wires and amplifies this difference. Since the induced noise level should be the same in both conductors, the amplifier will ignore the noise. For example, assume a sensor is producing a 50-millivolt signal. This signal is applied to the input module, but induced noise is at a level of 5 volts. In this case the noise level is 100 times greater than the signal level. The induced noise level, however, is the same for both of the input conductors. Therefore, the differential amplifier ignores the 5-volt noise and amplifies only the voltage difference, which is 50 millivolts.

Question 3

What are the advantages and disadvantages of controlling output with small relays?

Answer 3

One method of controlling the output is with small relays. The CPU controls the relay coil. The contacts connect the load to the line. The advantage of this type of output module is that it is not sensitive to whether the voltage is ac or dc and can control 120- or 24-volt circuits. The disadvantage is that it does contain moving parts that can wear. In this instance, the CPU is isolated from the external circuit by a magnetic field instead of a light beam.

Question 4

Electrical noise is such a problem in some installations that several manufacturers are using instead of wires for connection to the I/O rack.

A) transistors
B) coaxial cables
C) optical cables
D) radio signals

Question 5

The PLC is a special type of designed to perform a special function.

A) die
B) motor
C) robotic arm
D) computer

Question 6

What are timers and counters?

Question 7

A(n) is connected across the ac input to help eliminate any voltage spikes that may occur.

A) rheostat
B) metal-oxide varistor
C) avalanche diode
D) gas tube

Question 8

The is used to program the CPU.

A) I/O rack
B) power supply
C) tape deck
D) loading terminal

Question 9

In the relay method of isolation, the CPU is isolated through the use of a(n) .

A) LED
B) laser
C) electrostatic buildup
D) magnetic field

Question 10

What are some potential problems you might face when grounding equipment?

Question 11

The current rating of most outputs can range from 0.5 to about amperes, depending on the make of the controller and the type of output being used.

A) one
B) two
C) three
D) five

Question 12

The PLC recognizes only .

A) push-buttons
B) pressure switches
C) open or closed contacts
D) float switches

Question 13

The is used to connect the CPU to the load.

A) opto-isolator
B) power supply
C) input module
D) output module

Question 14

The is the "brains" of the PLC.

A) power supply
B) CPU
C) I/O rack
D) program loader

Question 15

An electronic device that is often used to help overcome the problem of induced noise is the .

A) float switch
B) LED
C) MOV
D) differential amplifier

Question 16

PLCs are designed to be programmed with .

A) ladder diagrams
B) BASIC
C) higher-order languages
D) relays

Question 17

The actual logic of the control circuit is performed by .

A) physical relays
B) internal relays
C) contacts
D) RAM

Question 18

PLCs were first used by the industry in the late 1960s.

A) railroad
B) defense
C) automotive
D) steel

Question 19

Describe the power supply of the PLC.

Question 20

Highlighting a contact means that there is a through that contact.

A) complete circuit
B) short circuit
C) change of state
D) troubleshooting element