Programmable Logic Controllers

Question 1

How do you usually select a PLC?

Answer 1

PLC’s are usually selected according to the number of input and output points (I/O points) they have

Question 2

What would you connect to the input points of a PLC?

Answer 2

Switches and sensing devices would be connected to the inputs of a PLC

Question 3

What would be connected to the output terminals of a PLC?

Answer 3

Pilot lights, relays, or contactors that operate motors, valves or other equipment would be connected to the output points of a PLC

Question 4

What does a Control System consist of?

Answer 4

1. Input Devices - switches, limits, thermostats or other devices that sense specific conditions of a process or environment
2. A Controller - a controller that receives this information from the input devices and decides what to do with it by using a hard-wired circuit arrangement or by a software program
3. Output Devices - relays, contactors, solenoids, or other devices that control operating devices like motors, pumps, heaters, fans or valves to bring about an appropriate change in the system that is being monitored by the input devices

Question 5

What several advantages do PLC’s have over relay control?

Answer 5

1. Lower Overall Cost - smaller installation and labour costs, fewer componets to install, maintenance costs are lower because there are no mechanical parts to wear out, trouble-shooting is easier which reduces down-time costs, and they operate more economically with a smaller energy requirement that a relay control panel
2. Programmability - PLC programs can be changed or updated without any changes to the hard-wiring, a program can be revised while the system is still in operation, the program can also be tested for proper logical function before it is implemented which reduces downtime, one program can be used for multiple identical processes
3. Reliability - Utilizes solid-state technology which is very reliable and allows them to be used in many different environments, uses static switching which means it requires no moving parts, these devices have long life spans and offer high speed switching
4. Expandability - Typically offered in a modular format or with some expansion capabilities so that additional componets may be added to the system. One way of expanding a PLC system is to add another PLC and link the two together with a Communication Data Highway

Question 6

What are the main hardware componets of a modular PLC system?

Answer 6

1. Power Suppy
2. Backplane
3. Processor (CPU)
4. Input/Output (I/O) Modules

Question 7

Which componets does the Power Supply of a PLC supply power to?

Answer 7

1. The Processor (CPU)
2. The Input and Output (I/O) Modules

Question 8

Does the PLC Power Supply provide power for the field I/O devices?

Answer 8

No, they require their own power supply. The PLC Power Supply provides power for the Processor (CPU) and the I/O Modules, not the field devices

Question 9

What does the Backplane of a Modular PLC do?

Answer 9

• The Backplane is the power and communication bus for the PLC componets

Question 10

What are the primary functions of a PLC Processor (CPU)?

Answer 10

1. Read and store the status of all the input devices
2. Execute the user program by solving the program’s logic
3. Update the status of all the outputs

Question 11

What does the Processor (CPU) of a PLC do?

Answer 11

• The brain of the PLC
• Governs the system activities regarding receiving, analyzing, processing, and transmitting data
• Contains a memory area for data storage, performs or executes user programs and communicates with other devices

Question 12

Whare the types of memory are used in PLC’s?

Answer 12

1. RAM - Random Access Memory
2. ROM - Read Only Memory
3. EEPROM - Electrically Erasable Programmable Memory

Question 13

What is the difference between the types of memory in a PLC?

Answer 13

• The Memory may be Volatile or Non Volatile
• Volatile memory is cleared of all data when it loses its electrical power supply, these devices require a backup battery to keep their stored data
• Non-Volatile memory retains its memory when it loses its power suppy

Question 14

What is Random Access Memory (RAM)?

Answer 14

• Designed so that information is easily stored and retrieved
• Most RAM is volatile memory and requires a battery backup to retain its stored data when the power supply is shut off
• Non-volatile RAM memory has a self-contained backup battery within the package of the memory chip

Question 15

What is Read-Only Memory (ROM)?

Answer 15

• Read-Only Memory is non-volatile memory and is designed to permanently store data and programs
• This information is designed to be read, not altered. The data or program is fixed at the time of manufacture
• The basic input/output system is stored in ROM memory so that the PLC recognizes its componets when it is turned on

Question 16

What is Electrically Erasable Programmable Memory (EEPROM) Memory?

Answer 16

• Integrated circuit memory that is considered non-volatile but can be reprogrammed using a PLC programming device
• The memory chip must be erased before the new program is loaded, flash drives and USB’s are an example of EEPROM

Question 17

Where does the voltage for the I/O field devices come from?

Answer 17

• Comes from a separate supply source
• PLC Power Supply only supplyes power to the Input and Output Modules, not the I/O devices
• The signal voltage out to these devices is usually a different value from that of the processor(CPU) and is isolated from the controller’s processor by optoisolators or small relays and filter circuitry

Question 18

What is a data highway?

Answer 18

• A data highway is Local Area Network (LAN) cabling that connects multiple PLC’s, or PLC’s to Personal Computers (PC’s), allowing them to exchange data and programs
• This allows one PC to change the programs of several PLC’s and facilitate coordination of processes controlled by different PLC’s

Question 19

What is a remote I/O Module?

Answer 19

• If input or output devices are located some distance from the PLC, a Remote I/O module can be installed to decrease wire and installation costs
• These modules can have several input or output devices connected and only require one cable back to the PLC instead of individual cables back to the PLC from each I/O device

Question 20

What is a Discrete signal?

Answer 20

• A signal that either on or off

Question 21

What is a Discrete Input device?

Answer 21

• Relays, contacts, sensors and switches are discrete input devices that are either on or off
• They are connected PLC input modules that recognize signals that are either on or off, not in between states

Question 22

What is a Discrete Output device?

Answer 22

• Pilot lights, contactor coils, relay coils, and solenoids are discrete devices that respond to being either energized or de-energized
• These devices are connected PLC output modules that deliver an output of only power on or off

Question 23

How are Discrete Input and Output circuits isolated from the internal circuitry of the control modules?

Answer 23

• This isolation is achieved by means of Optoisolating
• When a Discrete Input device sends a signal to an Input Module, the module converts this signal into a light beam. The module recognizes this light beam as the device being on or energized and sends a signal to the processor (CPU)
• When the processor (CPU) decides which output is to become energized, it sends a signal to the output module, this signal is converted into a light beam and a solid-state electronic switch (transistor for DC and Triac for AC) is activated by the light closing the circuit to a discrete output device
• This isolation protects the PLC from the high voltage of the input and output circuits
• When the output circuit requires larger currents than the solid-state electronic switch can handle, a relay coil is operated by the solid-state switch instead. This relay is called an Interposing Relay

Question 24

What is an Interposing Relay?

Answer 24

• When the output circuit solid-state electronic switch cannot handle the high currents of the circuit, an Interposing Relay is used to handle these currents
• The output module converts the signal it recieves from the Processor into a light beam, a solid-state switch is activated by this light beam closing the circuit, which then operates the Interposing Relay coil

Question 25

What is an Analog signal?

Answer 25

* An Analog signal is a proportional voltage or current signal back to the PLC *