IDIS 400-EXAM I-REVIEW QUESTIONS

3.5 Calculate the actual number of words in an 8K memory.

The number of words in 8K memory will be 8*1024 words. Since 1K means 1024 bytes or 1024 words, 8K means (1024*8) bytes or words (i.e., 8192 bytes).

4.3 Define the term BYTE

Byte is defined as the object or unit that contains 8 bits and is used widely digital communication or PLC systems or almost in all the computer oriented devices. The byte as it is consists of number of bits as a unit, useful in encoding a character of the data in the computer.

2.7 Briefly describe the function of an interposing relay.

Interposing relay is used when there is a necessity to control the machinery which is of large load than the connected output control circuit. This is a current relay, and in this relay there will be a small inrush value and a sealed output current vale. The capacity of this relay is 10 amperes, but it can control the load larger than 10 amps. The control relay used for this purpose is called interposing relay.

3.11 What is a WATCHDOG TIMER?

It is a diagnostic tool that scans the defects of the PLC system and tries to recover the faults of the system. Watchdog timers will be preset to certain value which is slightly higher than the normal scan time of the system. Whenever scan is started, watchdog timer will be turned ON. If the scan is completed within the prior time of the specified duration of the minimum scan time that means that system is at good condition and sets it timer to 0. If the scan takes more time, watchdog timer reaches its limit and states that PLC is at faulty state.

6.6 The two main vertical lines of a ladder diagram are often referred to as: a. Rungs b. Power ports c. Rails d. Tracks e. None of the above

Ladder diagrams are meant for providing easy understanding of the circuit behavior and to simplify circuit representation. Ladder diagrams contain circuit representation like a ladder. Two vertical lines will be drawn at the left and right side. These vertical lines are called RAILS and carry voltage potential. They are represented by L1 and L2. L1 is meant for AC high voltage for the peak over. L2 is meant for low AC circuits or neutral for the ground.

4.5 What word and bit number are represented by PLC-5 address O:010/01?

PLC address O:010/01 represents an output device that is connected to the terminal 01. The first letter in the given address location tells that it is related to an output device. The second number 010 represents the output image table word, and lastly the number after forward slash (/) (i.e., 01 represents the bit which contains the status (ON or OFF) of device. That means that 01 bit of output image table word 012 controls the status of connected output device.

3.12 What special precautions should be taken with lithium batteries?

Precautions to be taken while dealing with the lithium batteries. •They should not be exposed or dropped into water. •They should not be recharged. •They must be replaced periodically.

1.1 List the four main components of a programmable logic controller.

Programmable Logic Controller is a solid-state device. It is a system that comprises different components like: •Programming device •Processor unit •Power supply •Input / Output section The main purpose of the system is to perform logical functions which were earlier performed by components like electromechanical relays, mechanical timers/counters and drum switches. It's age meant for controlling and operating the machinery of the manufacturing process equipment.

2.6 Briefly describe why a hardwired emergency-stop circuit is recommended for PLC installations.

Programmable Logic Controllers (PLCs) are considered as solid-state devices. These solid-state devices generally tend to fail when they are shorted. This is an added hazard concerned with the safety of the equipment. So to avoid this, a hard-wired emergency STOP circuit will be incorporated with the PLC system. This is also called E-Stop which provides the user to take the suitable decision for the safety of the system.

2.12 Electromagnetic interference (EMI) can be reduced with the proper grounding of equipment. (TRUE/FALSE)

TRUE Electromagnetic Interference (EMI) can be reduced with the proper grounding of equipment. The above said statement is TRUE, since it is one of the EMI effect reduction methods. EMI can be reduced by the following methods: •Isolation •Suppression Grounding comes into the category of suppression. Proper grounding apart from the solid state control systems reduces the electromagnetic induction effects. Other reducing methods include I/O shielding, etc.

1.8 RELAY LADDER LOGIC is a high-level graphic computer language. (TRUE/FALSE)

TRUE Ladder logic is widely used computer language for the programming of programmable logic controllers (PLC). It consists of graphical symbols which in turn developed into programming language. The symbols of these diagrams will be in the form of ladder. Relay ladder logic means the ladder logic is being employed with the help of relays. It can be understood easily by the diagrams in the program. Hence the statement, "Relay Ladder Logic is a high level graphic computer language" is TRUE.

2.5 Triaces are susceptible to "dielectric-type" breakdown if the maximum peak voltage level is exceeded. (TRUE/FALSE)

TRUE Triaces are susceptible to "dielectric type" breakdown if the maximum peak voltage level is exceeded. - TRUE The above statement is true. The trial is crystalline material and is very sensitive to the voltage disturbances. If maximum peak voltage is applied to trial, it will smoothly break down. This breakdown is called "dielectric-type" and this causes the train to become short circuit permanently.

3.2 Briefly describe VOLATILE MEMORY.

Volatile memory is volatile in nature (i.e., data is temporary). Whenever power is gone, the data in the volatile memory also vanishes. It has to get it afresh. Random Access Memory (RAM) is a common type of volatile memory. One advantage is data can be read or written to the RAM by the user. Peculiar thing of RAM is any memory location of the RAM chip can be accessed. But the drawback is, as it is volatile in nature it must be supported by secondary memory storage since data can be stored in that storage. There are different types of RAM; they are MOS, HMOS, AND CMOS-RAM which is complementary metal oxide semiconductor.

2.1 Describe briefly the purpose of the I/O section.

I/O section is also called input/output section. This makes the Programmable Logic Controllers (PLC) a reliable and versatile device which can control the driven equipment as well as process machines. The primary purpose of this I/O section is 'to convert any input/output signals into signals that will be compatible with the processor'. Input signals may be any type of voltage or current through the input/output devices may be at different levels of voltage/current. Input section contains push buttons, limit switches, proximity switches. But the output section contains indicator lights, motor starter coils, solenoids, etc. Processor takes the inputs from the above devices in the form of ON or OFF (logic 1 or 0) and generates the output in the form of ON or OFF (logic 1 or 0) which in turn wakes up the output devices. The advantage of I/O section of the PLC's is it can convert any type of voltage or current signals in the controlling process.

2.9 Which of the following are NOT normally sources of electrical noise? a. Solenoid b. Relay c. Indicator Lamp d. Motor Starter e. Motor f. Overload Heaters

Indicator lamp and the overload heaters are not the sources of electrical noise. Indicator lamp generally meant for status display. This usually may consist of LED's. The overload heaters are meant for disconnecting the motor terminals from the supply whenever the motor experiences over currents. The overload heater then opens the contact such that supply will be disconnected. All the remaining elements such as solenoids, relays, motor starter, motors create electrical noise because these components are associated with the magnetic field alterations. Whenever supply is switched ON, there will be a sudden change of magnetic field which creates voltage spikes in the voltage signal which in turn may damage the equipment.

1.2 Define the term INTERFACE.

Input / Output section is called interface. Interface is nothing but the medium for transmission of the data between any two systems or between inputs and outputs or between the input elements or between hardware and software. In interface any signal which will be in the form of AC/DC voltage will be converted to low-voltage DC signal.

3.3 Briefly describe NON-VOLATILE MEMORY.

Whenever power is lost, these memories can retain the information that is in the memory chip. It does not require any battery backup. The data stored on this memory storage cannot be altered. The data stored on non-volatile memory is placed at the manufacturer and contains program of the PLC operations. One common type of this storage is Read Only Memory (ROM). As its name implies can be read only in this storage. There are other types of ROM also exists, they are other types of ROM also exists, they are: •PROM: Programmable Read Only Memory •UVPROM-EPROM: Ultra-Violet Programmable Read Only Memory/Erasable Programmable Read Only Memory •EAROM: Electrically Alterable Read Only Memory •EEPROM: Electrically Erasable Programmable Read Only Memory

3.7 Which of the following are types of non-volatile memory? a. EEPROM b. PROM c. RAM d. EAROM e. FLASH

a. EEPROM b. PROM d. EAROM

4.1 The following types of information are normally found and/or stored in one of the PLC's two memory categories (user and storage). Place an S (for storage memory) and a U (for user memory) before the information type to indicate in which category it is normally found and/or stored. a. Status of discrete input devices b. Preset values of timers and counters c. Numeric values of arithmetic d. Holding registers

a. Status of discrete input devices - Storage Memory b. Preset values of timers and counters - Storage Memory c. Numeric values of arithmetic - Storage Memory d. Holding registers - User Memory

2.11 E-Stop refers to: a. Extra Stop b. Emergency-Stop c. Every Stop d. Elevator Stop e. Energy Stop

b. Emergency-Stop E-Stop means emergency stop. This refers to the safety circuit of the Programmble Logic Controllers (PLC). This helps the system to start and stop the machine cycles automatically and intimates the status of the system. E-Stop is generally will be of hard-wired type.

6.4 Contacts wired in parallel have what relationship? a. AND b. OR

b. OR If the contacts are connected in parallel, it creates two paths in the energized circuit. Current tends to flow through both the paths. Even if one path is disconnected current reaches the last requirement. Likewise in OR gate any 1 of the both inputs yields resultant 1. If the either of the input may be 0, but the output will be 1. If two inputs are 0's, the resultant is also will be 0. Hence parallel circuits refers OR gate.

3.6 The most common type of volatile memory is: a. PROM b. EAROM c. EEPROM d. RAM

d. RAM (Random Access Memory)

1.5 Explain the following initials or acronyms. DC CPU PLC ADC DAC NEMA AC PC I/O

•DC: Direct Current •CPU: Central Processing Unit •PLC: Programming Logic Controller •ADC: Analog to Digital Converter •DAC: Digital to Analog Converter •NEMA: National Electrical Manufacturers Association •AC: Alternating Current •PC: Personal Computer

6.5 Contacts wired in series have what relationship? a. AND b. OR

AND If the contacts are connected in series, it creates only one path in the energized circuit. Current tends to flow through the path. Even if one pair of contacts are in open condition path, flow of current stops and circuit gets disconnected, current will not flow. Likewise in AND gate both 1's of the both inputs yields resultant 1. If the either of the input is 0, the output will be 0. If two inputs are 0's, the resultant is also will be 0. Hence circuit refers AND gate.

4.8 Using SLC 500 or MicroLogicix address format, what would address I:1.0/4 indicate?

According to SLC 500 and MICROLOGIX family, the address location I: 1.0/4 represents an input device 4 of word 0 in slot 2. It can be understood from the following format. X: X.X/X -X: File Type •O: Output •I: Input -X.: SLOT Number -.X: Word Number -/X: Bit Number/Term Number Slot number may be from 0-30 Word number will be either 0 or 1 Bit numbers from 0-15 as a word contains 16 bits starting from 0.

6.10 How are contacts that are associated with relays, motor starters, timers, and the like identified?

Contacts that are connected with the timers, motor, starters, relays are always represented with the starting letter of the device which they control. Suppose if the contacts are controlling motor starter they will be represented with M, suppose if the contacts are connected with a control relay they will be represented by C. Like that lettering of representation depends upon the connected device.

2.15 What type of tool or object should be used to change the position of DIP switches?

DIP switches are meant for communicating with processor. It is jumper or switch like element and it is used for rack configurations. It is a common switch. DIP switches are like ON or OFF switches. In order to change the DIP switch position, a non conduction point object can used to change the position. Pencil is strictly prohibited to change the position of the DIP but ball point pen can be used to change.

6.9 Devices that are intended to perform a START function are normally wired in _____ with each other.

Devices that are intended to perform a START function are normally wired in PARALLEL with each other. START functions are always preferred to be connected in parallel since it allows multiple facilities to switch ON the particular device. It resembles function of OR gate. Any one path of the two parallel paths provides the flow of current and enables the device to operate.

6.8 Devices that are intended to perform a STOP function are normally wired in _____ with each other.

Devices that are intended to perform a STOP function are normally wired in SERIES with each other. STOP function devices are always given more importance than that of START functioning devices. It is somewhat emergency case. So series connection ensures implementation of AND gate, that means whenever STOP button is operated, rest of the circuit will get isolated from STOP button.

6.2 Describe the difference between a wiring diagram and a ladder (schematic) diagram.

Differences between wiring diagrams and ladder diagrams WIRING DIAGRAMS: (a) Shows the circuit wiring with its components together. (b) Diagram will be similar to the physical location. (c) Helps to locate position of the components. Doesn't bother about the simple way of describing. (d) Concerned about the physical relationship of the circuit. LADDER DIAGRAMS: (a) Shows the working of the circuit. (b) Showing actual location is not at all concerned, circuit behavior matters here. And provides enhancement in understanding circuit behavior. (c) Simplifies the circuit, and mostly in a sequential method, it represents the components. (d) Concerned about electrical relationship of circuit.

1.4 Define the term DISCRETE.

Discrete means discontinuous. If the magnitude of a certain quantity varies with respects to the something it is called discontinuous quantity. Likewise in some signals or in some electrical or electronic signals the magnitude of the quantity varies with time. The other name of the discrete signal is also called digital signal. These digital or discrete signals play important role in the present electronics

6.1 Define the terms NORMALLY OPEN and NORMALLY CLOSED.

Electrical devices usually represented in their normal status. Normal status refers that circuit is de-energised and no forces are acting on it, the forces being any pressure or flow. Here Normally Open means the component will be in open state when the circuit is de-energised. An example is holding contacts. Whenever Normally Open (NO) holding contacts occurs, it means holding contacts are in open condition when there is no power supply or when the circuit is de-energised. Its state or position gets changed whenever circuit is energized. Normally Closed (NC) means at the time of de-energised moment, the component is in closed state. This can be visible in control relay contacts. These NC contacts change their state or position whenever circuit is energized. Such type of contacts or components is called Normally Closed (NC) contacts.

2.8 I/O modules are keyed to prevent unauthorized personnel from removing them from the I/O rack. (TRUE/FALSE)

FALSE I/O modules are keyed to prevent unauthorized personnel from removing them from the I/O rack - FALSE The main reason for keying the modules to the racks is to avoid the replacement issues i.e., (it avoids the accidental or inadvertent replacement with an incorrect module). In a way it can be explained like this: suppose an input module is to be connected to a slot in the rack, and that particular slot is meant for output module and contains the wiring that is connected to output device, then replacing with an input module causes erroneous results in the system. To avoid this type of problems I/O modules must be keyed to the rack slots. This is the prime aspect though unauthorized personnel interaction is also a considerable issue.

3.15 The actual scan time, or time it takes the PLC to complete a four-step scan, decreases as the number of program words increases. (TRUE/FALSE)

FALSE The statement above said is FALSE. Because the time taken to complete four steps scan of any PLC will be increasing as the program size increases. The time taken will be generally varies from small fraction of milliseconds to 100+ milliseconds. In a way it can be concluded that it takes 3-5 milliseconds for scanning a 1K program. Depending on that it can be concluded that if the program size increases, scan time increases.

2.10 To ensure maximum benefit of shielding, the shield of a shielded cable must be terminated and grounded at both ends. (TRUE/FALSE)

FALSE To ensure maximum benefit of shielding, the shield of a shielded cable must be terminated and ground at both ends - False •Shield cannot be terminated at the device end but can terminated at the I/O rack because I/O rack which is properly grounded will be already connected to the earth ground. •Shield cannot be grounded at both ends because it creates a ground loop which introduces ground currents and thus leads to faulty signals.

1.10 Draw a block diagram and label the main components of a typical DC power supply.

FIGURE 1.1 The power supply basically consists of four components as shown above. The first one is the transformer. It steps down the voltage levels to the required voltage which is to be rectified. Next component is the rectifier which converts the AC voltage into DC voltage. Then comes the filter network to remove any unwanted disturbances present in the rectified voltage. These disturbances may be harmonics caused by the Rectifier Bridge. The comes the voltage regulator which can be used to control the DC supply to provide the required output voltage.

6.7 The horizontal lines of a ladder diagram are referred to as: a. Rungs b. Power ports c. Rails d. Tracks e. None of the above

Ladder diagrams have some vertical lines and some horizontal lines. Vertical lines are called RAILS. Horizontal lines are called RUNGS. These rungs hold the electrical components of the circuit to connect to voltage potential.

3.8 List the two broad categories of memory (not volatile and non-volatile).

Memory storage of a PLC (Programmable Logic Controller) can be divided into two types based on the usage. -USER MEMORY: •Stores user program •Consists of control logic for the driven equipment-in the form of relay logic and instructions -STORAGE MEMORY: •Used as a storage for the input/output status, present values of timers and counters •Data of internal control relays •This data is very important in controlling the equipment

4.4 In a PLC-5, data file 5 is what type of file?

PLC-5 processors belong to Allen-Bradley, in these processors family memory areas are referred as files. Each file carries a number. The memory is broadly classified into two types (i.e., storage and user memory). Memory areas or files in storage memory will be assigned from 0 to 999. Likewise user memory areas also will be numbered from 0-999 for storing user data (i.e., program, subroutines, etc.). In storage memory from 0-8 are pre-allotted files, in user memory from 0-3 will be reallotted. Here file number 5 means it comes under storage memory area. The file with identification number 5 will represent counter designated by C. All the counters that have used in programming will be prefixed by C5.

1.7 List the two types or styles of programming devices.

Programming devices are the ones meant for programming the required task and that program will be placed in PLC's (Programmable Logic Controller). There are two types of programming devices, hand held programmer and personal computer. Hand held programmer is used when the program is to be done in the remote locations or in the fields. It is convenient in terms of size, weight, and mobility. But the display issues and ineffectiveness in reviewing of the task, limit its usage. Another type of programming method is using personal computer. It is the most general and common type of programming methodology. Personal computer is to have a PLC programming software and a communication cable if required. The advantage of the PC is the large screen, program storage. It provides convenient troubleshooting and memory access.

1.3 Define the term REAL WORLD.

Real world refers to the actual input and output devices where a hard wired connection to the system is required. In fact these real world devices such as push buttons, limit switches, selector switches, timers, counters, and some other functioning devices can be developed in the PLC system without actual devices and hard wiring. Hence there must be a different name to identify the real devices and virtual devices, which lead to the invention of real world that refers to the actually existing device.

2.2 State two reasons for employing optical isolation.

Reasons to employ optical isolation are: •To avoid any chance of having electrical contacts between the processor and the input device. •To protect the processor from the voltage disturbances like noise, transients, voltage spikes, sags, and swells, etc. In a single sentence it can be said the optical isolation provides the safety of eliminating any unwanted voltages that originate from the input devices to reach the processor.

3.14 Describe the four steps of a typical PLC processor scan.

Soon after the successful self test, processor will be available to the work. It undergoes a four step scan as part of its working. They are: •Identifying the status of input devices. •Interprets the logic of the program. •Updating the output devices •Communication and Housekeeping. Step 1: meant for finding out the status of connected input devices. Step 2: takes the responsibility for the interpreting the logic of the program. Step 3: deals with the identifying and updating the connected output devices. Step 4: meant for doing the required memory management.

3.13 When a PLC is first turned ON, it will run a self-diagnostic or self-check test. (TRUE/FALSE)

TRUE When a PLC is first turned ON, it will run a self-diagnostic or self-check test. The above said statement is TRUE. Whenever PLC is turned ON at the very first time, it calls for a self test. During this test it checks all the parts like memory for any defects, I/O section for improper communication, or remote rack failures. Once everything is OK, processor of the PLC system initiates the pre-programed tasks.

4.7 Using Allen-Bradley PLC-5 address format, what would address I:013/12 indicate?

The address I:013/12 according to Allen-Bradley PLC-5 address format indicates an input device which is connected to terminal 12 of module 3, which is held in rack 01. It is translated according to the following format. X: XXX / XX -X: File Type •O: Output •I: Input -XX: I/O Rack -X: Module Group -/XX: Terminal Number

4.6 Using SLC 500 addressing, what do the following addresses indicate? a. O:3/15 b. I:2.1/3 c. O:5/0 d. I:7/8

The address locations regarding SLC 500 addressing: (a) O:3/15 - it indicates that it is the address of an output device that is connected to terminal 15 of the 3rd slot. In terms of programming methodology, it can be understood that the bit number 15 of output image table word 3 controls the connected output device. (b) I:2.1/3 - it indicates an address where an input device is connected to the 2nd slot and 20th terminal of the slot. It can be translated that an input device is connected to input image data word 2 which is larger than 16 bits. As it is larger than 16 bits, it is divided into two words of 16 bits like word 0 and word 1. Hence 2.1 mean input device is connected to (16+3) 19th terminal, as word begins from 0, 19 means it is 20th connected device. So the address given represents an input device which is connected to slot 2 and 20th terminal. Or the 20th location of input image word 2 contains the status of connected input device. (c) O:5/0 - it gives the address of an output device that is connected to the slot number 5 and terminal 0. SO the first bit of output image word 5 consist the controlling information of connected output device. (d) I:7/8 - it indicates the input device that is connected to slot 7 and terminal 8. It indicates the 8th bit of input image word 7 contains the input device.

2.14 List three environmental considerations when installing PLC equipment.

The environmental considerations that must be taken while installing PLC equipment is given below: •Temperature •Dust •Humidity There are other issues that also exists but the above said three are environmental issues to consider.

4.2 Identify the following PLC-5 files: a. I b. O c. N d. S e. B f. T g. R h. F i. C

The given PLC-5 files are: a. I-Input File b. O-Output File c. N-Integer File d. S-Status File e. B-Bit or Binary File f. T-Timer File g. R-Control File h. F-Floating Point File i. C-Counter File

6.13 Identify the following logic gates.

The given gates are: •NOT gate •OR gate •AND gate •XOR gate

1.9 What is the major advantage of a PLC system over the traditional hardwired control system?

The major advantage of PLC systems over the traditionally hard wired systems is its quick response in controlling the process of the driven equipment and provision of compact, reliable control components. Other advantages include elimination of hard wiring which is needed to be replaced whenever it requires a change. As today's machinery consisted of highly automated and specialized and high-speed manufacturing processes, it requires the most sophisticated rather than electromechanical relays, drum switches, mechanical timers. Hence PLC replaces with effective control equipment with the help of solid-state devices. With the help of PLC, place occupied also will be reduced.

3.1 The processor is often referred to as the _____ of the programmable controller.

The processor is often referred to as the BRAIN of the programmable controller. The decision maker for the programmable logic controller is the processor. Depending on this decision output devices will be controlled. So it takes the task like the brain of humans. Hence processor was termed as the brain of the programmable controller.

1.6 Define the term ANALOG.

The term analog refers to the quantity which varies continuously with the time. Its characteristics changes continuously with respect to the time. In general the sine wave is considered as ideal representation of analog signal. In real world, the power will be supplied in analog signals (i.e., voltage and current quantities are analog).

6.3 Explain the operation of the circuit in Figure 6-9 if M contacts 2 and 3 do not close.

The topic of discussion is what will happen if contacts 2 and 3 do not close. The contacts 2 and 3 shown in the figure are called holding contacts. Its normal position is Normally Open (NO) when the circuit is de-energised. Their state changes whenever there is a change in the circuit (i.e., any supply of power or pressure etc.). Here holding contacts 2 and 3 are connected in parallel to the start button. It means that START button and holding contacts are in OR relation. When start button is pushed, the starter coil M energies, after few seconds the START button released, meanwhile the M contacts (2 and 3) should close to start the motor. It is not possible to provide the path for current flow through start button for entire operation. So motor cannot start if the contacts 2 and 3 do not close.

3.4 1K of memory is actually. a. 1,000 words b. 1,010 words c. 1,024 words d. 1,042 words

c. 1,024 words 1K means 1 Kilobyte. It contains 1024 bytes. Each byte is represented by 8 bits. 1 word is equal to 8 bits. Hence it can be said that 1 word is equal to 1 byte (i.e., 8 bits). Here 1K means 1024 bytes (i.e., 1024 words). Remember the difference 1k is different to 1K. 1k means 1000 bytes but 1K means 1024 bytes. The only difference is small and capital alphabet 'k'. Therefore, the correct option is c.

2.13 Solid-State output devices tend to: a. never fail b. fail in the open or OFF condition c. fail in the shorted or ON condition d. not be affected by overload

c. fail in the shorted or ON condition Usually the solid state devices fail when they are in short or ON state. Failing while it is in OFF condition is rare. It adds another safety hazard to the PLC (Programmable Logic Controller) system. Therefore, the correct option is c.