Chapter 3 - I/O System

Question 1

Push button?

Answer 1

A type of discrete input. Uses a simple human interface for controlling some aspect of a machine or process. When pushed it changes the electrical operation from on to off or vice versa. Ex: keyboard keys, e-stop button

Question 2

Selector switch?

Answer 2

A discrete input device that also requires force to turn the electrical operation from off to on or vice versa. Usually locks into position

Question 3

Limit Switches?

Answer 3

A discrete input device that is operated by the motion of a machine part or the presence of an object. They are used as safety interlocks, controlling machinery or to count objects passing. It consists of an actuator linked to a set of contacts, when the object comes into contact with the actuator the device operates the contacts to make or break an electrical connection.

Question 4

Relay?

Answer 4

Relays are switches that open and close circuits electromechanically or electronically. Relays control one circuit by opening and closing contacts in another. Open (NO) -not energized. Closed (NC)- energized. Applying an electrical current changes the contact state.

Relays provide an electrical connection between two or more points in response to the application of a control signal. (time delay, monitoring, latching, safety)

Question 5

Electromechanical Relays?

Answer 5

Contacts are opened or closed by a magnetic force. They tend to provide a cleaner ON or OFF condition and don’t have the current leakage that SSR’s do.

Question 6

Solid State Relays?

Answer 6

There are no contact and all switching is done totally electronic using devices such as silicon controlled rectifiers.

Question 7

Motor Starter Contacts?

Answer 7

An discrete input device. A contactor is an electrically controlled switch which uses an electromagnetic coil to pull in the contacts. A motor starter is a contactor plus an overload relay that will drop out the coil voltage is the motor overloads.

Question 8

Photoelectric sensors (eyes)?

Answer 8

An discrete input device. It uses a light transmitter (often infrared) and a photoelectric receiver to discover the distance, absence or presence of an object.

Question 9

Proximity switch?

Answer 9

A discrete input device that detects the presence of an object nearby without physical contact.

Question 10

Thumbwheel switches?

Answer 10

A discrete input device that converts the numeric value selected into binary, decimal, hexadecimal or other code using the combination of ON and OFF signals for multiple contacts circuits and that outputs the resulting code.

Question 11

Circuit Breakers?

Answer 11

A discrete input device. An automatically operated electrical switch that protects an electrical circuit from damage due to excess current from an overload or short circuit. It’s basic function is to interrupt current flow after a fault is detected.

Question 12

Solenoids?

Answer 12

A discrete output device. A coil of wire used as an electromagnet. It converts electrical energy to mechanical energy by creating a magnetic field from the electrical current and uses the magnetic field to create linear motion.

Question 13

Other discrete outputs?

Answer 13

Alarms
Valves
Fans 
Lights 
Motor Starters

Question 14

The I/O System?

Answer 14

Consists of the I/O Rack and I/O module. The location where the module is inserted defines the address to reference each connected device. 
A  module usually has 8 inputs or 8 outputs and a rack will (in most cases) hold 8 modules. Therefore, a standard number of I/O points for one rack is 64.

Question 15

I/O Addressing?

Answer 15

Addressing is the way the control program in the CPU relates to a particular real world sensor or actuator. Addresses are either in Decimal or Octal form.

Decimal method follows a sequential assignment of I/O points. Ex: Inputs 1-8 (module 1) 9-16 (module 2) etc

Octal method is based on 8 digits, 0 through 7. Ex: Inputs 0-7 (module 1) 10-17 (module 2) 20-27 (module 3)

Typical address is usually 4 to 5 digits. Ex: Allen-Bradley
110 00 (I or O, Rack #, Module #, Connection Points)

Question 16

Flexible Addressing?

Answer 16

With flexible addressing you can create control logic software without being constrained to sequential I/O assignment, resulting in a randomly addressed I/O system.
I/O is addressing is done physically with Dual Inline Package (DIP) switches, or using EEPROM memory to contain the I/O data and is permanent after installation.
Advantage: allows PLC systems to be designed and installed in several stages
Disadvantage: Makes it difficult for troubleshooting and diagnostics

Question 17

Non-Flexible Addressing?

Answer 17

Individual slots and points are determined by the sequence in which the I/O racks are connected together (fixed by manufacturer)

Question 18

Four Tasks of Input Modules?

Answer 18

1. Senses the presence or absence of an input signal at each of its input terminals
2. Converts the input signal to a level usable by the interface’s electronic circuit
3. Electronically isolates the input and output stages of the module
4. Produces an output to be sensed by the CPU.

Question 19

Bridge Rectifier Circuit?

Answer 19

Detects AC and converts to DC

Question 20

Signal Conditioning?

Answer 20

A filter that protects against noise interference. Causes a signal delay of 9-25ms.

Question 21

Threshold Decision Circuit?

Answer 21

Detects whether the incoming signal has reached the proper voltage level for the specified input rating. If the signal exceeds and remains at the threshold voltage for a time period of at least the filter delay, the signal will be recognized as valid.

Question 22

Optical Isolation?

Answer 22

Usually a phototransistor (LED) or some other type of opto-coupler device ensures there is no electrical connection between the field device (power) and PLC (logic), to prevent voltage spikes from damaging the logic side.

Question 23

Transistor-Transistor Logic?

Answer 23

A 5 Volt DC I/O point which allows for interfacing with TTL compatible devices such as solid-state controls and sensors. Sometimes requires an external +5 VDC supply.
TTL output interfaces allow the PLC to drive output devices that are TTL compatible (7 segment LED displays) and require an external +5 VDC power supply.
Input configuration is similar to AC/DC inputs except the time delay from filtering is shorter.

Question 24

Mixed I/O modules?

Answer 24

Allows each point in the module to be either an input or an output. This saves in installation costs and allows the detection of failures both in the module as well as the sensors and actuators connected to it.

Question 25

Triac?

Answer 25

A semiconductor device capable of switching alternating current

Question 26

Solid-State Device?

Answer 26

An electronic component that controls current flow by use of solid materials such as semiconductors.

Question 27

Silicon Controlled Rectifier (SCR)?

Answer 27

A semiconductor device used to control power in high voltage applications.

Question 28

Components of a Circuit?

Answer 28

The incoming signal, the opto-coupler which isolates it, the signal rectifier which converts it into AC or DC, the signal conditioner which filters out noise and either a threshold circuit to validate signals or a control relay to control the signal load. Depending on if its an I/O.

Question 29

Binary Coded Decimal (BCD)

Answer 29

A multibit input module that allows groups of bits to be input as a unit to accommodate devices that require that the bits be handled in parallel form.

Question 30

Numerical Data I/O interfaces

Answer 30

Comes in two classes providing interface to multibit digital devices and analog devices. It is used to input data into specific register or word locations in memory.

Question 31

Register Input Modules

Answer 31

Input modules that temporarily store data

Question 32

Multiplexing?

Answer 32

Allows one transmission line to carry several digital signals, and is accomplished by each line being used in short intervals at prescribed times by a signal source. A strobe signal is used to indicate the timing.

Question 33

Transducer?

Answer 33

Converts one form of energy into another.

Question 34

Thermocouple?

Answer 34

Type of transducer that converts temperature to voltage. As the temperature in the metal varies, so does the voltage. A voltage amplifier (temp-measuring transmitter) is used to increase the signal strength to a level the PLC can understand. They are extremely susceptible to noise, twisting the lead wires or applying a foil shield helps alleviate this.

Question 35

Resistance Temperature Detectors (RTDs)?

Answer 35

Type of transducer that detects a change in resistance of metal over a range in temperature. As the temperature of the metal increases, the resistance also increases. The resistance signal is converted to voltage using a converter, such as a differential amplifier.

Question 36

Pressure Transducers?

Answer 36

Type of transducer used to measure and control fluids such as a liquid and a gas. Pressure is the force that a fluid exerts on its surroundings. Static pressure - fluid not in motion. Dynamic pressure - fluid in motion.
Differential transformers are used to measure pressure by measuring the force of displacement (ex LVDT). Also resistance, inductance and capacitance are used to measure displacement via diaphragms or bellows.

Question 37

Linear Variable Differential Transformer (LVDT)?

Answer 37

Used by pressure transducers to measure pressure. It measures force in terms of the displacement of the ferromagnetic core of the transformer. The movement or displacement of the core affects the emf. The polarity of the voltage is affected by the direction the core moved, the magnitude is determined by the distance it moved

Question 38

Bellows?

Answer 38

Used by pressure transducers to convert a pressure differential into a physical displacement. If the pressure increases or decreases the bellows will either expand or contract, moving the core and changing its inductance value. This is change is measured and converted into a proportional voltage signal.

Question 39

Flow Transducers?

Answer 39

Measures solid flow and liquid flow. Solid flow measurement involves a conveyor and a load cell. The load measures displacement, the greater the force on the load cell the greater the displacement of the movable core. As the conveyor speed increases, so does the flow rate.
Liquid flow is measured using Bernoulli’s principle. As the velocity of a fluid increases, its pressure decreases. When a fluid flows through a restriction its velocity will increase and it’s pressure will decrease.

Question 40

Humidity Transducers?

Answer 40

To control the environmental conditions PLC’s must be able to compare the amount of moisture required to the amount present. Relative humidity is the ratio of moisture to the maximum amount of water vapor that air can possibly hold.

Hygrometers measure the change in length caused by humidity. By placing hydroscopic material under tension, an LVDT transmits a signal as the material length varies.

Psychrometers use wet/dry bulbs to measure the relative humidity based on the rate or evaporation.

Question 41

Photovoltaic Transducers?

Answer 41

A transducer that will generate a voltage proportional to light intensity. As the light shining on the device increases, the voltage increases.

Question 42

Four Applications of Advanced I/O

Answer 42

1. Applications requiring a faster response than the CPU scan time.
2. Applications requiring the sending and receiving of alphanumeric data between peripheral devices and PLC
3. Motion control problems
4. Complex control problems such as three-mode (PID) process control.

Question 43

Four Factors in Evaluating Analog I/O?

Answer 43

1. Power requirements: large amount? external supply?
2. Temperature coefficient and total output drift: Modules ability to maintain accuracy or temperature ranges
3. Input impedance and capacitance: values must be know to match the external device to the module, measured in megohms and picofarads
4. Common mode rejection ratio: Modules ability to prevent noise from interfering with data integrity.

Question 44

Fast Response Module?

Answer 44

Type of Advanced I/O with a microprocessor that detects input pulses of very short duration. It counts and accumulates the pulse and provides the CPU the results during the normal I/O scan.

Question 45

ASCII Module?

Answer 45

Type of Advanced I/O used for sending and receiving alphanumeric data between peripheral equipment and the PLC. Can have limited or advanced circuitry. The advanced circuitry has a RAM buffer that temporarily stores data and a microprocessor and is used in network communications.

Question 46

Proportional Integral Derivative (PID)?

Answer 46

Type of Advanced I/O used to process control applications in which PID algorithms are used, aka three mode closed loop control. A PID module allows process control to take place outside the CPU to lessen the burden. Its microprocessor compares data to setpoints and determines the appropriate output signal.

Question 47

Motion Control Modules?

Answer 47

Type of Advanced I/O used to control linear and rotary motion control applications. It’s basic requirement is to provide an intelligent closed-loop motion control capability to effectively interface the PLC sequencing operation with precisely executed positioning commands to a servo drive positioning system. Open loop - the PLC has no way of determining the accuracy of commands sent by the module to the output device.

Question 48

Automatic ID Modules?

Answer 48

A type of Advanced I/O. Automatic Identification and Data Collection (AIDC) describes the automatic direct entry of data into a PLC system. Provides a reliable means of identifying and tracking items. Ex. RFID, Bar Codes.

Question 49

Resolver Interface Module?

Answer 49

A type of Advanced I/O. It is used to input the angular or rotary shaft position, of a resolver rotor into a PLC system. The resolver cannot rotate while the PLC is off or the processor may not be able to calculate the correct position. The resolver cannot rotate more that 1/2 turn in one PLC scan.

Question 50

Optical Shaft Encoder?

Answer 50

A digital transducer that converts angular shaft position into a binary coded signal. This type of sensor uses an encoder disk to provide rotary displacement measurement.
Absolute encoders can maintain position information even during a power failure. Incremental encoders (tachometers) which measure position and velocity, cannot.