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41 Cards in this Set
- Front
- Back
- 3rd side (hint)
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What language is used in PLC’s? |
Ladder logic |
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Why are PLC’s more reliable than hardwired circuits? (3) |
- solid state -fewer parts (cheaper) -smaller -faster -easily expandable |
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What is downloading? |
A process that uses a programming terminal to copy a user program into the PLC memory |
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What are the two main sections of PLC memory? |
Executive and user |
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In the above diagram, what is the purpose of the bridge rectifier? |
Converts AC signal to a DC signal |
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In the above diagram, what is the purpose of #2 portion of the circuit? |
Noise debounce filter- removes unwanted AC noise from the incoming signal |
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In the above diagram, what is the function of the device labeled “optical coupler”? |
It optically isolates the input signal from the PLC’s processor signals. |
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1: What are 4 basic functions provided on an output card? 2: What does the input card have that the output card does not have? |
1: termination, noise suppression, isolation, indication. 2: signal conditioning. The input device however, does not have noise suppression |
Output card and input cards have 3 of the same functions, but they both have 1 different function. |
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How can you tell the difference between and AC output card and input card? How can you tell the difference between AC And DC output cards? |
Output card has a Triac, whereas the input card has a bridge rectifier. The output card also has an Metal oxide varistor(MOV), as opposed to the input module which has a zener diode Ac card has triac compared to DC card which has a transistor |
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What is the purpose of the “MOV” on the above sinking type output card? |
Noise suppression to protect the card |
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When might interposing relays be required? |
If the loads to be switched are of higher voltage or current |
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What type of input is opposite of a discrete input? |
Analogue input |
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Why are triacs commonly used in PLC outputs rather than relays? |
Because they are solid state therefore they are more reliable |
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What are 5 factors to consider during the installation of PLC’s? |
-permissible limits of temperature (5-40 degrees C) -humidity -dust -location -shock and vibration |
Physical environment |
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According to the module, how far away must communication cables be kept from power cables, and how must communication cables cross power cables? |
-Must be kept 300mm from power cables (600mm if power cable is 300V or higher according to CEC 60-308 (1) ) -Must cross power cables at a right angle |
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Grounding and shielding is done in a radial fashion to prevent what? |
Ground loops |
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In the above diagram, two systems are in the same cable. Under what circumstances would CEC allow this? |
12-3030 (1)(b) must be of the same insulation rating of the highest voltage |
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What is the main reason for installing a master control relay in a PLC system? |
Acts as a safety shut down for the whole system |
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What is the main reason for installing a master control relay in a PLC system? |
Acts as a safety shut down for the whole system |
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During an operating cycle in a PLC, what are the two basic parts of a processor scan? What is the scan time? |
Program scan and I/O update. The scan time is the amount of time it takes to complete the entire process scan |
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If that state of the inputs change during the program scan, when does the change take effect? |
During the start of the next program scan |
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What are 3 items which the scan time of the processor depends on? |
-number if I/O’s -how fast the processor is -the length of the ladder logic |
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What portion of the operation cycle does the processor read the input status and write to the outputs? |
During the I/O update |
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What problem can a slow scan time cause? |
It may miss a high speed input |
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What are the 3 main ladder logic instructions that we use? |
-output energize instruction -examine if on instruction (XIC) -examine if off instruction (XIO) |
-( )- -] [- -]/[- |
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What is the last instruction to be entered in a logic rung? Coil, contact, overload or XIO instruction? |
Coil |
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When will an examine if on instruction be true? |
When a bit in the memory is 1 |
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Are internal relays accessible at the I/O rack? |
No |
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When is a PLC timer said to be enabled? |
When the rung is true |
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How is the essential part of the volatile memory maintained in the CPU |
Battery |
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What is the basic function of an I/O module? |
Provides a physical link between actual field equipment and the CPU |
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What are the 3 main types of I/O modules? What is an example of each? |
Input: Discrete: contacts, electronic switches Analogue: potentiometer, thermistors Pulse generated: shaft encoder Output: Discrete: solenoid valves, pilot lights Analogue: variable speed drive input references, servo-valve coils Pulse-driven: stepper motor |
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How are the I/O modules protected from damage due to accidental connection to incorrect voltage? |
Optical isolation |
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According to CEC, can you use a PLC as a disconnecting means? |
14-700 no, you can not use a solid state device as a disconnecting means |
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How is a PLC timer reset? |
When the power to the rung is killed (or reset) |
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Which PLC timer bit would you use to see if the timer was on? What bit do you use to see if the timer is done? |
Enable bit Done bit |
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Is there a physical connection between the input devices and the output devices? |
No |
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What does “fail-safe” mean? |
If the circuit loses power, the program will fail in its normal state (usually off) |
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To stop a conveyor, which switch would be considered fail safe- NC or NO limit switch? |
NO |
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Which would be considered fail-safe, using a NC or NO limit switch to start a conveyor? |
NC |
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How many milliseconds is a typical scan time? What is the range in milliseconds of a scan time? |
Typical scan time is 20msec but can be anywhere from 5milli-200milli seconds depending upon speed of CPU |
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