PLC's Flashcards
What allowed the development of PLC?
Microprocessors
What was the first PLC?
Modicon GM
How are PLC’s programmed?
using ladder logic diagrams or english
What do ladder logic programs use in their schematics compared to motor control schematics?
Ladder uses instructions
Motor control uses N.C. & N.O. contacts
What is ladder logic used to represent?
AND, OR, NOT plus arithmetic, timing, counting and memory functions
What are some advantages of PLC’s?
- Good for noisy industrial environments
- Has isolation
- Accepts inputs from pilot devices
- Outputs to contactors
- Work in wide temp. ranges
What does EEPROM stand for?
Electrically Erasable Programmable Read Only Memory
What are opto-isolators used for?
So there is no electrical connection between input & output devices
How many bits is 1 Byte?
8 bits
how many bits is a nibble?
4 bits
What does the input image table keep track of?
the input status of each input
what does the output image table control?
the status of each output
How does processor scanning work?
- Read input status
- Solve ladder logic program
- update the outputs
- repeat
what is the approx time for a scan?
1 to 200 milliseconds
what does scan time depend on? (3)
- # of I/O
- Program length
- Types of instructions
When is a device “sourcing” and the module “sinking” ?
if an input device switches +VDC to the input module (module supplies the -Ve)
When is a module “sourcing” and device “sinking” ?
if an input device switches -VDC and the module supples the +Ve
What will happen if a faulty program is downloaded?
PLC may be bricked (locked-up)
What are internal bits?
Registers internal to the PLC (virtual coils)
Where are internal bits used?
in programming for “one-shots” or where the status of one rung is required for a subsequent rung
What does latching do?
Sets an output on even when the logic in the rung becomes false
What would happen to latches during a power outage? Is low voltage protection provided?
They remain true, and low voltage protection would not be provided unless a MCR is provided
What is a One Shot Rising Instruction?
An instruction that will be true for only one scan
Where is an OSR typically placed?
In a rung after other “trigger” input instructions
What happens to the output instruction associated with the OSR?
it will only be true for the duration of the scan
What does the OSR do after the completion of the scan?
It will hold the rung false even with prior instructions still true
What happens to an OSR when the input logic goes false?
it will be triggered again when the rung conditions become true
What are some applications for an OSR?
- incrementing speed of a motor equally with one press of a button
- One button used to toggle a condition on or off
- Capturing a fast counter which would otherwise be unreadable
what are the 3 words for each timer?
Enable bit, timer timing bit, Done bit
What does the Enable Bit do?
is set (to 1) when the rung becomes true and reset (to 0) when the rung becomes false
What does the Timer Timing Bit do?
is set when rung becomes true and accumulated is less than preset value
What does the Done Bit do? What is it used for?
is set when accumulated value is greater than or equal to preset value. Used to control output devices
How does a Retentive timer work?
- RTO timer retains the accumulated value when the rung conditions are false and when the timer is done
- Must have a reset instruction addressed to it
List 5 basic sections of a PLC?
- Input module
- Output module
- Opto-Isolators
- Central Processing Unit
- Power Supply
Inputs wired to the input module control the _____________?
Input Image Table
List 3 basic operations that the CPU section of the PLC performs?
- Read the inputs
- Solve the logic program
- Write to the outputs
When will the output instruction in a rung become true?
If the rung conditions become true up to that point
What is the max current for a Triac Output Module?
2 A
What may Triac Outputs require and why?
May require a Burden Resistors in parallel with load or interposing relays. When off it may give enough leakage current to activate high impedance (small) loads
What do analog output modules have?
A digital to analog converter
Where can you not use a reset instruction?
Timer Off Delay
What must be done for fail safe wiring?
Input devices shall not be wired normally open in the field. Fail safe = normally closed or NOHC
What does shielded input wiring for sensors do?
Reduces ill effects of plant noise, EMI and RFI
How must you bond shielded input wiring? Why?
Bond one end only to prevent ground loops
How is brute force bonding achieved?
With large bonding conductors
What does a differential amplifier do?
Provides electrical noise cancellation
What can using interposing relays in Triac outputs do?
To avoid leakage current issue, and control loads with higher power requirements (higher voltage/current). Also used to interface different voltages (AC to DC)
List the power supply options from least to most desirable (5)
- Raw power
- Isolating transformer
- Voltage regulating transformer
- Conditioned power
- Uninterruptible power supply
Where does the main program run in a PLC?
In ladder file #2
Which files are designated for subroutines?
Ladder files 3 thru 255
What do subroutines do?
- Make programs more efficient
- improve organization for ease of understanding, interpretation and future editing
What does a JSR instruction do?
Redirects logic execution from current ladder file to specified subroutine file
What is a SBR instruction?
The 1st instruction of a subroutine
What is a RET instruction?
Directs the processor scan to return to where it came from
Why does the CPU have a key switch?
To prevent the programmed logic from being changed accidentally
What voltage do internal logic circuits of PLCs operate on? What must it be free from and regulated to?
5 - 15V DC
- must be free of voltage spikes and other electrical noise
- regulated to within 5% of the required voltage
What do discrete and analog mean?
Discrete - on or off
Analog - variable
What operating voltages are generally used for I/O modules?
120 Vac or 24 Vdc
What should have surge suppressors to prolong the life of a PLC?
Inductive loads such as relays, contactors and solenoid valves
What do surge suppressors do?
Reduce voltage transients and prevent electrical noise
What must an enclosure for a PLC provide?
Method of cooling either by convection or cooling fans
Where is it recommended a PLC be installed in a cabinet?
At the bottom
What is shielded cable used for?
Signal wiring
Where must signal cable never be ran?
In same conduit as power wiring
How should input and output devices never be wired?
In series or parallel arrangements
When is a PLC input said to be a sinking input?
If the input energizes when a high level voltage is applied
When is a PLC input said to be a sourcing input?
If the input energizes when a low-level voltage is applied
Why is there a problem using a sourcing input module?
If a ground develops between sensor and input module, the input will energize if the negative side of the power supply is grounded
