PLC/Control Flashcards
what are central PLC logics?
1- FBD
2- LD
3- Sequential
4- Structural text
main logics?
1- AND
2- OR
3- NOT
Performance Level (PL)
In machine building, this refers to manageable units on which life safety systems are frequently used. ISO 13849-1 was developed with this context in mind. The standard takes into account the machinery directive and considers safety functions from both a qualitative and quantitative perspective. To classify various technical safety performance capabilities, it defines five Performance Levels (PL a, b, c, d, e), representing the average probability of dangerous failure per hour (PFH). ISO 13849-1 specifies an iterative process for designing and validating the safety-related parts of a control system (SRP/CS).
Safety Integrity Level (SIL)
The chemical industry was the driving force behind basic safety standard IEC 61508. As a result, the focus was on basing safety statements on how likely the reliable response to the function requirements is to occur, should a safe response be required. In terms of architecture, chemical and process-technical facilities tend to be large and complex. The life safety systems here are designed to rarely have to take action. The term SIL (SIL 1, 2, 3, 4) is derived from IEC 61508. IEC 62061 also defines the SIL claim limit, and describes the iterative process for determining and validating the SIL of safety-related electrical, electronic and programmable control systems (SRECS).
PL and SIL table
PL Mean probability of a critical outage [1/h] SIL
a ≥ 10-5 to < 10-4 –
b ≥ 3 x 10-6 to < 10-5 1
c ≥ 10-6 to < 3 x 10-6 1
d ≥ 10-7 to < 10-6 2
e ≥ 10-8 to < 10-7 3
PNP and NPN
PNP and NPN are abbreviations for two types of bipolar junction transistors that differ in polarity and how they work:
PNP
Stands for “Positive-Negative-Positive”. PNP transistors have a positive terminal and a negative terminal. They switch the positive side of a circuit and are sometimes called “sourcing sensors” because they provide positive power to the output. PNP transistors are less common in PLCs but can handle high voltage loads and provide good isolation between the input and output terminals.
NPN
Stands for “Negative-Positive-Negative”. NPN transistors have a negative terminal and a positive terminal. They switch the negative side of a circuit and are sometimes called “sinking sensors” because they sink ground to the output. NPN transistors are the most common type in PLCs and are known for their ability to switch quickly and handle high current loads.
PNP vs NPN in PLC applications
The main difference between PNP and NPN transistors in a programmable logic controller (PLC) is the polarity of the input they provide:
NPN
Stands for negative-positive-negative, and provides a negative input to the PLC. NPN transistors are more common in PLCs, and are often used in low-side switches, high-speed digital circuits, and high-frequency analog circuits.
PNP
Stands for positive-negative-positive, and provides a positive input to the PLC. PNP transistors are less common in PLCs, but are often used in high-side switches, high-voltage applications, and complementary push-pull amplifiers.
IEC 61131-2
The IEC 61131-2 standard applies to Programmable Logic Controllers (PLCs) and related software tools. It addresses electrical safety, but EMC requirements are covered in the IEC 61326 series standards.
The IEC 61131-2 standard for types of digital inputs for industrial applications
Type 1: Electromechanical circuits, such as pushbuttons and relays
Type 2: Discrete, high current semiconductor circuits, such as 2-wire proximity switches
Type 3: Integrated (low power) semiconductor circuits, which can be 2- or 3-connection sensors
The main differences between the types are the voltage and currents they produce:
Type 1: These inputs can’t be used for 2-wire semiconductor switches.
Type 2: These inputs have higher power consumption and are more suitable for low channel density.
Type 3: These inputs have lower power consumption than Type 2 inputs, even at high voltages. They are suitable for small modules with a high channel density.
