WHAT COMPONENTS MAKE A SIEMENS PLC S7-300 (PROGRAMABLE LOGIC CONTROL)?
Table of Contents
In this SIEMENS PLC S7-300 course, different modules will be used in some specific order. The order of the modules is very important because it tells the PLC what to do with these modules. If a particular order is not followed then there can error in the communication of PLC with other modules present in the complete setup.
The sequence or Order of these modules in PLCs is as follows:
PS → CPU→ DI → AI → DO → AO → FM → CP
PS: Power Supply Module
CPU: Central Processing Unit Module
DI: Digital Input Module
AI: Analog Input Module
DO: Digital Output Module
AO: Analog Output Module
FM: Function Module
CP: Communication Processor Module
BRIEF INTRODUCTION OF SIEMENS PLC S7-300 MODULES:
Input is 220V AC and Output is 24V DC for all modules.
Output current could be 2A, 5A, or 10A according to the requirement of modules connected to that rack.
Brain of the PLC. Does the same functions as CPU in computers / laptops.
Depending on the system inputs and outputs, these modules are used.
To achieve higher accuracy in some specific task, we need function modules. These specific signals cannot be handled with input/output modules to attain high accuracy. Funciton modules bypass the main CPU module to process the signals for higher accuracy using their own built in CPU’s.
These module contains industrial communication interface like MPI, Profibus and Profinet ports. In some exceptional cases, we need additional communication ports which can be provided using CP modules.
These are typical modules used in SIEMENS PLC S7-300 but not necessarily are all used. It depends on the control plant/process by PLC and which modules are essential for automation and control purposes.
POWER SUPPLY MODULE:
It takes 120-220 V single-phase AC power and converts it into 24 V DC supply which is used for all other modules. All other modules are designed specifically on 24 V DC supply.
We can connect 24V DC output power to the CPU using two following methods:
It doesn’t need any data communications. So, we don’t have any data communication port at the rear side of the power supply module because its only purpose is to provide a regulated supply of 24V DC power.
SIEMENS PLC S7-300 CPU MODULE TASK:
It is used to process the instructions given by the computer for operation of the plant. If the CPU stops working, all the working of the plant will stop immediately.
New and old CPU differs from each other in the following things:
- Mode of selectors switch (See the Old S7-300 module image)
- Place of memory card holder (See the image comparison)
- Size of the module (See the new S7-300 module image)
In old CPUs, there are four modes of operation called:
- Run-P
- Run
- Stop
- MRes (Memory Reset)
Old S7-300 CPU (See selector switch)
New S7-300 CPU (See selector switch)
RUN-P MODE:
This mode is used to send the data into the CPU from the computer. In this mode CPU processes that data. We can download data from the computer and also upload existing data from the CPU to the computer in Run-P mode.
RUN MODE:
This mode is exactly same as Run-P mode but only exemption of downloading data from computer.
STOP MODE:
This mode stops the processing of the data.
MRes MODE:
This mode is used to reset or erase the CPU memory.
CPU MEMORY TYPES:
Before digging into the topic of CPU memory types, I will present a review of general CPU memory types used in computers.
General Memory Types:
Following are the general types of memories used in all computers:
- Random Access Memory (RAM)
- Read Only Memory (ROM)
- Erasable Programable ROM (EPROM)
- Electrically Erasable Programmable ROM (EEPROM)
Random Access Memory ( RAM: )
RAM is the most common memory type used in all computers. It is faster than other conventional memories like CD, DVD, and audio tape because RAM needs the same amount of time to access the stored data from any location which is not the case for conventional storage. RAM is volatile memory that means all the data will be loss if power goes off.
Read Only Memory ( ROM: )
ROM stands for Read Only Memory which means that data can only be readable and cannot be modified. ROM is non-volatile memory that means it store all the data even after the power goes off.
Erasable Programable ROM ( EPROM: )
EPROM is an Erasable Programable ROM which is also non-volatile memory but we can do erase and modification work in stored data. Ultraviolet light is used to erase the stored data in EPROM which is a difficult and time-consuming task to re-write ROM.
Electrically Erasable Programable ROM ( EEPROM: )
EEPROM is the solution to the difficulty of the time-consuming process of re-write ROM. Although, the rest of the functions are the same as EPROM. It has the drawback of limited number of time erasing capability. This drawback is converted in Flash EPROM which is much faster than EEPROM and also has the greater storage capacity.
Flash Electrically Erasable Programable ROM ( EEPROM: )
It is the advanced version of EEPROM and has more storage space, much faster
