Table of Contents
Memory types of SIEMENS PLC S7-300 Central Processing Unit CPU)
We have studied in our class-01 about the basics of SIEMENS PLC. Now, we will differentiate between different modules of S7-300 PLC. We also learned about the general types of storage memories used in our computers or mobile phones. Now in this class-02, we will see the details of specific memories which used in PLC. To understand the workings of SIEMENS PLC, we have to get familiar with the basic workings of these memories present in Step-7 PLCs.
BRIEF INTRODUCTION OF SIEMENS PLC S7-300 CPU Memory Types
Step-7 PLC storage memories will be volatile or non-volatile in general nature. Similarly, these can be EEPROM type or Flash type as well.
PLC Load Memory:
It stores whatever we download in PLC from our computer. Load memory can be internal or external.
The internal type was used in old Step-7 PLC which was type of RAM memory. To protect data from load memory, a backup battery was used in case of power failure.
External type is used in new S7 PLCs as a micro memory card (MMC) which is a flash type of memory. CPU cannot run without this MMC.
PLC Working Memory:
It stores executive data which is already downloaded into PLC load memory. It works pretty much similarly to Random Access Memory (RAM).
PLC System Memory
It stores the Input/Output addresses of the physical system
PLC Retentive Memory:
It stores data permanently like in BIOS of computer. We can store special data in this memory which required long-term use.
SIEMENS PLC S7-300 CPU PORTS for DATA TRANSFER
There are series of ports which can be used to connect one PLC to other PLC or an automation facility present on the site.
Multi Point Interface (MPI) Port:
Most of the time, we use this port to transfer the exiting program to PLC from computer. We need PC adapter to perform this function. This PLC adapter has one usb-port end which is connected to PC and other has port which can be connected to MPI port.
Similarly, we can use MPI port to form the MPI network. One network connection can be used as connection between CPU and HMI panel. We use RS-485 cable for this CPU to HMI connection.
Decentralized Peripherals (DP) Port:
We can use this port in PLC-DP network. This port is used to connect other automation facilities using profibus (Process Field Bus) DP network. RS-485 cable is used to setup this DP network. Interface Module (IM) is used to form this DP network.
Point to Point (PtP) Port:
This port is used in MODBUS network. We can directly connect a printer or bar-code reader to this PtP port as well.
Profi-Net (PN) or Eathernet Port:
This ort can be used to build-up profi-net network. This is common port present on other automation facilities like IM and HMI. We can also built profi-net network using this PN port and communicate through networking cable.
S7-300 CPU STATUS & ERROR INDICATORS
There are series of LED’s on SIEMENS S7-300 PLC which shows the modes of operation in different situations.
System Fault (SF) LED (Red):
It indicates error in the system. It could be software or hardware error. It will also turns ON when supply of Data Module gets OFF.
Bus Fault (BF) LED (Red):
It turns ON when we have error in network we have used. If we used Profi-Bus DP network and some of connector is not properly connected then BF LED will glow red. Disconnection of DP network cable can also be one of the cause of BF. An overlap of addresses in network also light-up BF LED.
If we use 2 as CPU address of slot and also use 2 for CP module then it will cause overlap of addresses.
Maintenance Mode (MAINT) LED (Orange/Yellow):
Whenever this LED light-up, it means that CPU cannot run and needs troubleshooting process. This happens very rarely in the system.
Force Mode (FRCE) LED (Yellow):
When this LED light-up, it means that one of input or output is in force mode.
Run Mode (RUN) LED (Green):
It turns ON when CPU is in running mode and everything is working fine;
Stop Mode (STOP) LED (Yellow):
It turns ON whenever we stop CPU processing from the CPU selector switch or when any software fault occurs while processing the program.
