What is Inside Siemens S7 300 PLC

Have you ever wonder what is inside a Siemens S7 300 PLC ? . You came to the right place . We are going to discuss what is inside S7 300 PLC and also we are going to discuss how each unit inside this PLC works . The design of this PLCs . You may have heard some people say that PLCs cannot be repaired . But , We will show you how to troubleshoot and repair these PLCs . After reading this article you will realize that how simple is the design of these PLC .


Before see what is inside Siemens S7 300 PLC , We have to disassemble the PLC.  The disassembly process is very easy . The whole thing is held by plastic tabs or locks . There are three locks in top and three locks in bottom of the PLC . To unlock these locks follow these steps .

Disassembly S7 300 Plastic Locks

Unlocking Plastic Locks

• Use a flat screw driver or a tweezer and put it between the locks opening hole.

• Turn the tweezer or screw driver to detach the lock.

• Once the lock is detached put some thin wadge between it . This will prevent re-locking  .

• Follow above steps on each lock at top and bottom of the PLC.

• Once all the locks are open , Pull the top section of the PLC out .The top housing part of the PLC will come out with the removable PLC power connector.

Siemens S7 300 Inside

Siemens S7 300 Inside

• It’s time to remove each card inside this base model . Pull the input and output cards out shake it while removing it .

• You can pull the power/Interface card in the same way you removed . But ,In this case there are two parts together .The board itself and the metal cover next to the board . You can remove the screw nuts on the metal cover and remove it separately .

Siemens S7 300 Plastic tabs

Siemens S7 300 Plastic tabs

• Once all sub boards are removed the mother board needs to be removed . It’s held by three plastic tabs you can easily push these locks and pull the motherboard out. But , Be care full these plastic locks can break easily .

What is Inside Siemens s7 300 PLC

In this section we will look into what is inside s7 300 PLC . We have four PCBs in this PLC base module Motherboard , Input card , Output card and Power / Interface card .


S7 300 PLC Motherboard

S7 300 PLC Motherboard

The motherboard is where the processor or the ASIC is situated . All the expansion modules or cards connects to the motherboard via ports. There are bus buffer ICs near each of these ports . The processor communicates with these cards via bus buffer ICs .There are two types of buffer ICs in this case Octal buffer IC (part no – HA541) and quad buffer IC (part no – HB125). There is a Real time clock IC with its external  oscilator  . This real-time clock IC is made by Siemens (Part no : RTC-132A0517) . Right next to the processor you will see the RAM IC (Part no : K6F1616U6C) .It’s a 16Mb(1M x 16 bit) Low Power SRAM IC in BGA package manufactured by Samsung Electronics.  Right next to the RAM IC you can find the ROM IC (Part no : PL032J70BA) .Its a 32Mbit ROM IC in BGA package manufactured by SPANISON . In the back side of this PCB , You will find a power circuit with both linear (L1117) and switching regulator (L4971D) ICs both manufactured by ST Microelectronics . There are also the expansion module port available on this motherboard.


SXA1020B2 Siemens Processor



Part number – SXA1020B2-1

Custom made for Siemens by Infineon .

External 12 Mhz Oscillator being used.

Package – BGA


Input Card

This base model have 16 input digital card . This card have seperate power input (24v) . So , In order to this card to work you have to supply 24 volt dc through power input pin. Here is the function and what is on this s7 300 PLC base model’s input module.In this input card there are two sections with separate power input . Each section  have duplicate components but also few common parts .

Block Diagram Siemens S7 300 Input card

Input card

Working Principle – The input from each terminal goes through a series of resisters an the end it connects to the anode of the LED input indicator . The voltages taken from different stages of this series of resistors . One of these voltage is given to comparator input pin of quad comparator IC (Part no : 2901 by ST microelectronics) . Each of these ICs will compare four inputs with a reference voltage .If any input exceed the reference voltage the output pin of the comparator changes the voltage level .This output of the comparator connected to input of eight bit parallel to serial IC (Part no : 74HC165 ). There are four of these parallel to serial ICs which will send the status of input pins via serial out pin . The clock signal for these parallel to serial ICs comes from the processor . This clock signal is common for all parallel to series ICs . The clock signal allow the IC to shift the input status of the pins as serial data through serial out pin .  Even though , These ICs have eight bit input only for inputs from each IC being used. So in order to process all the inputs they used  four of these IC’s . The serial out of these ICs forwarded to the processor through optocoupler ICs (part no : HCPL-611). There are six of these optocoupler ICs to isolate the motherboard side from the input card side.

Output Card

The Digital output card of this PLC base module have a simple design when compared to the Input card we discussed in This section. As in the digital input card there are two separate sections. Each have eight outputs . Each section have a separate power input pin to power up the components inside . The output pin is directly connected to the output pins of High side drive IC (Part no : BTS 4880) . This ICs output pin can drive a load up to 0.675 A . These output pins are driven by an array of FET Transistors inside the IC. The IC have overload protection , Short circuit protection to prevent any damages to the IC and the connected components . However , It’s always good thing to remember to avoid connecting any component that draws more than 0.675 A current . If such load need to be driven you can use a external relay or external load driver circuit. The LED indicator is directly connected to the output through a resistor . The Output logic status of this high side switch IC depends on the logic status of input pins.


Output card

Output card

Working Principle – The inputs for these ICs are connected to the output pins of serial to parallel (75HC595) IC through a series of optocoupler ICs (D217E1). The serial data contains each pins output state was transmitted to the 74hc595 IC and it will convert this serial data into parallel data which will be available on the eight output pins of these ICs . These outputs where connected to the LED side of the optocoupler IC . The other side of that optocoupler is connected to the input of the high side switching IC . The output logic status will change according to the input logic status .Thus , changing the logic levels at the output terminals .


Power / Interface Card

• Power Section – This card is where the power input terminals for the PLC base model is situated . There are six regulators in this board four of them are switching regulators . These regulators provide the power required for this board and also for the motherboard. There also some linear regulators like LM317 present in this board .

• Interface Section – When it comes to interface there are mainly two interfaces the DP or the profibus interface and the MPI interface . These interfaces use the separate RS232 serial ports . DP port have a isolated power source . This power source includes a transformer and a transformer drive IC (Max232) . Data from Both of these ports are goes through transceiver ICs (Part no : 65176B and VP1176) before reaching to the processor . This is due to voltage differences . This transceiver ICs  will change the voltage levels of the data signals .

• Memory card  – The memory card port is directly connected to the motherboard port .However , The power for the memory card is supplied by on board regulator IC .The SD card port is manufactured by the famous company Molex .

Conclusion – What is Inside Siemens S7 300 PLC

The Siemens S7 300 PLCs have a very simple and compact design . Most of the time it’s very easy to repair . Majority of components are available in the market you can replace them easily . The PCB in this case is multi-layer . Use proper tools to avoid damages to components or PCB itself .However , The processor or ASIC  is not available in the market . You can find the separate cards in Ebay and other online auction and shopping websites .