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Siemens has broadly 3 PLC ranges ie Siemens S7 200 , 300 and 400
Special Memory Byte 0 (SM0.0 – SM0.7) provides eight bits that are updated by the S7-200 CPU at the end of each scan cycle. Your program can read the status of these bits and then make decisions based on a bit’s value.
Always_On : SM0.0 This bit is always ON.
First_Scan_On : SM0.1 This bit is ON for the first scan cycle only. One use is to call an initialization subroutine.
Retentive_Lost : SM0.2 This bit is turned ON for one scan cycle if retentive data was lost. This bit can be used as either an error memory bit or as a mechanism to invoke a special startup sequence.
RUN_Power_Up : SM0.3 This bit is turned ON for one scan cycle when RUN mode is entered from a power-up condition. This bit can be used to provide machine warm-up time before starting an operation.
Clock_60s : SM0.4 This bit provides a clock pulse that is OFF for 30 seconds and ON for 30 seconds, for a cycle time of 1-minute. It provides an easy-to-use delay or a 1-minute clock pulse.
Clock_1s : SM0.5 This bit provides a clock pulse that is OFF for 0.5 seconds and then ON for 0.5 seconds for a cycle time of 1 second. It provides an easy-to-use delay or a 1-second clock pulse.
Clock_Scan : SM0.6 This bit is a scan cycle clock that is ON for one scan and then OFF for the next scan. This bit can be used as a scan counter input.
Mode_Switch : SM0.7 This bit indicates the current position of the Mode switch (OFF=TERM position and ON=RUN position). You can use this bit to enable Freeport mode only when the switch is in the RUN position. Normal communication with the PC/programming device can be re-enabled by switching to the TERM position.
This warning message indicates that the timestamps for the project do not match the timestamps for the program in the PLC. This may indicate that the programs are different, in which case it would be dangerous to continue the current operation. However, the programs may be functionally identical and still have different timestamps.
Each program contains two distinct timestamps; the “Created” timestamp and the “Last Modified” timestamp. The created timestamp is set when the project is created by the New Project option. The Created timestamp is not affected by any user edits or program compilation.
The Last Modified timestamp is used to indicate when the user last modified the program. There are many conditions that cause the Last Modified timestamp to be set, it includes Edit of instructions or operands in the program, Adding, deleting, or modifying a Local or Global Symbol, Adding or deleting a POU, Compiling the program block and Downloading the program block (this automatically compiles the program block and therefore sets the last modified timestamp).
STEP 7-Micro/WIN 32 provides the “Compare” option, to allow you to determine whether the programs are same or different.
Fatal errors cause the PLC to stop the execution of your program. Depending on the severity of the error, a fatal error can render the PLC incapable of performing any or all functions.
To enable Write and Force functions while in STOP mode, choose the Debug > Write-Force
Outputs in STOP menu command.
The S7-200 PLCs support writing and forcing outputs (both analog and digital) while the PLC is in STOP mode. As a safety precaution, you must specifically request this functionality to be enabled in STEP 7-Micro/WIN 32. The menu option Debug>Write-Force Outputs in STOP enables you to Write or Force outputs while the PLC is in STOP mode.
Warning If the S7-200 PLC is connected to equipment when you write or force an output, these changes may be transmitted to the equipment. This could result in unanticipated activity in the equipment, which could also cause death or serious injury to personnel, and/ or damage to equipment.
Each time STEP 7-Micro/WIN 32 is opened, the menu option defaults to unchecked, and you are prevented from writing or forcing outputs while the PLC is in STOP mode. Checking the menu option enables writing and forcing for the current editing session with the current project. When a different project is opened, the menu option returns to its default state and you are prevented from either writing or forcing output addresses while the PLC is in STOP mode.
The example program below shows formatting in the STL editor according to the guidelines given in How to Enter a Statement in STL. You might want to set up your program similarly, by having the network comments give a general overview of the function of the network and the statement comments identify the function of the statement. Note that the sample is written with symbolic addresses.
If you would like to see this program (in either STL or LAD) in STEP 7-Micro/WIN 32, select File>Open, browse to the STEP 7-MicroWINSamplesEnglish directory, highlight the sample program and click the Open button.
Sample Program for a Paint Mixer:
NETWORK 1 //Fill the tank with ingredient 1 and monitor tank level.
//Maintain Pump Status if Start Switch Opens.
LD Start_1 //Load value of Start_1 I0.0.
O Pump_1 //Or with Pump_1 Q0.0 value. A Stop_1 //And with Stop_1 I0.2.
AN High_Level //And Not with High_Level I0.4.
= Pump_1 //Assign result to Pump_1 Q0.0.
NETWORK 2 //Fill the tank with paint ingredient 2 and monitor tank.
//level. Maintain Pump Status if Start Switch Opens.
LD Start_2 //Load value of Start_2 I0.1.
O Pump_2 //Or with Pump_2 Q0.1 value. A Stop_2 //And with Stop_2 I0.3.
AN High_Level //And Not with High_Level I0.4.
= Pump_2 //Assign result to Pump_2 Q0.1.
NETWORK 3 //Set memory bit if High Level is reached.
LD High_Level //Load value of High_Level I0.4.
S High_Lev_Reached, 1 //Set High_Lev_Reached M0.1 to 1 (on).
NETWORK 4 //Start Timer if High Level is reached.
LD High_Lev_Reached //Load value of High_Level_Reached M0.1.
TON Mix_Timer, +100 //Timer 37, Preset = 100 (@0.1 s).
NETWORK 5 //Turn on Mixer Motor.
LDN Mix_Timer //Load Not value of Mix_Timer T37.
A High_Lev_Reached //And with High_Lev_Reached M0.1.
= Mixer_Motor //Assign result to Mixer_Motor Q0.2.
= Steam_Valve //Assign result to Steam_Valve Q0.3.
NETWORK 6 //Drain mixing tank.
LD Mix_Timer //Load value of Mix_Timer T37.
AN Low_Level //And Not with Low_Level I0.5.
= Drain_Valve //Assign result to Drain_Valve Q0.4.
= Drian_Pump //Assign result to Drain_Pump Q0.5.
NETWORK 7 //Count each cycle.
LD Low_Level //Load value of Low_Level I0.5.
A Mix_Timer //And value of Mix_Timer T37.
LD Reset //Load value of Reset I0.7.
CTU Cycle_Counter, +12 //Counter 30, Preset = 12.
NETWORK 8 //Reset memory bit if Low Level reached and Timer //timed out.
LD Low_Level //Load value of Low_Level I0.5.
A Mix_Timer //And value of Mix_Timer T37.
R High_Lev_Reached, 1 //Reset value of High_Lev_Reached M0.1 to 0.
A. Protocol used in the in S7-200 PLC is PPI Protocol
B. 32 station Can Be Possible in the S7-200 Network
S7 300 PLC Questions
In a STEP 7 program you work with addresses such as I/O signals, bit memory, counters, timers, data blocks, and function blocks. You can access these addresses in your program absolutely, but your programs will be much easier to read if you use symbols for the addresses (for example, Motor_A_On, or other identifiers according to the code system used within your company or industry). An address in your user program can then be accessed via this symbol.
An absolute address comprises an address identifier and a memory location (for example, Q 4.0, I 1.1, M 2.0,FB21).
You can make your program easier to read and simplify troubleshooting if you assign symbolic names to the absolute addresses.
STEP 7 can translate the symbolic names into the required absolute addresses automatically. If you would prefer to access ARRAYs, STRUCTs, data blocks, local data, logic blocks, and user-defined data types using symbolic names, you must first assign symbolic names to the absolute addresses before you can address the data symbolically.
You can, for example, assign the symbolic name MOTOR_ON to the address Q 4.0 and then use MOTOR_ON as an address in a program statement. Using symbolic addresses it is easier to recognize to what extent the elements in the program match the components of your process control project.
An instance data block is assigned to every function block call that transfers parameters. The actual parameters and the static data of the FB are saved in the instance DB. The variables declared in the FB determine the structure of the instance data block. Instance means a function block call. If, for example, a function block is called five times in the S7 user program, there are five instances of this block.
Creating an Instance DB
Before you create an instance data block, the corresponding FB must already exist. You specify the number of the FB when you create the instance data block.
One Instance DB for Each Separate Instance
If you assign several instance data blocks to a function block (FB) that controls a motor, you can use this FB to control different motors.
The data for each specific motor (for example, speed, runup time, total operating time) are saved in different data blocks. The DB associated with the FB when it is called determines which motor is controlled. With this technique, only one function block is necessary for several motors.
A system function is a preprogrammed function that is integrated on the S7 CPU. You can call the SFC in your program. SFCs are part of the operating system and are not loaded as part of the program. Like FCs, SFCs are blocks ”without memory.”
S7 CPUs provide SFCs for the following functions:
If you using HW Config to revise a station configuration and you want to exchange a module for one with a new order number for example,
proceed as follows:
This procedure is faster than exchanging modules by deleting the old module and then inserting the new one and assigning parameters to it.
You can turn this function on or off in HW Config by means of the menu command Options > Settings (“Enable Module Swapping”)
Complex data types define data groups that are larger than 32 bits or data groups consisting of other data types.
The following table describes the complex data types. You define structures and arrays either in the variable declaration of the logic block or in a data block.
Data Types :
Defines an area with 64 bits (8 bytes). This data type saves in binary coded decimal format: DT
Defines a group with a maximum of 254 characters (data type CHAR). The standard area reserved for a character string is 256 bytes long. This is the space required to save 254 characters and a header of 2 bytes. You can reduce the memory required for a string by defining the number of characters that will be stored in the character string (for example: string ‘Siemens’).
Defines a multidimensional grouping of one data type (either elementary or complex). For example: ”ARRAY [1..2,1..3] OF INT” defines an array in the format 2 x 3 consisting of integers. You access the data stored in an array using the Index (”[2,2]”). You can define up to a maximum of 6 dimensions in one array. The index can be any integer (-32768 to 32767).
Defines a grouping of any combination of data types. You can, for example, define an array of structures or a structure of structures and arrays.
Simplifies the structuring of large quantities of data and entering data types when creating data blocks or declaring variables in the variable declaration. In STEP 7, you can combine complex and elementary data types to create your own ”userdefined” data type. UDTs have their own name and can therefore be used more than once.
You determine the structure of the assigned instance data block and allow the transfer of instance data for several FB calls in one instance DB.
Structured data types are saved in accordance with word limits (WORD aligned).
Multi-Point Interface ( MPI ) :
Data Transfer – 187.5 kbits to 15 Mbit/s,
Distance – 50 m without RS 485 repeater / 10 Km with repeater
Number of nodes – up to 32
Data Transfer – 12 Mbit/s,
Distance – 23 Km with fibre optic cable
Number of nodes – up to 125
Simantic S7 manager uses DB, OB, FC, PB and FB
OBs : Determine the structure of the user program
Data Block : These are the blocks used by logic blocks in CPU program for storing the data. DB’s doesnot contain any instructions and it take up space in the user memory. The user program can access a data block with bit, byte, word or double word operations.
Global data block : These contains information that can be accessed by all the logic block in the user program.
Instance data block : These DBs are always assigned to a particular FB.
FC Functions : It is a logic block without memory. An FC is always executed by calling in another block. FC is used either for returning a function value to a calling function or executing a technological function. Temporary variable belonging to FC are saved in local stack and this data is lost when the FC has been executed.
Function Blocks (FBs) : A function block is block with a memory. A FB contains a program that is always executed when a different logic block calls the FB. FB make it much easier to program frequently occurring complex functions.
SFBs and SFCs are integrated in the S7 CPU and allow you access to some important system functions.
Statement List (STL) is a textual programming language that can be used to create the code section of logic blocks. Its syntax for statements is similar to assembler language and consists of instructions followed by addresses on which the instructions acts.
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