Frequently Asked Questions about Siemens S7300 PLC

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1: How can one avoid the “communication failure” message when using the CPU 315F and ET 200S? By using the CPU S7 315F, ET 200S, and fail-safe DI/DO modules, you will call the fail-safe program of OB35. Furthermore, you have accepted the default values for all monitoring times and are willing to receive “communication failure” messages. OB 35 is set to 100 milliseconds by default. You have set the F monitoring time for the F I/O module to 100 milliseconds; therefore, the I/O module must be addressed at least once every 100 milliseconds. However, since OB 35 is called only once every 100 milliseconds, communication failures occur. To ensure that there is a difference between the scanning interval of OB35 and the F monitoring time, make sure that the F monitoring time is greater than the scanning interval of OB35. This issue occurs with the S7 distributed safety system, starting from V5.2 SP1, as well as with models 6ES7138-4FA00-0AB0, 6ES7138-4FB00-0AB0, and 6ES7138-4CF00-0AB0. In the new module, the monitoring time for F is set to 150 milliseconds. 2: What is the monitoring time for the S7-300 CPU on PROFIBUS when the DP slave is unavailable? When a DP slave on a PROFIBUS interface using a CPU operates a PROFIBUS network, it is desirable to check during startup whether the desired configuration matches the actual configuration. Two different times are given on the Startup tab of the CPU properties dialog box. 3: How to determine if there is an error with the power supply or buffer, such as a battery failure? If an error in the power supply (only S7-400) or the buffer triggers an event, the CPU operating system accesses OB81. After correcting the error, access OB81 again. In the event of a battery fault, if the BATT.INDIC switch in battery detection is activated, the S7-400 accesses only OB81. If OB81 is not configured, the CPU will not enter the STOP operating state. If OB81 is unavailable, the CPU will still remain operational in the event of a power supply error. 4: What considerations should be taken into account when assigning addresses to the I/O modules (centralized or distributed) on an S7 CPU? Please note that the created data areas (such as a double word) cannot be located at the boundary of the process image, because in such a data block only the area below the boundary can be read into the process image; therefore, it is not possible to access the data from the process image. Therefore, these configuration rules do not support this scenario: for example, configuring an input double word at address 254 in a 256-byte process image with input data. If such a location is absolutely necessary, the size of the process image must be adjusted accordingly (in the CPU’s Properties). 5: How is basic communication of global data carried out in S7 CPUs? What should be noted when communicating? Global data communication is used for exchanging small amounts of data. Global data (GD) can include: input and output tags, data within data blocks, as well as timer and counter functions. Data exchange refers to the transfer of data in the form of packets between CPUs connected to a unidirectional or bidirectional GD network. The GD ring is identified by its GD ring number. One-way connection: A single CPU can send GD packets to multiple CPUs. Bidirectional connection: The connection between two CPUs; each CPU can send and receive a GD packet. It must be ensured that the receiver’s CPU has not confirmed the receipt of the global data. If data is to be exchanged via the corresponding communication blocks (SFB, FB, or FC), it is necessary to establish connections between these blocks. By defining a connection, the design of communication blocks can be greatly simplified. This definition is valid for all called communication blocks and does not need to be redefined each time. 6: Can an S7-400 memory card be used with a CPU 318-2DP? In normal operation, only the “short” memory cards with part numbers 6ES7951-1K... (Flash EPROM) and 6ES7951-1A... (RAM) can be used. 7: Despite the LED being on, why can’t the CPU 31xC read the complete inputs from addresses 124 and 125 by default? For the following CPU models, please check whether the 24V voltage is connected to pin 1. The LED is controlled by the input current. The 24V voltage on pin 1 requires further processing. 313C (6ES7 313-5BE0.-0AB0), 313C-2DP (6ES7 313-6CE0.-0AB0), 313C-2PTP (6ES7 313-6BE0.-0AB0), 314C-2DP (6ES7 314-6CF0.-0AB0), 314C-2PTP (6ES7 314-6BF0.-0AB0). 8: When configuring the PN interface of the CPU 31x-2 PN/DP, how should communication errors that occur occasionally on the PROFINET interface be handled? Please ensure that all components (converters) in the Ethernet (PROFINET) support 100 Mbit/s full-duplex basic operation. Avoid having central distributors split the network, as these devices can only operate in half-duplex mode. 9: In the hardware configuration editor, what does the “clock” correction factor mean? In the hardware configuration, by going to CPU > Properties > Diagnostics/Clock, you can access the “Clock” section and specify a correction factor there. This correction factor only affects the CPU’s hardware clock. Time interruptions originate from the system clock and have nothing to do with the settings of the hardware clock. 10: How can two-way data transmission between master and slave stations be achieved using function blocks via PROFIBUS DP? At the master station PLC, data exchange with the slave stations can be accomplished by calling SFC14 “DPRD_DAT” and SFC15 “DPWR_DAT”; whereas for the slave stations, FC1 “DP_SEND” and FC2 “DP_RECV” can be used to carry out data exchange. 11: What identification data can be read from the S7 CPU? The following identification data can be read using SFC 51 “RDSYSST”: the order number and CPU version number can be read. To this end, use SFC 51 and SSL ID 0111 along with the following indices: 1 = module identifier, 6 = basic hardware identifier, 7 = basic firmware identifier. 12: On an S7-300 equipped with a CPU 317-2PN/DP, how can communication function blocks FB14 (“GET”) and FB15 (“PUT”) be programmed for data exchange? To enable data exchange between two S7-300 stations using a CPU 317-2PN/DP through an S7 connection, which is configured using NetPro, communication function blocks must be called in S7 communication. Module FB14 (“GET”) is used to retrieve data from a remote CPU, while module FB15 (“PUT”) is used to write data to a remote CPU. The function blocks are included in the standard library of STEP 7 V5.3. Properties of the communication modules FB14 (“GET”) and FB15 (“PUT”) of the CPU 317-2PN/DP: FB14 and FB15 provide asynchronous communication functions. The execution of these modules may span multiple OB1 loops. Activate FB14 or FB15 by entering the parameter REQ. DONE, NDR, or ERROR indicate that the job has ended. PUT and GET can communicate over the connection simultaneously. Note: The communication blocks in the SIMATIC_NET_CP library cannot be used with CPU317-2PN/DP. 13: What considerations are needed for synchronized processing of tasks on the compact CPUs 313C-2 PtP and CPU 314-2 PtP? In the user program, it is not possible to program both SEND jobs and FETCH jobs simultaneously. In other words, as long as the SEND job (SFB 63) has not been completely terminated (with a status of DONE or ERROR), the FETCH job (SFB 64) cannot be called (even when REQ=0). As long as the FETCH job (SFB 64) has not been completely terminated (DONE or ERROR), the SEND job (SFB 63) cannot be called (even when REQ=0). When processing an active job (SEND job, SFB 63 or FETCH job, SFB 64), a passive job (SERVE job, SFB 65) can also be processed simultaneously. 14: Can the MICROMASTER 420 to 440 be used as configuration axes (for external position detection) and operated together with the CPU 317T? Yes, but in terms of power and precision, the requirements for the configured axes vary greatly. Under demanding conditions, the servo drives SIMODRIVE 611U, MASTERDRIVES MC, or SINAMICS S must be used in conjunction with the CPU 317T. Even under low-demand conditions, the MICROMASTER series can meet the requirements for power and precision. 15: How to configure direct data exchange (inter-node communication) between two CPU modules that have been set up as DP slaves? The two CPU stations are configured as DP slaves and are operated by the same DP master; direct data exchange between them can be achieved by configuring the switching mode to DX. 16: How to use SFC65, SFC66, SFC67, and SFC68 for communication? For one-way basic communication, the system function SFC67 (X_GET) is used to read data from a passive station, while the system function SFC68 (X_PUT) is used to write data to a passive station (server). These blocks are only invoked in the active station. For a two-way basic communication, the system function SFC65 (X_SEND) in the calling station is used, where data is to be sent to another active station. In the similarly active active receiving station, the data will be recorded through the system function SFC66 (X_RCV). In the basic communication of these two types, each block call can handle up to 76 bytes of user data. For the S7-300 CPU, the data consistency for data transfer is 8 bytes, while for the S7-400 CPU it is the full length. If connected to an S7-200, it must be taken into account that the S7-200 can only be used as a passive station. 17: What is free allocation of I/O addresses? Free address allocation means that you can freely assign an address to each type of module (SM/FM/CP). Address allocation is carried out in STEP 7. First, define the starting address; the other addresses of this module are based on it. Advantages of free address allocation: Since there are no address gaps between modules, the available address space can be utilized optimally. When creating standard software, the configuration of the involved S7-300 can be ignored during address allocation. 18: What can a diagnostic buffer do? Identify the source of faults more quickly, thereby improving system availability. Evaluate the last event before STOP and identify the cause of STOP. The diagnostic buffer is a circular buffer with a single diagnostic entry, which are displayed in the sequence of events that occur ； The first entry shows the most recent events. If the buffer is full, the earliest occurring event will be overwritten by the new entry. Depending on the CPU, the size of the diagnostic buffer is either fixed or can be set via parameters in HW Config. 19: What are the entries in the diagnostic buffer? 1) Fault events 2) Changes in operation mode and other operation events that are important to the user 3) User-defined diagnostic events (using SFC52 WR_USMSG). In STOP operation mode, as few events as possible are stored in the diagnostic buffer, so that the user can easily identify the cause of the STOP state within the buffer. Therefore, entries are stored in the diagnostic buffer only when an event requires the user to take action (such as scheduling a system memory reset, battery charging) or when important information must be recorded (such as firmware updates, station failures). 20: How to determine the size of MMC to store a STEP 7 project completely? To select the appropriate MMC for a project, it is necessary to know the size of the entire project as well as the size of the blocks to be loaded. The size of a project can be determined using the method described below: 1) First, archive the STEP 7 project. Then open the archived item in the Windows Resource Explorer and determine its size (select the item and right-click). This will tell you the size of the archived file. 2) Load the block into the CPU. It is still necessary to select "PLC > Module Information > Memory". Here, in “Load memory RAM + EPROM”, the size of the allocated load memory can be seen. 3) This value must be added to the size of the archived items that have already been determined. This allows one to determine the total amount of memory required to store the entire project on an MMC. 21: Which settings are retained after a complete CPU reset? When the CPU is reset, the memory is not completely erased. The entire main memory is completely erased, but the data loaded into memory, as well as the data stored on Flash-EPROM memory cards (MC) or micromemory cards (MMC), remains intact. In addition to loading into memory, timers (except CPU 312 IFM) and diagnostic buffers are also retained. CPUs with an MPI interface or a combined MPI/DP interface retain the current address and baud rate used by the interface only until a full reset occurs. On the other hand, another PROFIBUS address was also completely deleted and can no longer be accessed. Important: After resetting PG/PC, communication with the CPU can only be established through the MPI or MPI/DP interface. 22: Why can’t the CPU be accessed online via MPI? If the MPI parameters have already been changed on the CPU, please check the hardware configuration. These values can be compared with the parameters under “Set PG/PC interface” to check for any discrepancies. Alternatively, you can do this: open a new project and create a new hardware configuration. Set the respective values for address and transfer speed in the properties of the CPU’s MPI interface. Write the “empty” item to the memory card. Insert the memory card into the CPU and then reapply voltage to it, in order to transfer the settings located on the memory card to the CPU. The current settings of the MPI interface have now been transmitted, and in this way, a connection can be established as long as the interface is not faulty. This method is applicable to all S7-CPUs with a memory card interface. 23: What is the purpose of the error OB? If the described error occurs (see Document 1), the corresponding OB will be called and processed. If the OB is not loaded, the CPU enters STOP mode (exceptions: OB70, 72, 73, and 81). The S7-CPU can identify two types of errors: 1) Synchronization errors: These errors occur during the execution of specific operations and can be attributed to certain parts of the user program. 2) Asynchronous errors: These errors cannot be directly attributed to the running program. These errors include priority-related errors, errors in automated systems (faulty modules), or redundancy-related errors. 24: Which “fault OBs” should be programmed in the DP slave or the CPU315-2DP master? When configuring a CPU315-2DP station as a slave, the following OBs must be programmed in the STEP7 program in order to evaluate error messages of the distributed I/O type: OB 82 for diagnostic interrupts, OB 86 for sub-rack faults, and OB 122 for I/O access errors. 1) Diagnostic OB82: If a module that supports diagnostics and for which a diagnostic interrupt has been enabled detects an error, it sends a request for a diagnostic interrupt to the CPU both for incoming events and for outgoing events. The operating system then calls OB82. OB82’s own local variables contain the logical base address of the defective module and 4 bytes of diagnostic data. If you have not programmed the OB82 yet, the CPU enters the “stop” mode. You can block or delay the diagnosis interruption of OB, and release it again through SFC 39 - 42. 2) Sub-rack failure OB86: If a DP master system or a distributed I/O station is detected to be faulty (for both incoming and outgoing events), the operating system of that CPU invokes OB 86. If there is no programming OB 86 but such an error occurs, the CPU enters the “stop” mode. You can block or delay OB86 and release it again through SFC 39 - 42. 3) I/O access error OB122: When an error occurs while accessing the data of a module, the operating system of that CPU calls OB 122. For example, if the CPU detects a read error while accessing the data of a single module, the operating system then calls OB 122. This OB 122 operates with the same priority class as the interrupt block. If programming OB 122 is not performed, the CPU changes from the “running” mode to the “stopped” mode. 25: Why are reserved areas rewritten in some cases? In STEP 7’s hardware configuration, several operand areas can be defined as “reserved areas”. This allows the content in these areas to be retained even after a power loss, even in the absence of a backup battery. If a block is defined as a “reserved block” and it does not exist in the CPU or has only been temporarily installed, then part of the content in those areas will be overwritten. After the power is turned on/off, other information can be found in the relevant sections. 26: Why can’t the contents of a flash card be loaded into an S7 300 CPU? Your project is on the flash card. Now use it to load the S7 300. But after the loading was complete, it was found that the RAM in the CPU was still empty. The reason for this problem is that there are \"incorrect\" organizational blocks in your program that cannot be processed (for example, OB86 lacks a DP interface). After resetting and restarting the CPU, the RAM remains empty. The diagnostic buffer will provide some information regarding this \"unable to load\" block. 27: The diagnostic address when using the CPU315-2DP as a slave and as a master. When configuring a CPU315-2DP station, you use the S7 tool “H/W CONFIG” to assign the diagnostic address. If a fault occurs, these diagnostic addresses are added to the variable “OB82_MDL_ADDR” in the diagnostic OB. You can analyze this variable in OB82 to identify the faulty station and take appropriate action. Below is an example of how to assign a diagnostic address: Step 1: Configure the slave device using the CPU315-2DP and assign it a diagnostic address, such as 422. Step 2: Configure the master station using the CPU315-2DP. Step 3: Connect the configured slave stations to the master station and assign them a diagnostic address, such as 1022. 28: What settings are required for the DP slave interface of the S7-300 CPU in order to use it for routing? If a CPU is used as the I-Slave and also acts as an S7 router, please note the following: The DP interface of the slave used for routing selection must be set to active. This can be done in HW Config: in the properties dialog for the DP interface, the option “Commissioning/Test operation” or “Programming, status/modify...” must be activated. The precautions regarding these settings can be found in the table below. For S7 routing connections, there are 4 available connection resources – independent of any other connection resources. No connection resources using PG/OP or S7 basic communication. If it is necessary to establish a connection with a communication partner located on its rack through the DP interface (as in CP 343-1), a routing connection must also be used. For connections to a communication partner located on the same rack via the MPI interface, routing connection resources are not used, as it is possible to reach the partner directly in this case. Note: This does not apply to CPU 318. 29: Why is there no return value when using the internal timing table of the S7-300 CPU? When parameterizing the system function blocks SFC2, SFC3, and SFC4 for CPUs 312IFM to 316-2DP, if an identifier greater than “B#16#0” is specified for a runtime schedule, an error will occur and the desired functions will not be available. In this case, the identifier “8080h” will be output at the “RETVAL” output of the block. Note: For these CPUs, only one timer is available. Therefore, you should use only the identifier “B#16#0”. Within a cycle block (OB1, OB35), the system function SFC2 \"SET_RTM\" must not be called; instead, it should be invoked during the restart of OB (OB100). You can also initiate this block using an external trigger. Otherwise, the block will keep resetting the running timer and will never be able to complete the counting. 30: How are variables stored in temporary local data? The L stack always starts at address “0”. In the L stack, the same number of bytes is reserved for each data block to store the static or local data associated with that block. When a block is terminated, its space is released as well. The pointer always points to the first byte of the currently open block. 31: Is the runtime counter also reset after the CPU has been fully reset? When using the S7-300, there is a difference between CPUs with a hardware clock (an internal \"real-time clock\") and those with a software clock. For CPUs with software clocks that lack a backup battery, the value of the runtime counter is erased after the CPU is completely reset. For CPUs with hardware clocks that have a backup battery, the final value of the runtime counter is retained after the CPU is completely reset. Similarly, the runtime counters of CPU 318 and all S7-400 CPUs retain their last value after the CPU is completely reset. 32: How to configure an S7 CPU that is not part of the same project as a DP slave for my S7 DP master module? By default, in STEP 7, only one S7 CPU can be configured as a slave, provided that it is within the same project. The station then appears as “CPU 31x-2 DP” in the hardware list under “PROFIBUS-DP > Configured Stations”. Using this approach, a connection between the DP master station and the DP slave station can be established. There is also an option to configure an S7 CPU that is not in the same project as the master station as a slave station. Proceed as follows: Configure the DP slave in the conventional manner. Download the GSD file for the S7-300 CPU to be used as a slave from the Internet. This file is located under “PROFIBUS GSD Files / SIMATIC” on the customer support website. Open SIMATIC Manager and hardware configuration. Open “Options > Install new GSD...”, and insert the newly downloaded GSD file into the hardware directory. (Note: No windows need to be opened in HW Config during this process.) Update the hardware directory by going to “Options > Update Directory”. You can now configure your DP master station. The S7-300 CPU as a slave can be found under “PROFIBUS-DP > More Field Devices > SPS”. Note: If the DP slave is connected manually, make sure that the bus parameters, the PROFIBUS address of the DP slave, and its I/O configuration are identical in both projects. 33: Is the impact of a power outage without a backup battery the same as that of a complete reset? It’s different. When the CPU is completely reset, its hardware configuration information is deleted (except for the MPI address), the programs are removed, and the remaining magnetic memory is also cleared. If the power is turned off without a spare battery and memory card, the hardware configuration information (except for the MPI address) and the programs are deleted. However, remanent magnet memory is unaffected. If the program is reloaded under these conditions, it will use the old values stored in the residual magnetism memory when operating. For example, these values usually come from the first 8 counters. Failing to take this into account can lead to dangerous system conditions. Recommendation: Without a spare battery and memory card, always perform a complete reset after turning off the power. 34: To connect a 2-wire sensor to the analog input of a compact CPU? 2-wire and 4-wire sensors can be connected to the analog input terminals of the CPU 300C. When using a 2-wire sensor, set “I = Current” as the measurement type in the hardware configuration, just as with 4-wire sensors. Note: Please be aware that the compact CPU only supports active sensors (4-wire sensors). If passive sensors (2-wire sensors) are used, an external power supply is required. Warning: Please pay attention to the maximum allowable input current. 2-wire sensors may exceed the maximum allowable current in the event of a short circuit. The maximum allowable current specified in the technical data is 50mA (breakdown limit). For such situations (for example, applying current limiting to 2-wire sensors or connecting a PTC thermistor in series with the sensor), ensure that sufficient protection is provided. 35: Can SM322-1HH01 also operate at a load voltage of 24 V AC? Yes, you can also use the SM322-1HH01 with a load voltage of 24 V AC. 36: What is the minimum load voltage and current required to ensure that the SM322-1HF01 is powered on? The SM322-1HF01 relay module requires 17 V and 8 mA to ensure proper operation. For the lifespan of the contacts, such a value is better than the value given in the manual for this module (10 V and 5 mA). The specified values in the manual should be considered as the minimum requirements. 37: Which 24V digital input modules (6ES7 321-xBxxx- ...) need to be powered? The table below shows the power connector connections (L+ / M) for the 24V digital input module. 38: Can the SM321 module (DI16 x 24V) also be used in the ET200M? The module SM321 (MLFB 6ES7 321-7BH00-0AB0) can also be used in the ET200M. Among them, the CPU 31x-2DP serves as the DP master station, or the communication processor CP CP342-5 functions as the DP master station. Similarly, this module can be connected to an S7-400 CPU via an ET200M and the S7-400 communication processor CP443-5. 39: What is the address occupied by the SM323 digital card? The SM323 module is available in a 16-bit version (6ES7 323-1BL00-0AA0) and an 8-bit version (6ES7 323-1BH00-0AA0). For 16-bit type modules, the input and output occupy the addresses “X” and “X+1”. If the base address of SM323 is 4 (i.e., X=4 ； If the slot is 5), then the inputs are addressed at addresses 4 and 5, and the output addresses are also addressed at addresses 4 and 5. In the module’s wiring view, the input byte “X” is located at the top on the left, while the output byte “X” is at the top on the right. For 8-bit type modules, the input and output each occupy one byte, and they share the same byte address. If fixed slot addressing is used, with the SM323 inserted into slot 4, the input addresses are I 4.0 to I 4.7, and the output addresses are Q 4.0 to Q 4.7. 40: Can SM321-1CH20 be used in place of SM321-1CH80 without changing the hardware configuration? The technical specifications of the SM321-1CH20 and SM321-1CH80 modules are identical. The only difference is that the SM321-1CH80 can be used in a wider range of environmental conditions. Therefore, you do not need to change the hardware configuration.