50 Classic Q&A About Siemens S7-300 PLC
1: How to Avoid “Communication Error” Message When Using CPU 315F and ET 200S?
When using CPU S7 315F, ET 200S, and safety-related DI/DO modules, you will call the safety program in OB35. Moreover, you have accepted all the default monitoring time values and are willing to receive the “communication error” message. OB 35 is set to 100 milliseconds by default. You have set the monitoring time for the F I/O module to 100 milliseconds, which means the I/O module must be addressed at least every 100 milliseconds. However, since OB 35 is only called every 100 milliseconds, a communication error will occur. To ensure that the scanning interval of OB35 differs from the monitoring time of F, make sure the monitoring time of F is greater than the scanning interval of OB35.
This issue occurs in the S7 distributed safety system, up to V5.2 SP1 and modules 6ES7138-4FA00-0AB0, 6ES7138-4FB00-0AB0, 6ES7138-4CF00-0AB0. In the new modules, the monitoring time for F is set to 150 milliseconds.
2: What is the monitoring time of S7-300 CPU on PROFIBUS when the DP slave is unavailable?When operating the PROFIBUS network with the DP slave on the CPU’s PROFIBUS interface, it is desired to check whether the expected configuration matches the actual configuration during startup. Two different times are provided on the Startup tab in the CPU properties dialog.
3: How to Determine Power or Buffer Errors, Such as Battery Failure?If a power (only S7-400) or buffer error triggers an event, the CPU operating system accesses OB81. After the error is corrected, OB81 is accessed again. In case of battery failure, if the BATT.INDIC switch in the battery detection is activated, the S7-400 only accesses OB81. If OB81 is not configured, the CPU does not enter the STOP operating state. If OB81 is unavailable, the CPU remains running when there is a power error.
4: What Issues Should Be Noted When Assigning Addresses to I/O Modules on S7 CPU (Centralized or Distributed)?Please note that the created data area (such as a double word) cannot be configured at the boundary of the process image, as only the area below the boundary can be read into the process image, making it impossible to access data from the process image. Therefore, these configuration rules do not support this situation: for example, configuring an input double word at address 254 of a 256-byte input process image. If such addressing is necessary, the size of the process image must be adjusted accordingly (in the CPU’s Properties).
5: How to Perform Basic Communication of Global Data in S7 CPU? What Should Be Noted During Communication?Global data communication is used to exchange small amounts of data. Global data (GD) can include: inputs and outputs, markers, data in data blocks, timer and counter functions. Data exchange refers to the exchange of data packets between CPUs connected in a unidirectional or bidirectional GD ring. The GD ring is identified by its GD ring number. Unidirectional connection: one CPU can send GD data packets to multiple CPUs. Bidirectional connection: a connection between two CPUs: each CPU can send and receive a GD data packet. It must be ensured that the receiving CPU does not confirm the receipt of the global data. If you want to exchange data through the corresponding communication blocks (SFB, FB, or FC), a connection between communication blocks must be established. 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 the S7-400 Storage Card Be Used for CPU 318-2DP?In normal operation, only storage cards with order numbers 6ES7951-1K… (Flash EPROM) and 6ES7951-1A… (RAM) can be used.
7: Why Can’t CPU 31xC Read Full Input from Default Addresses 124 and 125 Even Though the LED is On?For the following models of CPU, 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 needs to be further processed.
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: How to Handle Occasional Communication Errors at the PN Interface When Configuring CPU 31x-2 PN/DP?Please ensure that all components (switches) in the Ethernet (PROFINET) support 100 Mbit/s full-duplex basic operation. Avoid central distributors that split the network, as these devices can only operate in half-duplex mode.
9: What Does the “Clock” Correction Factor Mean in the Hardware Configuration Editor?In hardware configuration, you can access the “Clock” domain through CPU > Properties > Diagnostics/Clock to specify a correction factor. This correction factor only affects the CPU’s hardware clock. Time interrupts originate from the system clock and are unrelated to the setting of the hardware clock.
10: How to Implement Bidirectional Data Transfer Between Master and Slave Using Function Blocks via PROFIBUS DP?The master PLC can complete data exchange with the slave by calling SFC14 “DPRD_DAT” and SFC15 “DPWR_DAT”, while the slave can call FC1 “DP_SEND” and FC2 “DP_RECV” to complete the data exchange.
11: What Identification Data Can Be Read from the S7 CPU?Identification data such as order number and CPU version number can be read using SFC 51 “RDSYSST”. For this, use SFC 51 and SSL ID 0111 with the following indices: 1 = Module Identification, 6 = Basic Hardware Identification, 7 = Basic Firmware Identification.
12: How to Program Communication Function Blocks FB14 (“GET”) and FB15 (“PUT”) for Data Exchange on S7-300 with CPU 317-2PN/DP?To exchange data between two S7-300 workstations using a CPU 317-2PN/DP via an S7 connection configured using NetPro, communication function blocks must be called in S7 communication. Module FB14 (“GET”) is used to retrieve data from the remote CPU, while module FB15 (“PUT”) is used to write data to the remote CPU. The function blocks are included in the standard library of STEP 7 V5.3.
Attributes of communication modules FB14 (“GET”) and FB15 (“PUT”) of CPU 317-2PN/DP:
FB14 and FB15 are asynchronous communication functions. The operation of these modules may span multiple OB1 cycles. The input parameter REQ activates FB14 or FB15. DONE, NDR, or ERROR indicate the completion of the job. PUT and GET can communicate simultaneously through the connection.
Note: The communication blocks in the SIMATIC_NET_CP library cannot be used for CPU 317-2PN/DP.
13: What Should Be Noted for Synchronized Processing Between Compact CPUs 313C-2 PtP and 314-2 PtP?In the user program, SEND and FETCH operations cannot be programmed simultaneously. That is, as long as the SEND operation (SFB 63) has not completely terminated (DONE or ERROR), the FETCH operation (SFB 64) cannot be called (even when REQ=0). Similarly, as long as the FETCH operation (SFB 64) has not completely terminated (DONE or ERROR), the SEND operation (SFB 63) cannot be called (even when REQ=0). While processing a main action (SEND operation, SFB 63 or FETCH operation, SFB 64), a subordinate action (SERVE operation, SFB 65) can be processed simultaneously.
14: Can MICR.master420 to 440 Run as Configured Axes (External Position Detection) with CPU 317T?Yes, but the requirements for power and precision for configured axes differ greatly. In high-demand situations, servo drives SIMODRIVE 611U, MASTERDRIVES MC, or SINAMICS S must run with CPU 317T. In low-demand situations, the MICROMASTER series can also meet power and precision requirements.
15: How to Configure Direct Data Exchange (Node-to-Node Communication) Between Two CPU Modules Configured as DP Slaves?Two CPU stations configured as DP slaves, operated by the same DP master station, can achieve direct data exchange by configuring the exchange mode to DX.
16: How to Use SFC65, SFC66, SFC67, and SFC68 for Communication?For unidirectional basic communication, use the system function SFC67 (X_GET) to read data from a passive station, and use the system function SFC68 (X_PUT) to write data to a passive station (server). These blocks are only called in the active station. For bidirectional basic communication, call the system function SFC65 (X_SEND) in the calling station to send data to another active station. In the active receiving station, data will be recorded via the system function SFC66 (X_RCV).
In both types of basic communication, each block call can handle a maximum of 76 bytes of user data. For S7-300 CPU, the data transfer consistency is 8 bytes, while for S7-400 CPU, it is full length. If connected to S7-200, it must be noted that S7-200 can only serve as a passive station.
17: What is Free Allocation of I/O Addresses?Free allocation of addresses means you can freely allocate an address for each module (SM/FM/CP). Address allocation is done in STEP 7. First, define the starting address, and the other addresses of the module are based on it.
The advantages of free allocation of addresses: Since there are no address gaps between modules, the available address space can be optimized. During the creation of standard software, the address allocation process can ignore the configuration of the involved S7-300.
18: What Can the Diagnostic Buffer Do?It helps to identify fault sources more quickly, thus improving system availability. It evaluates the last events before a STOP and looks for the causes of the STOP.
The diagnostic buffer is a circular buffer with individual diagnostic entries, which display in the sequence of events occurring; the first entry shows the most recently occurred event. If the buffer is full, the earliest event will be overwritten by new entries. Depending on the CPU, the size of the diagnostic buffer is either fixed or can be set via parameters in HW Config.
19: What Entries Are Included in the Diagnostic Buffer?1) Fault events; 2) Mode transitions and other operational events important to the user; 3) User-defined diagnostic events (using SFC52 WR_USMSG). In STOP mode, the diagnostic buffer stores as few events as possible to allow users to easily find the cause of the STOP. Therefore, entries are only stored in the diagnostic buffer when events require a user response (e.g., planned system memory reset, battery needs charging) or must register important information (e.g., firmware updates, station failures).
20: How to Determine the Size of MMC to Fully Store STEP 7 Projects?To select an appropriate MMC for the project, it is necessary to know the total size of the entire project and the size of the blocks to be loaded. The size of the project can be determined as follows:
1) First archive the STEP 7 project. Then open the archived project in Windows Explorer and determine its size (select the project and right-click). This will tell you the size of the archive file.
2) Load the blocks into the CPU. Now, still select “PLC > Module Information > Memory”. Here, in “Load memory RAM + EPROM”, you can see the size of the allocated load memory.
3) The value must be added to the already determined size of the archived project. This will give you the total memory size required to save the entire project on one MMC.
21: What Settings Will Be Retained After a Complete Reset of the CPU?When resetting the CPU, the memory is not completely erased. The entire main memory is completely erased, but the data in the load memory, as well as data saved on the Flash-EPROM storage card (MC) or micro storage card (MMC), will be retained. Besides load memory, timers (except for CPU 312 IFM) and diagnostic buffers are also retained. A CPU with MPI interface or a combined MPI/DP interface only retains the current address and baud rate used by the interface before the complete reset. On the other hand, another PROFIBUS address is completely erased and cannot be accessed anymore.
Important Note: After resetting the PG/PC, communication with the CPU can only be established via MPI or MPI/DP interface.
22: Why Can’t Access the CPU Online via MPI?If the MPI parameters have been changed on the CPU, please check the hardware configuration. These values can be compared with the parameters under “Set PG/PC interface” to see if there are inconsistencies.
Alternatively, you can do this: Open a new project, create a new hardware configuration. In the properties of the CPU’s MPI interface, set the respective values for address and transmission speed. Write the “empty” project to the storage card. Insert the storage card into the CPU and then power up the CPU again, transferring the settings on the storage card to the CPU. Now the current settings of the MPI interface have been transferred, and as long as the interface is not faulty, a connection can be established. This method applies to all S7 CPUs with a storage card interface.
23: What is the Purpose of Error OB?If an error described occurs (see File 1), the corresponding OB will be called and processed. If the OB is not loaded, the CPU enters STOP (exceptions: OB70, 72, 73, and 81).
S7-CPU can identify two types of errors: 1) Synchronous errors: These errors are triggered during the processing of specific operations and can be attributed to specific parts of the user program.
2) Asynchronous errors: These errors cannot be directly attributed to the running program. These errors include priority class errors, errors in the automation system (faulty modules), or redundancy errors.
24: Which “Fault OBs” Should Be Programmed in a DP Slave or CPU 315-2DP Master?When configuring a CPU 315-2DP station as a slave, the following OBs must be programmed in the STEP7 program to evaluate the error information of distributed I/O types: OB 82 Diagnostic Interrupt OB, OB 86 Subrack Fault OB, OB 122 I/O Access Error.
1) Diagnostic OB82: If a diagnostic-supporting module that has been released for diagnostic interrupts identifies an error, it issues a diagnostic interrupt request to the CPU for both incoming and outgoing events. The operating system then calls OB82. OB82 contains the logical base address of the defective module and 4 bytes of diagnostic data in its local variables. If you have not programmed OB82, the CPU enters “stop” mode. You can block or delay the diagnostic interrupt OB and release it again using SFC 39 – 42.
2) Subrack Fault OB86: If a fault is identified in a DP master system or a distributed I/O station (for both incoming and outgoing events), the CPU’s operating system calls OB86. If OB86 is not programmed but such an error occurs, the CPU enters “stop” mode. You can block or delay OB86 and release it again using SFC 39 – 42.
3) I/O Access Error OB122: When there is an error accessing a module’s data, the CPU’s operating system calls OB122. For example, if the CPU identifies a read error when accessing a single module’s data, the operating system calls OB122. OB122 runs with the same priority class as the interrupt block. If OB122 is not programmed, the CPU changes from “run” mode to “stop” mode.
25: Why Are Retained Areas Overwritten in Some Cases?In STEP 7 hardware configuration, several operand areas can be defined as “retained areas”. This allows the contents of these areas to be maintained even without a backup battery after a power failure. If a block is defined as a “retained block” and it does not exist in the CPU or has only been temporarily installed, some contents of these areas may be overwritten. After the power is turned on/off, other contents can be found in the relevant areas.
26: Why Can’t the Contents of the Flash Card Be Loaded into S7 300 CPU?Your project is on the flash card. Now you want to load it into S7 300. But after loading, the RAM of the CPU is still empty. The reason for this issue is that your program contains unmanageable, “faulty” organization blocks (for example, OB86 without DP interface). After resetting and restarting the CPU, RAM remains empty. The diagnostic buffer provides some information about this “unable to load” block.
27: Diagnostic Address When Using CPU 315-2DP as Slave and CPU 315-2DP as MasterWhen configuring a CPU 315-2DP station, you use the S7 tool “H/W CONFIG” to assign diagnostic addresses. If a fault occurs, these diagnostic addresses are included in the diagnostic OB’s variable “OB82_MDL_ADDR”. You can analyze this variable in OB82 to determine the faulty station and respond accordingly.
Here is an example of how to assign diagnostic addresses: Step 1: Configure the CPU 315-2DP as a slave and assign a diagnostic address, for example, 422.
Step 2: Configure the CPU 315-2DP as the master.
Step 3: Link the configured slave to the master and assign a diagnostic address, for example, 1022.
28: What Settings Are Required for the DP Slave Interface of S7-300 CPU to Use It for Routing?If using the CPU as an I-Slave and the CPU also serves as an S7 router, please note the following:
The DP interface of the slave used for routing must be set to active state. This can be done in HW Config: In the properties dialog of the DP interface, the option “Commissioning/Test operation” or “Programming, status/modify…” must be activated. Notes 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. Connection resources using PG/OP or S7 basic communication are not used.
If a communication partner located in its rack needs to be connected via the DP interface (e.g., in CP 343-1), a routing connection must be used. For a connection with a communication partner located in its rack via the MPI interface, routing connection resources are not used, as in this case, the partner can be directly reached. Note: This does not apply to CPU 318.
29: Why Is There No Return Value When Using the Internal Runtime Table of S7-300 CPU?When parameterizing system function blocks SFC2, SFC3, and SFC4 for CPU 312IFM to 316-2DP, if an identifier greater than “B#16#0” is specified for a runtime table, an error will occur and the desired functionality will not be available. In this case, the identifier “8080h” will be output at the block’s “RETVAL” output.
Note: For these CPUs, only one timer is available. Therefore, you should only use the identifier “B#16#0”. In a cyclic block (OB1, OB35), system function SFC2 “SET_RTM” must not be called, but should be called in a restart OB (OB100). You can also trigger this block via an external trigger. Otherwise, this block will always reset the runtime table, and counting will never be completed.
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 owned by each block.
When a block terminates, its space is released. The pointer always points to the first byte of the currently opened block.
31: Is the Runtime Counter Also Reset After a Complete Reset of the CPU?When using S7-300, there is a distinction between CPUs with hardware clocks (built-in “real-time clock”) and those with software clocks. For those software clock CPUs without backup batteries, the runtime counter’s last value is deleted after the CPU is completely reset. For those hardware clock CPUs with backup batteries, the last value of the runtime counter is retained after the CPU is completely reset. Similarly, the runtime counter of CPU 318 and all S7-400 CPUs is retained after the CPU is completely reset.
32: How to Configure an S7 CPU Not in the Same Project as My S7 DP Master Module as a DP Slave?By default, in STEP 7, only one S7 CPU can be configured as a slave if the station is in the same project. The station then appears as “CPU 31x-2 DP” under “PROFIBUS-DP > Configured Stations” in the hardware directory. In this way, a link between the DP master and DP slave can be set up.
There is also an option to configure an S7 CPU not in the same project as the master as a slave. Proceed as follows:
Regularly configure the DP slave.Download the GSD file for the S7-300 CPU to be used as a slave from the customer support website under “PROFIBUS GSD Files / SIMATIC”.
Open SIMATIC Manager and Hardware Configuration.Open “Options; Install New GSD…” and insert the downloaded GSD file into the hardware directory. (Note: During this process, no window in HW Config needs to be opened.)
Update the hardware directory via “Options; Update Directory”.Now you can configure your DP master station. The S7-300 CPU configured as a slave can be found under “PROFIBUS-DP > More Field Devices > SPS”.
Note: If manually combining this DP slave, ensure that the bus parameters, the PROFIBUS address of this DP slave, and its I/O configuration are the same in both projects.
33: Are the Effects of Power Outage Without Backup Battery the Same as Complete Reset?No. In the case of a complete reset of the CPU, its hardware configuration information is deleted (except for MPI address), the program is deleted, and the remaining memory is cleared.
In the case of a power outage without a backup battery and storage card, the hardware configuration information (except for MPI address) and the program are deleted. However, the remaining memory is not affected. If the program is reloaded in this case, it will use the old values from the remaining memory during operation. For example, these values usually come from the first 8 counters. If this point is not considered, it can lead to dangerous system states.
Recommendation: After a power outage without a backup battery and storage card, always perform a complete reset.
34: Can 2-Wire Sensors Be Connected to the Analog Input of a Compact CPU?Yes, both 2-wire and 4-wire sensors can be connected to the analog input of CPU 300C. When using a 2-wire sensor, set “I = Current” as the measurement type in the hardware configuration, just like the setting for a 4-wire sensor.
Note: Please note that compact CPUs only support active sensors (4-wire sensors). If passive sensors (2-wire sensors) are used, an external power supply must be used.
Warning: Please note the maximum allowable input current. 2-wire sensors may exceed the maximum allowable current in case of a short circuit. The maximum allowable current specified in the technical data is 50mA (destruction limit). For this situation (e.g., applying current limitation to 2-wire sensors or connecting a PTC thermistor in series with the sensor), ensure adequate protection.
35: Can SM322-1HH01 Also Work with a Load Voltage of AC 24 V?Yes, you can also use SM322-1HH01 with a load voltage of AC 24 V.
36: What Is the Minimum Load Voltage and Current Required to Ensure SM322-1HF01 is Activated?SM322-1HF01 relay module requires 17 V and 8 mA to ensure normal opening and closing. Such values are better than those provided in the module’s manual (10 V and 5 mA) for contact lifespan. The values specified in the manual should be regarded as minimum requirements.
37: Which 24V Digital Input Modules (6ES7 321-xBxxx- …) Require Power Connection?Power connection (L+/M) is required for 24V digital input modules.
38: Can SM321 Modules (DI16 x 24V) Also Be Used in ET200M?Module SM321 (MLFB 6ES7 321-7BH00-0AB0) can also be used in ET200M. In this case, CPU 31x-2DP serves as the DP master or communication processor CP342-5 serves as the DP master. Similarly, this module can be connected to an S7-400 CPU via ET200M and communication processor CP443-5.
39: What Addresses Are Occupied by SM323 Digital Cards?SM323 module has 16-bit type (6ES7 323-1BL00-0AA0) and 8-bit type (6ES7 323-1BH00-0AA0). For 16-bit type modules, inputs and outputs occupy two addresses “X” and “X+1”. If the base address of SM323 is 4 (i.e., X=4; inserted in slot 5), then the input is assigned to addresses 4 and 5, and the output addresses are also assigned to addresses 4 and 5. In the wiring view of the module, the input byte “X” is located at the top left, and the output byte “X” is at the top right.
For 8-bit type modules, inputs and outputs each occupy one byte, and they have the same byte address. If SM323 is inserted in slot 4 with fixed addressing, the input address is I 4.0 to I 4.7, and the output address is Q 4.0 to Q 4.7.
40: Can SM321-1CH20 Replace SM321-1CH80 Without Changing Hardware Configuration?SM321-1CH20 and SM321-1CH80 modules have the same technical parameters. The only difference is that SM321-1CH80 can be used under a wider range of environmental conditions. Therefore, you do not need to change the hardware configuration.
41: What Must Be Noted When Directly Accessing I/O?It must be noted that in an S7-300 configuration, if direct read access to I/O across modules (using the command to read several bytes at once) is performed, incorrect values may be read. You can view the specific addresses in hardware.
42: Does the SM321 Module Need to Be Connected to DC 24V?No, if the MLFB is 6ES7 321-1BH02-0AA0, then the SM321 module no longer needs to be connected to DC 24V.
43: How to Plan Analog Module SM374 in STEP 7 Hardware Configuration? How to Find This Module in the Hardware Directory?Analog module SM374 can be used in three modes: as a 16-channel digital input module, as a 16-channel digital output module, or as a mixed digital input/output module with 8 inputs and 8 outputs.
Now configure SM374 according to the module you need to simulate, that is:
If using SM 374 as a 16-channel input module, configure it as a 16-channel input module – recommended use: SM 321: 6ES7321-1BH01-0AA0,
If using SM 374 as a 16-channel output module, configure it as a 16-channel output module – recommended use: SM 322: 6ES7322-1BH01-0AA0,
If using SM 374 as a mixed input/output module, configure it as a mixed input/output module (8 inputs, 8 outputs) – recommended use: SM 323: 6ES7323-1BH01-0AA0.
44: Will the Analog Input I+ of Module 6ES7 331-1KF0.-0AB0 Be Damaged When Short-Circuited During Current Measurement?No, when measuring current, the analog input I+ of module 6ES7 331-1KF0.-0AB0 will not be damaged. This module has built-in overcurrent protection. Each 50-ohm resistor in the module has a PTC component in front to prevent the input channel of the module from being damaged.
Please note that the maximum permissible input voltage for the long term is 12V, and the transient (up to 1 second) value is 30V.
45: If the CPU is Powered Off, Will the 2-Wire Measurement Transmitter Continue to Be Powered?If the transmitter module is inserted in position “D”, and the module is powered by external voltage at pins 1 and 20, the 2-wire measurement transmitter continues to be powered. Even if the CPU is powered off, its supply current remains unchanged.
46: When Measuring Temperature (Fahrenheit) with the S7-300 Analog Input Module, Can the Absolute Error Limits Listed in the Module Documentation Be Used Directly?No, the specified error limits cannot be used directly. Both basic error and operational error are described in absolute temperature and Celsius temperature. They must be multiplied by a factor of 1.8 to convert to Fahrenheit temperature units.
Example: S7-300 AI 8 x RTD: The specified operational error for temperature input is +/-1.0 degrees Celsius. When measuring in Fahrenheit, the acceptable maximum error is +/-1.8 degrees Fahrenheit.
47: Why Can’t a Commercial Digital Multimeter Read the Constant Current Used to Read Impedance on the Analog Input Block?Almost all S5/S7 analog input devices still operate in a complex way, meaning that all channels are sequentially plugged into only one AD converter. This principle also applies to the constant current necessary for reading impedance. Therefore, the current flowing through the resistor to be read is only available for short-term readings. For the SM331-7KF02-0AB0 with a selected interface suppressing “50Hz” and 8 parameterized channels, this means that the current will flow approximately every 180ms, with 20ms available for reading impedance each time.
48: Why Is the Voltage Output of the S7-300 Analog Output Group Outside Tolerance? What Are Terminals S+ and S- Used For?The following description applies to all analog output modules SM 332: When using the analog output module SM 332, attention must be paid to the allocation of return inputs S+ and S-. They serve to compensate for performance impedance. When connecting actuators with independent wires having S+ and S-, the analog output will adjust the output voltage so that the voltage actually present on the actuator is the expected voltage.
If compensation is desired, the actuator must be connected with 4 wires. This means that for the first channel, the output voltage is connected to the actuator via pins 3 and 6. Allocate the actuator’s pins 4 and 5. If no compensation is desired, simply bridge pins 3-4 and 5-6 on the front switch.
Note: Because of the open sensor terminals (S+ and S-), the output voltage is adjusted to a maximum of 140 mV (for 10V). For this allocation, it is impossible to maintain a 0.5% voltage output using error limits.
49: How to Connect a Potentiometer to 6ES7 331-1KF0-0AB0?The sampling end and the start end of the potentiometer connect to M+, and the end connects to M-, and S- and M- are connected together.
Note: The maximum allowable resistance is 6K. If the potentiometer supports directly outputting a variable voltage, the start end of the potentiometer should connect to V+, and the M end connects to M-.
50: How to Connect a PT100 Temperature Sensor to the Analog Input Module SM331?The PT100 thermistor changes its resistance value with temperature. If a constant current flows through this thermistor, the voltage drop across the thermistor changes with temperature. A constant current is applied at the connections Ic+ and Ic-. The analog module SM331 measures the change in current at M+ and M-. By measuring the voltage, the temperature can be determined.
PT100 can be connected to the analog input group in three ways: 4-wire connection can achieve the most accurate measurements.
* Note: The formula for the 3-wire connection only indicates the actual measurement process of the analog input module SM331 (MLFB number 6ES7 331-7Kxxx-0AB0).
2) In the S7-300 series, there are some analog input terminals that allow multiple measurements. They specify the line resistance of the common return line and perform mathematical compensation. The achieved accuracy is almost comparable to that of a 4-wire connection. An example of such a module is SM331 (MLFB number 6ES7 331-7PF00-0AB0).
3) The provided formulas still apply to the main physical relationships but do not include the effective measurement process to determine the PT100 resistance.
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