1: How to avoid “communication failure” messages when using CPU 315F and ET 200S?
When using CPU S7 315F, ET 200S, and fault-tolerant DI/DO modules, you will call the fault-tolerant program OB35. Moreover, you have accepted all default monitoring time values and are willing to receive “communication failure” messages. OB 35 is set to 100 milliseconds by default. You have set the F monitoring time of the I/O module to 100 milliseconds, so the I/O module must be addressed at least every 100 milliseconds. However, since OB 35 is only called once every 100 milliseconds, communication failure occurs. To ensure that the scanning interval of OB35 differs from the F monitoring time, make sure that the F monitoring time is greater than the scanning interval of OB35. This issue occurs in the S7 distributed safety system, up to V5.2 SP1 and models 6ES7138-4FA00-0AB0, 6ES7138-4FB00-0AB0, 6ES7138-4CF00-0AB0. In new modules, the F monitoring time is set to 150 milliseconds.
2: What is the monitoring time of the 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 if there is a power or buffer error, such as a battery failure?
If an error in the power supply (only S7-400) or buffer triggers an event, the CPU operating system accesses OB81. After the error is corrected, OB81 is accessed again. In the case of a 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 will not enter the STOP operating state. If OB81 is unavailable, the CPU will still remain running when there is a power error.
4: What issues should be considered when assigning addresses to I/O modules (centralized or distributed) on S7 CPU?
Please note that the created data area (such as a double word) cannot be configured on 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 on S7 CPU? What should be noted during communication?
Global data communication is used to exchange small amounts of data. Global data (GD) can be: input and output markers, data in data blocks, timer and counter functions. Data exchange refers to the exchange of data in packet form between CPUs connected in a unidirectional or bidirectional GD ring. The GD ring is identified by the 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 global data. If data is to be exchanged via 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 complete inputs from default addresses 124 and 125 even though the LED lights up?
For the following CPU models, please check if 24V voltage is applied to pin 1. The LED is controlled by the input current. The 24V voltage on pin 1 needs 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 CPU 31x-2 PN/DP, how to handle occasional communication errors on the PROFINET interface?
Please ensure that all components (converters) 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 the hardware configuration, you can access the “clock” field through CPU > Properties > Diagnostics/Clock to specify a correction factor. This correction factor only affects the hardware clock of the CPU. Time interrupts originate from the system clock and have no relation to the settings of the hardware clock.
10: How to implement bidirectional data transfer between master and slave stations via PROFIBUS DP using function blocks?
Data exchange can be completed on the master PLC 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 CPU 317-2PN/DP via an S7 connection configured with 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. < CPU 317-2PN/DP communication module FB14 (“GET”) and FB15 (“PUT”) properties: 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 end of the job. PUT and GET can communicate simultaneously through the connection. Note: Communication blocks from the library SIMATIC_NET_CP cannot be used for CPU 317-2PN/DP.
13: What should be noted for synchronized processing of jobs between compact CPU 313C-2 PtP and CPU 314-2 PtP?
In the user program, SEND jobs and FETCH jobs cannot be programmed simultaneously. That is, as long as the SEND job (SFB 63) has not completely terminated (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 completely terminated (DONE or ERROR), the SEND job (SFB 63) cannot be called (even when REQ=0). While processing a main job (SEND job, SFB 63 or FETCH job, SFB 64), a secondary job (SERVE job, SFB 65) can be processed simultaneously.
14: Can MICR.master420 to 440 be operated as configured axes (external position detection) with CPU 317T?
Yes, but the requirements for power and precision for the configured axes differ significantly. In high-demand situations, servo drives SIMODRIVE 611U, MASTERDRIVES MC, or SINAMICS S must operate 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 and operated by the same DP master 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 system function SFC67 (X_GET) to read data from a passive station, and use 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 same active receiving station, the data will be recorded through 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 data consistency is 8 bytes, while for S7-400 CPU, it is full length. If connected to S7-200, it must be considered that S7-200 can only be used as a passive station.
17: What is free allocation of I/O addresses?
Free allocation of addresses means that you can freely assign an address to 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 advantage of free allocation of addresses is that since there are no address gaps between modules, the available address space can be used optimally. When creating standard software, the address allocation process can be done without considering the configuration of the involved S7-300.
18: What can the diagnostic buffer do?
It can quickly identify the source of faults, thereby improving system availability. It evaluates the last events before a STOP and looks for the cause of the STOP. The diagnostic buffer is a circular buffer with individual diagnostic entries that display in the sequence of events; the first entry shows the most recent 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, as few events as possible should be stored in the diagnostic buffer so that the user can easily find the cause of the STOP. Therefore, entries are only stored in the diagnostic buffer when the event requires a user response (e.g., planned system memory reset, battery needs charging) or important information must be registered (e.g., firmware update, station failure).
20: How to determine the size of MMC to completely store STEP 7 projects?
To select the appropriate MMC for the project, it is necessary to know the total size of the 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 you still need to select “PLC > Module Information > Memory”. Here, in “Load memory RAM + EPROM”, you can see the size of the allocated load memory.
3) This value must be added to the size of the archived project that has been determined. This way, you can derive the total memory size required to save the entire project on one MMC.
21: Which settings are retained after a complete reset of the CPU?
When resetting the CPU, memory is not completely deleted. The entire main memory is completely deleted, but data in the load memory, as well as data stored 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 an MPI interface or a combination 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 deleted and cannot be accessed anymore.
Important note: After resetting the PG/PC, communication with the CPU can only be established via 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. You can compare these values with the parameters under “Set PG/PC interface” to see if there are any 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 turn on the CPU’s power 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 storage card interfaces.
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: What “fault OBs” should be programmed in the DP slave or CPU 315-2DP master station?
When configuring a CPU 315-2DP station as a slave, the following OBs must be programmed in the STEP7 program to evaluate distributed I/O type error information:
OB 82 diagnostic interrupt OB, OB 86 subrack fault OB, OB 122 I/O access error
1) Diagnostic OB82: If a module that supports diagnostics and 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 via 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 OB 86. If OB 86 is not programmed but such an error occurs, the CPU enters “stop” mode. You can block or delay OB86 and release it again via SFC 39 – 42.
3) I/O access error OB122: When an error occurs while accessing data from a module, the CPU’s operating system calls OB 122. For example, if the CPU identifies a read error while accessing data from a single module, the operating system calls OB 122. This OB 122 runs with the same priority class as the interrupt block. If OB 122 is not programmed, the CPU changes from “run” mode to “stop” mode.
25: Why are certain areas retained being rewritten?
In the hardware configuration of STEP 7, several operand areas can be defined as “retained areas”. This allows the contents of these areas to be retained even after a power failure, even without a backup battery. If a block is defined as a “retained block” and it does not exist in the CPU or has only been temporarily installed, then part of the contents of these areas will be rewritten. After power is turned on/off, other contents will be found in the relevant area.
26: Why can’t the contents of the flash memory card be loaded into the S7 300 CPU?
Your project is on the flash memory card. Now you want to load it into S7 300. But after loading, the CPU’s RAM is still empty. The reason for this issue is that your program contains unprocessable, “faulty” organization blocks (for example, OB86 without a 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 addresses when using CPU 315-2DP as a slave and CPU 315-2DP as a master?
When 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 added to the diagnostic OB 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 a 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 acts as an S7 router, please note the following: The DP interface of the slave used for routing must be set to active. 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 for PG/OP or S7 basic communication are not used. If a connection must be established via the DP interface to a communication partner located in its rack (such as in CP 343-1), a routing connection must also be used. However, for a connection to 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 reached directly. 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 required function 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, never completing the count.
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 CPUs without backup batteries, the last value of the runtime counter is deleted after the CPU is completely reset. For those 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 that is 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 that station is in the same project. That station then appears as “CPU 31x-2 DP” in the hardware directory under “PROFIBUS-DP > Configured Stations”. 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 that is not in the same project as the master as a slave. Proceed as follows:
Configure the DP slave as usual.
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 windows need to be opened in HW Config) Update the hardware directory via “Options; Update Directory”. Now you can configure your DP master. The S7-300 CPU configured as a slave will be found under “PROFIBUS-DP > More Field Devices > SPS”. Note: If manually linking the DP slave, ensure that the bus parameters, the PROFIBUS address of the DP slave, and its I/O configuration must be the same in both projects.
33: Is the impact of a power failure without a backup battery the same as a complete reset?
No. In the case of a complete reset of the CPU, its hardware configuration information is deleted (except for the MPI address), the program is deleted, and the remaining memory is cleared. In the case of a power failure without a backup battery and storage card, the hardware configuration information (except for the 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. 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 failure without a backup battery and storage card, always perform a complete reset.
34: Can a 2-wire sensor be connected to the analog input of a compact CPU?
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, the same as for the 4-wire sensor. Note: Please note that the compact CPU only supports active sensors (4-wire sensors). If using passive sensors (2-wire sensors), 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 the event of a short circuit. The maximum allowable current specified in the technical data is 50mA (destruction limit). For this case (e.g., applying current limiting to the 2-wire sensor 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 operates correctly?
SM322-1HF01 relay module requires 17 V and 8 mA to ensure proper opening and closing. Such values are better for the lifespan of the contacts than the values provided in the manual for this module (10 V and 5 mA). The values specified in the manual should be considered as minimum requirements.
37: For which 24V digital input modules (6ES7 321-xBxxx- …) is power supply required?
Power supply pins (L+ / M) of 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. Where CPU 31x-2DP acts as the DP master or communication processor CP CP342-5 acts 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 does the SM323 digital card occupy?
SM323 module has 16-bit type (6ES7 323-1BL00-0AA0) and 8-bit type (6ES7 323-1BH00-0AA0). For the 16-bit type module, input and output occupy two addresses “X” and “X+1”. If the base address of SM323 is 4 (i.e., X=4; 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 the 8-bit type module, input and output each occupy one byte, and they have the same byte address. If assigned to a fixed slot, SM323 inserted in slot 4 will have input addresses from I 4.0 to I 4.7, and output addresses from Q 4.0 to Q 4.7.
40: Can SM321-1CH20 replace SM321-1CH80 without changing the hardware configuration?
SM321-1CH20 and SM321-1CH80 modules have the same technical parameters. The only difference is that SM321-1CH80 can be used in a wider range of environmental conditions. Therefore, you do not need to change the hardware configuration.
41: What must be noted when performing direct access to I/O?
It should be noted that in an S7-300 configuration, if direct read access to I/O across modules is performed (using the command to read several bytes at once), 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, the SM321 module does not need to be connected to DC 24V.
43: How to plan the analog module SM374 in STEP 7 hardware configuration? How to find this module in the hardware directory?
The 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 a 16-channel input module – recommended: SM 321: 6ES7321-1BH01-0AA0, if using SM 374 as a 16-channel output module, configure a 16-channel output module – recommended: SM 322: 6ES7322-1BH01-0AA0, if using SM 374 as a mixed input/output module, configure a mixed input/output module (8 inputs, 8 outputs) – recommended: SM 323: 6ES7323-1BH01-0AA0.
44: When measuring current, will the analog input I+ of module 6ES7 331-1KF0.-0AB0 be damaged in case of a sensor short circuit?
When measuring current, the analog input I+ of module 6ES7 331-1KF0.-0AB0 will not be damaged in case of a sensor short circuit. The module has built-in overcurrent protection. Each 50-ohm resistor in the module has a PTC element in front to prevent the input channel of the module from being damaged. Please note that the maximum allowable long-term input voltage is 12V, and the transient (up to 1 second) value is 30V.
45: If the CPU is cut off, will the 2-wire measuring transmitter continue to be powered?
If the transmitter module is inserted in position “D”, and the module is powered by external voltage on pins 1 and 20, the 2-wire measuring transmitter will continue to be powered. Even if the CPU is cut 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. The basic error and operational error are specified 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 work in a complex way, that is, all channels are sequentially plugged into the only AD converter. This principle also applies to the constant current required to read impedance. Therefore, the current flowing through the resistor to be read is only used 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 does the voltage output of the S7-300 analog output group exceed tolerance? What are the uses of terminals S+ and S-?
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 desired 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 pins 5-6 on the front switch. Note: Because 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 not possible to maintain a voltage output using an error limit of 0.5%.
49: How to connect a potentiometer to 6ES7 331-1KF0-0AB0?
The sampling end and the first end of the potentiometer are connected to M+, the last end is connected to M-, and S- and M- are connected together. Note: The maximum allowable resistance is 6K. If the potentiometer directly outputs a variable voltage, the first end of the potentiometer should be connected to V+, and the M end should be connected to M-.
50: How to connect a PT100 temperature sensor to the analog input module SM331?
The resistance of the PT100 thermistor changes with temperature. If a constant current flows through this thermistor, the voltage drop across the thermistor changes with temperature. The constant current is applied to the contacts Ic+ and Ic-. The analog module SM331 measures the change in current between M+ and M-. By measuring the voltage, the temperature can be determined. There are three types of connections for PT100 to the analog input group: 4-wire connection provides the most accurate measurement. * Note: 1) The formula used 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 are measured multiple times. They specify the line resistance of the common return line and perform mathematical compensation. The accuracy obtained is almost comparable to that of the 4-wire connection. An example of such a module is SM331 (MLFB number 6ES7 331-7PF00-0AB0). 3) The formulas provided still apply to the main physical relationships but do not include the effective measurement process for determining the resistance of PT100.
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