71 Examples of Siemens PLC's Most Common Faults and Solutions

1. Under what environment can Siemens Step7Micro/WINV4.0 be installed to work properly?

The installation and operation environment of Step7Micro/WINV4.0 is:

WINOOWS2000SP3 or later

WINOOWsXPHome

WINOOWsXPProfessional

 

Siemens PLC has not been tested under other operating systems and is not guaranteed to work.

 

2. What is the compatibility between Step7Micro/WINV4.0 and other versions?

Project files generated by Micro/WINV4.0 cannot be opened or uploaded by older versions of Micro/WIN.

 

3. What are the differences between Siemens 200 PLC hardware versions?

The second-generation S7-200 (CPU22x) series is also divided into several major hardware versions.

6ES721x-xxx21-xxxx is version 21; 6ES721x-xxx22-xxxx is version 22.

Compared with version 21, version 22 has improved hardware and software. Version 22 is backward compatible with the functions of version 21.

The main differences between version 22 and 21 are: http://www. plcs.cn

The free port communication rates 300 and 600 of the 21st version CPU are replaced by 57600 and 115200 of the 22nd version.

Version 22 no longer supports 300 and 600 baud rates, and version 22 no longer has restrictions on the location of the smart module

 

4. How to connect the power supply of Siemens PLC?

When wiring the CPU, you must be especially careful to distinguish which power supply method it is. If you connect 220VAC to a 24VDC powered CPU, or accidentally connect it to a 24VDC sensor output power supply, the CPU will be damaged.

 

5: How many bits does the S7-200PLC processor have?

The data length of the central processing chip of S7-200CPU is 32 bits. This can also be seen from the data length of CPU accumulator AC0/AC1/AC2/AC3.

 

6. How to calculate the power supply requirements of S7-200?

The S7-200CPU module provides 5VDC and 24VDC power supplies:

 

When there is an expansion module, the CPU provides it with 5V power through the I/O bus. The sum of the 5V power consumption of all expansion modules cannot exceed the power rating provided by the CPU. If it is not enough, an external 5V power supply cannot be connected.

 

Each CPU has a 24VDC sensor power supply, which provides 24VDC for the local input points and expansion module input points and expansion module relay coils. If the power requirement exceeds the power rating of the CPU module, you can add an external 24VDC power supply to provide it to the expansion module.

 

The so-called power calculation is to use the power capacity that the CPU can provide, minus the power consumption required by each module.

 

Notice:

The M277 module itself does not require a 24VDC power supply, which is dedicated to the communication port. The 24VDC power supply requirement depends on the load on the communication port. The communication port on the CPU can connect the PC/PPI cable and TD200 and power them, and this power consumption does not need to be included in the calculation.

 

7. Can 200PLC work at minus 20 degrees?

The working environment requirements of S7-200 are:

0°C-55°C, horizontal installation

0°C-45°C, vertical installation

Relative humidity 95%, non-condensing

Siemens also provides S7-200 wide temperature range products (SIPLUSS7-200):

Operating temperature range: -25°C to +70°C

Relative humidity: 98% at 55°C, 45% at 70°C

Other parameters are the same as those of ordinary S7-200 products

Each wide temperature range product of S7-200 has its own order number, which can be found on the SIPLUS product homepage. If you cannot find it, it means that there is no corresponding SIPLUS product at present.

There are no wide temperature models for text and graphic display panels.

Please also note that there is no stock in China. If you need it, please contact your local Siemens office or dealer.

 

8. How fast does the digital input/output (DI/DO) respond? Can it be used for high-speed input and output?

S7-200 has hardware circuits (chips, etc.) on the CPU unit to process high-speed digital I/O, such as high-speed counters (inputs) and high-speed pulse outputs. These hardware circuits work under the control of user programs and can reach very high frequencies; but the number of points is limited by hardware resources.

 

The S7-200 CPU works cyclically according to the following mechanism:

Read the status of the input point to the input image area

Execute the user program, perform logical operations, and obtain the new state of the output signal

Write the output signal to the output image area

As long as the CPU is in operation, the above steps are repeated. In the second step, the CPU also performs communication, self-checking and other tasks.

The above three steps are the software processing of S7-200CPU, which can be regarded as the program scanning time.

In fact, the processing speed of digital quantity by S7-200 is limited by the following factors:

Input hardware delay (the time from the moment the input signal changes state to the moment the CPU can recognize the change when refreshing the input image area)

The internal processing time of the CPU includes:

Read the status of the input point to the input image area

Execute the user program, perform logical operations, and obtain the new state of the output signal

Write the output signal to the output image area

Output hardware delay (the time from when the output buffer status changes to when the actual level of the output point changes)

 

The above three time periods A, B, and C are the main factors that limit the response speed of Siemens PLC in processing digital quantities.

 

An actual system may also need to consider the delay of input and output devices, such as the action time of the intermediate relay connected to the output point.

 

The above data are all marked in the "S7-200 System Manual", and here is just a list comparison. The delay (filter) time of some input points on the CPU can be set in the "System Block" of the programming software Micro/WIN, and the default filter time is 6.4ms.

 

If a signal that is susceptible to interference is connected to a DI point on the CPU that can change the filter time, adjusting the filter time may improve the quality of signal detection.

 

The input points that support high-speed counter function are not subject to this filter time constraint when the corresponding function is enabled. The filter setting is also effective for the refresh of input image area, switch input interruption, and pulse capture function.

 

Some output points are faster than others because they can be used for high-speed output functions and have special hardware designs. When the hardware high-speed output function is not used, they are just processed like ordinary points.

 

The relay output switching frequency is 1Hz.

 

9. What are the countermeasures for S7-200 to handle fast response signals?

Use the CPU's built-in high-speed counter and high-speed pulse generator to process the sequence pulse signal;

 

Use the hardware interrupt function of some CPU digital input points and process them in the interrupt service program; the delay of entering the interrupt can be ignored;

 

The S7-200 has "direct read input" and "direct write output" instructions that can bypass the time limit of the program scan cycle;

 

Use the "pulse capture" function of some CPU digital input points to capture short pulses;

 

Note: The minimum period of a scheduled task in the S7-200 system is 1ms.

All measures to achieve fast signal processing must take into account the impact of all limiting factors. For example, it is obviously unreasonable to choose hardware with a 500μs output delay for a signal that requires a millisecond response speed.

 

10. Is there any relationship between S7-200 program scan time and program size?

Program scan time is proportional to the size of the user program.

 

The S7-200 System Manual contains data on the execution time required for each instruction. In practice, it is difficult to accurately calculate the program scan time in advance, especially before starting to program.

 

It can be seen that the conventional PLC processing mode is not suitable for digital signals with high time response requirements. It may be necessary to adopt some special methods according to the specific task.

 

11. What is the fastest speed that CPU224XP high-speed pulse output can reach?

The high-speed pulse outputs Q0.0 and Q0.1 of CPU224XP support frequencies up to 100KHz.

 

Q0.0 and Q0.1 support 5-24VDC output. http://www.plcs.cn But they must be grouped with Q0.2-Q0.4 to output the same voltage. High-speed output can only be used in CPU224XPDC/DC/DC model .

 

12. Does the analog input on the CPU224XP body also respond at high speed?

Its response speed is 250ms, which is different from the data of the analog expansion module. The analog I/O chip on the CPU224XP body is different from that used in the analog module, and the conversion principle used is different, so the accuracy and speed are different.

 

13: How to assign the address of the analog module behind CPU224XP

The analog I/O addresses of S7-200 always increase by 2 channels/modules. So the address of the first analog input channel after CPU224XP is AIW4; the address of the first output channel is AQW4, and AQW2 cannot be used.

 

14. What communication protocols does the communication port on the S7-200CPU support?

1) PPI protocol: a communication protocol developed by Siemens specifically for S7-200;

2) MPI protocol: not fully supported, can only be used as a slave

3) Free port mode: A user-defined communication protocol used to communicate with other serial communication devices (such as serial printers, etc.).

 

The S7-200 programming software Micro/WIN provides communication functions implemented through the free port mode:

 

1) USS instruction library: for S7-200 and Siemens inverters (MM4 series, SINAMICS G110 and old MM3 series)

 

2) ModbusRTU instruction library: used to communicate with devices that support the ModbusRTU master protocol

 

The two communication ports on the S7-200 CPU are basically the same, with no special differences. They can work in different modes and communication rates; their port addresses can even be the same. Devices connected to the two communication ports on the CPU do not belong to the same network. The S7-200 CPU cannot act as a bridge.

 

15. What can the communication port on the S7-200 CPU be used for?

1) A programming computer with the programming software Micro/WIN installed can program the PLC;

2) Can connect to the communication ports of other S7-200CPUs to form a network;

3) Can communicate with the MPI communication port of S7-300/400;

4) Can connect to Siemens HMI devices (such as TD200, TP170micro, TP170, TP270, etc.);

5) Data can be published through: OPC Server (PCAccess V1.0);

6) Can connect to other serial communication devices;

7) Can communicate with third-party HMI;

 

16. Can the communication port on the S7-200 CPU be expanded?

It is not possible to expand a communication port with the same function as the CPU communication port.

If there are not enough communication ports on the CPU, you can consider:

1) Purchase a CPU with more communication ports;

2) Check the types of connected devices. If there is a Siemens human-machine interface (HMI, operation panel), consider adding an EM277 module and connecting the panel to the EM277.

 

17. What is the actual communication distance of the communication port on the S7-200 CPU?

The data given in the "S7-200 System Manual" is a network segment of 50m, which is the communication distance that can be guaranteed under the network conditions that meet the specifications. For any distance exceeding 50m, a repeater should be added. Adding a repeater can extend the communication network by 50 meters. If a pair of repeaters are added, and there is no S7-200CPU station between them (EM277 can be used), the distance between repeaters can reach 1000 meters. Meeting the above requirements can achieve very reliable communication. In fact, some users have achieved communication over a distance of more than 50m without adding repeaters. Siemens cannot guarantee that such communication will be successful.

 

18. What factors should users consider when designing a network?

1) The communication port on the S7-200 CPU is electrically an RS-485 port, and the distance supported by RS-485 is 1000m;

2) The communication port on the S7-200CPU is non-isolated, so you need to ensure that the potential of each communication port on the network is equal;

3) Signal transmission conditions (network hardware such as cables, connectors, and external electromagnetic environment) have a great impact on the success of communication;

 

19. Does S7-200 have a real-time clock?

CPU221 and CPU222 do not have a built-in real-time clock and require an external "clock/battery card" to obtain this function. CPU224, CPU226 and CPU226XM all have a built-in real-time clock.

 

20. How to set the date and time values to start moving?

1) Use the menu command PLC> Time of Day Clock... in the programming software (Micro/WIN) to set it through online connection with the CPU. After completion, the clock starts to move;

2) Write a user program and use the Set_RTC (set clock) instruction to set it.

 

21. How are the addresses of smart modules assigned?

 

In addition to the digital and analog I/O expansion modules occupying input/output addresses in the S7-200 system, some intelligent modules (special function modules) also need to occupy addresses in the address range. These data addresses are used by the modules for functional control and are generally not directly connected to external signals.

 

In addition to using IB/QB as status and control bytes, CP243-2 (AS-Interface module) uses AI and AQ for address mapping of AS-Interface slaves.

 

22. What is the compatibility of Step7-Micro/WIN?

The most common Micro/WIN versions are V4.0 and V3.2. Older versions, such as V2.1, are no longer valuable except for converting old project files.

 

Different versions of Micro/WIN generate different project files. A higher version of Micro/WIN is backward compatible with project files generated by lower versions of software; lower versions of software cannot open higher versions.

 

Saved project files. It is recommended that users always use the latest version, which is currently Step7-Micro/WIN V4.0 SP1.

 

23. How to set the communication port parameters?

By default, the communication port of S7-200CPU is in PPI slave mode, the address is 2, and the communication rate is 9.6K.

 

To change the address or communication rate of the communication port, you must set it in the CommunicaitonPorts tab in the system block, and then download the system block to the CPU for the new settings to take effect.

 

 

24. How to set the communication port parameters to improve the network performance?

Assume that there are stations 2 and 10 as master stations in a network, and the highest address (of station 10) is set to 15. For station 2, the so-called address gap is the range from 3 to 9; for station 10, the address gap is the range from 11 to the highest station address 15, and also includes stations 0 and 1.

 

The master stations in the network communication will pass tokens between each other to control the communication activities on the entire network in a time-sharing manner. All the master stations on the network will not join the token passing ring at the same time, so a master station holding a token must regularly check whether there are new master stations joining the station address higher than itself. The refresh factor refers to the number of times the higher station address is checked after obtaining the token.

 

If the address gap factor 3 is set for station 2, when station 2 gets the token for the third time, it will check an address in the address gap to see if there is a new master station joining.

 

Setting a larger factor will improve network performance (because there are fewer unnecessary site checks), but it will affect the speed at which new master sites are added. The following settings will improve network performance:

 

1) Set the highest address that is closest to the actual highest station address

2) Make all master station addresses arranged continuously so that new master station detection will not be performed in the address gap.

 

25. How to set the data hold function?

Data retention settings define how the CPU handles the data retention tasks of each data area. The data area selected in the data retention setting area is the data area whose data content is to be "retained". The so-called "retention" means whether the content of the data area remains in the state before the power failure after the CPU is powered off and then powered on.

 

The data retention function set here is implemented in the following ways:

 

The data retention function set here is realized by the supercapacitor built into the CPU. After the supercapacitor is discharged, if an external battery (or clock/battery for CPU221/222) card is installed, the battery card will continue to supply power for data retention until the discharge is completed. The data will be automatically written into the corresponding EEPROM data area before power failure (if MB0-MB13 is set to retention).

 

26. What is the relationship between data retention settings and EEPROM?

1) If the storage units in the 14-byte range of MB0-MB13 are set to "keep", the CPU will automatically write their contents to the corresponding areas of the EEPROM when the power is off, and overwrite these storage areas with the contents of the EEPROM after power is restored;

2) If the range of other data areas is set to "not retained", the CPU will copy the values in the EEPROM to the corresponding addresses after power is turned on again;

3) If the data area range is set to "Retain", if the built-in super capacitor (+ battery card) fails to successfully retain the data, the contents of the EEPROM will overwrite the corresponding data area, otherwise it will not be overwritten.

 

27: What are the different types of passwords?

Set the CPU password in the system block to restrict user access to the CPU. Passwords can be set in different levels to give other people different levels of authority.

 

28. After setting the CPU password, why can't I see that the password has taken effect?

After setting the CPU password in the system block and downloading it, because you still maintain the communication connection between Micro/WIN and the CPU, the CPU will not protect the Micro/WIN with the password set.

 

To verify that the password is valid, you can:

 

1) Stop the communication between Micro/WIN and CPU for more than one minute;

2) Close the Micro/WIN program and then reopen it;

3) Stop the power supply to the CPU and then supply power again;

 

29. Is there a freeze function for digital/analog quantities?

The digital/analog output table specifies how the digital output points or analog output channels operate when the CPU is in the STOP state.

 

This function is very important for some equipment that must keep moving and running, such as brakes or some key valves, which are not allowed to stop when debugging Siemens PLC, so they must be set in the output table of the system block.

 

Digital quantity:

 

After selecting "Freezeoutputinlaststate", the last state is frozen. When the CPU enters the STOP state, the digital output point maintains the state before the shutdown (if it is 1, it remains 1, if it is 0, it remains 0). At the same time, the b. table below will not take effect. If it is not selected, the selected output point will remain in the ON (1) state, and the unselected ones will remain at 0.

 

Analog quantity:

 

After selecting "Freeze output in last state", the last state is frozen. When the CPU enters the STOP state, the analog output channel maintains the state before shutdown. At the same time, the table below does not work. When it is not selected, the output value of each analog output channel specified in the table below when the CPU enters the STOP state.

 

30. What is the function of the digital input filter and how to set it?

You can select different input filter times for the digital input points on the CPU. If the input signal has interference or noise, you can adjust the input filter time to filter out the interference to avoid false operation. The filter time can be selected in several levels within the range of 0.20~12.8ms. If the filter time is set to 6.40ms, the CPU will ignore the digital input signal when the effective level (high or low) lasts for less than 6.4ms; it can only be recognized when it lasts longer than 6.4ms.

 

In addition: the input points that support high-speed counter function are not subject to this filter time constraint when the corresponding function is enabled. The filter setting is effective for the refresh of input image area, switch input interruption, and pulse capture function.

 

31. What is the effect of analog filtering?

In general, if you use the analog filtering function of the S7-200 Siemens PLC, you don't need to compile a separate user filtering program.

 

If analog filtering is selected for a channel, the CPU will automatically read the analog input value before each program scan cycle. This value is the filtered value and the average value of the set sampling number. The analog parameter setting (sampling number and dead zone value) is valid for all analog signal input channels.

 

If a channel is not filtered, the CPU will not read the average filtered value at the beginning of the program scan cycle, but will directly read the actual value at that time when the user program accesses this analog channel.

 

32. How to set the analog filter dead zone value?

The dead zone value defines the range of values for calculating the average value of the analog quantity.

 

If the sampled values are all within this range, the average value set by the number of samples is calculated; if the current latest sampled value exceeds the upper or lower limit of the dead zone, the value is immediately adopted as the current new value and used as the starting value for subsequent average value calculations.

 

This allows the filter to respond quickly to large changes in analog values. Setting the deadband value to 0 disables the deadband function, that is, all values are averaged, regardless of how much the value changes. For fast response requirements, do not set the deadband value to 0, but set it to the maximum expected disturbance value (320 is 1% of the full scale of 32000).

 

33. What should we pay attention to when setting analog filtering?

1) Selecting a filter for analog inputs that change slowly can suppress fluctuations;

2) Selecting a smaller sampling number and dead zone value for analog inputs that change faster will speed up the response;

3) Do not use filters for analog values that change at high speed;

4) If you use analog quantity to transmit digital signal, or use thermal resistor (EM231RTD), thermocouple (EM231TC), AS-Interface (CP243-2) module, you cannot use the filter;

 

34. How to make the monitoring response in Micro/WIN faster?

You can set the background communication time, which specifies the percentage of the communication time between Micro/WIN and the CPU used for "run mode programming" and program and data monitoring in the entire program scan cycle. Increasing this time can increase the communication opportunities for monitoring, and the response in Micro/WIN will feel faster, but at the same time it will lengthen the program scan time.

 

35. Can the indicator light on the CPU be customized?

The indicator light can be customized by the user.

 

The LED indicator light (SF/DIAG) of the 23 version CPU can display two colors (red/yellow). Red indicates SF (system fault), and the yellow DIAG indicator light can be customized by the user.

 

Custom LED indicators can be controlled by the following methods:

 

1) Set in the "Configure LED" tab of the system block;

2) Use the DIAG_LED instruction in the user program to light it up;

 

The above conditions are in an OR relationship. If both SF and DIAG indications appear at the same time, the red and yellow lights will flash alternately.

 

36. Can I use the entire program storage area at any time?

The new function (runtime programming) of version 23 CPU requires a part of the program storage space. If you want to use the entire program storage area, for some specific CPU models, you need to disable the "run mode programming" function.

 

37. How do I access a password-protected CPU if I forget the password?

Even if the CPU is password protected, you can use the following functions without restriction:

1) Read and write user data http://www.plcs.cn

2) Start and stop the CPU

3) Read and set the real-time clock

 

If the password is not known, the user cannot read or modify the program in a CPU with three-level password protection.

 

38. How to clear the set password?

If you do not know the CPU password, you must clear the CPU memory before you can re-download the program. Executing the clear CPU command will not change the original network address, baud rate, and real-time clock of the CPU; if there is an external program storage card, its content will not change. After clearing the password, the original program in the CPU will no longer exist.

 

To clear the password, you can follow the 3 methods below:

 

1) In Micro/WIN, select the menu "PLC>Clear", select all three blocks and press "OK" to confirm.

2) Another method is to restore the CPU to its default settings using the program "wipeout.exe". This program can be found on the STEP7-Micro/WIN installation CD.

3) In addition, you can also insert an external memory card containing an unencrypted program into the CPU. After power-on, this program will be automatically loaded into the CPU and overwrite the original password-protected program. The CPU can then be accessed freely.

 

39. Can I still use the POU normally after it is encrypted?

POU is the program organization unit, which includes the main program (OB1), subroutine and interrupt service program in the S7-200 project file.

 

POUs can be encrypted individually. After encryption, a lock mark will be displayed on the POU, and the program content cannot be opened. The program is downloaded to the CPU and remains encrypted after being uploaded.

 

The library instructions, subroutines generated by the instruction wizard, and interrupt programs provided by Siemens with the programming software Micro/WIN are all encrypted. Encryption does not prevent them from being used.

 

40. Can I encrypt the entire project file?

Using Step7-Micro/WINV4.0 or above, users can encrypt the entire Project file so that people who do not know the password cannot open the project.

 

In the SetPassword command in the File menu of Micro/WIN, enter a project file password of up to 16 characters in the pop-up dialog box.

 

The password can be a combination of letters or numbers and is case sensitive.

 

41. How to open project files created by old versions of Micro/Win?

In the genuine STEP7Micro/WIN software CD, you can find the V2.1 version of Micro/WIN installation software in the OldRealeses folder. This version of Micro/WIN can open project files created by the previous old version. Using it as a bridge, after saving the old version of the software, you can open it in the latest version of STEP7Micro/WIN software.

 

Note: If you find that some networks are displayed as invalid in red after opening, it may be that the PLC model is too low or the version is too old. In this case, you can select a higher model or a newer version of the CPU. For example, change CPU222 to CPU224 in PLC>Type in the command menu.

 

42. How do I know the size of the program I wrote?

After executing PLC>Compile in the command menu in Micro/WIN, you can find the size of your program, the size of the occupied data block, etc. in the display window (message output window) below Micro/WIN.

 

43. What should I do if a compilation error occurs?

After compiling, if there is an error, the program cannot be downloaded to the CPU. You can view the error in the window below Micro/WIN, double-click the error to enter the error in the program, and modify it according to the instructions in the system manual.

 

44. How do I know the scan time of my program?

After the program has been run once, you can view the scan time of the program in the CPU online by viewing PLC>Information in the command menu in Micro/WIN.

 

45. How to find out whether the program address space used is reused?

After compiling the program, you can click the Cross Reference button in the View bar to enter the detailed cross reference information of the elements used in the program and the usage of bytes and bits. In the cross reference, you can directly click the address to enter the address in the program.

 

46. During online monitoring, why is the instruction function block in the program block red?

If you monitor online in the program editor and find a red instruction function block, it means that an error or problem has occurred. You can find the error that caused ENO=0 in the system manual. If it is a "non-fatal" fault, you can check the error type in the menu PLC>Information dialog box.

 

For instructions related to the PLC operating system or hardware settings, such as NetR/NetW (network read/write), XMT/RCV (free port send/receive), PLS, etc., which turn red during operation, the most likely reason is that the instruction is called multiple times while it is still being executed, or the communication port is busy at the time.

 

47. How to use the high-speed input and output of S7-200?

The wiring of the high-speed input and output terminals on the S7-200 CPU is the same as that of ordinary digital I/O. However, high-speed pulse output must use a CPU with DC transistor output (i.e. DC/DC/DC type).

 

48. Can rotary encoders (and other sensors) with NPN/PNP outputs be connected to the S7-200 CPU?

Yes. The digital inputs on the S7-200 CPU and expansion modules can be connected to source or sink sensor outputs. When connecting, just change the connection method of the common terminal accordingly (whether the power supply L+ is connected to the input common terminal, or the power supply M is connected to the common terminal).

 

49. Can S7-200 use two-wire digital (switch) sensors?

Yes, but the static operating current (leakage current) of the sensor must be less than 1mA. Siemens has related products, such as proximity switches (BERO) for PLC.

 

50. Does S7-200 have modules with reused input and output points?

The digital and analog input/output points of S7-200 cannot be multiplexed (i.e., they can be used as both input and output).

 

51. Can the high-speed input and output of CPU224XP reach 100K or 200K?

The two high-speed inputs of the new product CPU224XP support even higher speeds. When used as a single-phase pulse input, it can reach 200KHz; when used as a two-phase 90° orthogonal pulse input, the speed can reach 100KHz.

 

The two-way high-speed digital output rate of CPU224XP can reach 100KHz.

 

52. The high-speed input (I0.3/4/5) of CPU224XP is a 5VDC signal. Can other input points be connected to 24VDC signals?

Yes. Simply connect the common terminals of both signal power supplies to the 1M terminal. Both signals must be sink or source input signals at the same time.

 

53. The high-speed output points Q0.0 and Q0.1 of CPU224XP are connected to a 5V power supply. Can other points such as Q0.2/3/4 be connected to a 24V voltage?

No. They must be connected in groups at the same voltage level.

 

54. Are there analog quantities that cannot be filtered?

Since the principle of the analog conversion chip on the CPU224XP body is different from that of the extended analog module, there is no need to select filtering.

 

55. What are unipolarity and bipolarity?

Bipolar means that the signal will pass through "zero" during the change process, while unipolar does not pass through zero. Since analog quantity converted to digital quantity is a signed integer, the value corresponding to the bipolar signal will be negative. In S7-200, the value range of unipolar analog input/output signal is 0-32000; the value range of bipolar analog signal is -32000－+32000.

 

56. How should analog quantities be converted into expected engineering quantity values?

Analog input/output can be converted using the following general conversion formula:

Ov=【(Osh-Osl)*(Iv-Isl)/(Ish-Isl)】+Osl

Where :  HYPERLINK "https://link.zhihu.com/?target=http://www.plcs.cn" \t "https://zhuanlan.zhihu.com/p/_blank" http://www.plcs.cn

Ov: Conversion result

Iv: Conversion object

Osh: upper limit of conversion result

Osl: The lower limit of the conversion result

Ish: Upper limit of conversion object

Isl: The lower limit of the conversion object

 

57. What is the accuracy of the S7-200 analog input signal?

The pseudo input module has two parameters that are easy to confuse:

1) Resolution of analog conversion;

2) Accuracy (error) of analog conversion;

 

Resolution is the conversion accuracy of the A/D analog conversion chip, that is, how many bits are used to represent the analog quantity. The conversion resolution of the S7-200 analog module is 12 bits, and the smallest unit that can reflect the change of the analog quantity is 1/4096 of the full scale.

 

The accuracy of analog conversion depends not only on the resolution of A/D conversion, but also on the peripheral circuit of the conversion chip. In practical applications, the input analog signal will have fluctuations, noise and interference, and the internal analog circuit will also produce noise and drift, which will affect the final accuracy of the conversion. The error caused by these factors is greater than the conversion error of the A/D chip.

 

58. Why is the analog quantity an unstable value with large changes?

The possible reasons are as follows:

 

1) You may have used a self-powered or isolated sensor power supply, and the two power supplies are not connected to each other, that is, the power ground of the analog input module and the signal ground of the sensor are not connected. This will generate a very high common-mode voltage with up and down vibrations, affecting the analog input value.

2) Another reason may be that the analog input module wiring is too long or the insulation is poor.

 

This can be solved by:

 

1) Connect the negative terminal of the sensor input to the common M terminal on the module to compensate for this fluctuation. (But be careful to ensure that this is the only connection between the two power systems.)

The background is: the analog input module is not isolated inside; the common mode voltage should not be greater than 12V; the common mode rejection ratio for 60Hz interference signals is 40dB.

2) Use analog input filter.

 

59. Why does the SF red light on the EM231 module flash?

There are two reasons why the SF red light flashes: the module's internal software detects that the external thermal resistor is disconnected, or the input is out of range. Since the above detection is shared by two input channels, the SF light will inevitably flash when only one channel is connected to an external thermal resistor. The solution is to connect a 100Ohm resistor to the empty channel in the same wiring method as the used channel; or connect all the leads of the already connected thermal resistor to the empty channel one by one.

 

60. What is positive calibration and negative calibration?

The positive calibration value is 3276.7 degrees (Fahrenheit or Celsius), and the negative calibration value is -3276.8 degrees. If a disconnection or input out of range is detected, the value of the corresponding channel is automatically set to the above calibration value.

 

61. The technical parameters of the thermal resistor are not very clear. How to set the type on the DIP switch?

You should try to clear the parameters of the thermal resistor. Otherwise, you can use the default settings.

 

62. Can EM235 be used for resistance temperature measurement?

EM235 is not a module for connecting to a thermal resistor to measure temperature. Using it with difficulty may cause problems. It is recommended to use the EM231RTD module.

 

63. Does the analog input/output module of S7-200 have signal isolation?

Without isolation. If isolation is required in the user's system, please purchase signal isolation components separately.

 

64. How far is the transmission distance of analog signals?

Voltage-type analog signals are very easy to introduce interference due to the high internal resistance of the input end (10 megohms for the analog module of S7-200), so it is meaningless to discuss the transmission distance of voltage signals. Generally, voltage signals are used to set potentiometers in control equipment cabinets, or in situations where the distance is very close and the electromagnetic environment is good.

 

Current-type signals are not easily affected by electromagnetic interference along the transmission line, and therefore are widely used in industrial fields.

 

Current signals can be transmitted over much longer distances than voltage signals. In theory, the transmission distance of current signals is limited by the following factors:

1) The load capacity of the signal output terminal, expressed in ohms (e.g. 700Ω)

2) Internal resistance of the signal input terminal

3) Static resistance value of the transmission line (two lines going back and forth)

 

The load capacity of the signal output end must be greater than the sum of the internal resistance of the signal input end and the transmission line resistance. Of course, the actual situation will not completely conform to the ideal calculation result. Too long a transmission distance will cause signal attenuation and introduce interference.

 

65. What is the input/output impedance specification of the S7-200 analog module?

Analog input impedance:

Voltage signal: ≥10MΩ

Current signal: 250Ω

Analog output impedance:

Voltage signal: ≥5KΩ

Current signal: ≤500Ω

 

66: The power indicator light of the analog module is normal, why is the signal input light not on?

The housing of the analog module is designed and manufactured in a universal form, and there is actually no analog input signal indicator light. All light windows without printed marks are useless and empty.

 

67.  Why do the lowest three digits of the analog value have non-zero value changes?

The conversion accuracy of analog quantity is 12 bits, but the module shifts the converted value to the higher bit by three bits. If this channel is set to use analog quantity filtering, the current value is the average value of several samples, and the lowest three bits are the calculated values; if analog quantity filtering is disabled, the lowest three bits are all zero.

 

68. Does EM231TC require compensation wires?

The EM231TC can be set up to achieve cold junction compensation by the module, but compensation wires are still required to compensate the free ends of the thermocouples.

 

69. Why does the SF light on the EM231TC module flash?

If wire break detection is selected, the wire may be broken. The unused channel should be shorted or connected in parallel to the actual wiring channel next to it. Or the input is out of range.

 

70. What should I do if the data in Zone M is insufficient?

Some users are used to using the M area as the intermediate address, but the M area address space in the S7-200CPU is very small, only 32 bytes, which is often not enough. The S7-200CPU provides a large amount of V area storage space, that is, user data space. The V storage area is relatively large, and its usage is similar to that of the M area. V area data can be accessed by bit, byte, word or double word. For example: V10.1, VB20, VW100, VD200, etc.

 

71. How do I know the integrated I/O and extended I/O addressing of the S7-200 CPU?

There is no need to configure I/O addresses when programming S7-200.

 

The I/O addresses on the S7-200 expansion modules are arranged in ascending order according to the distance from the CPU. The closer to the CPU, the smaller the address number.

 

Between modules, the address of digital signals always increases by 8 bits (1 byte). If the physical input point on the CPU does not completely occupy a byte, the remaining unused bits cannot be allocated to the same signal of the subsequent module.

 

Analog output modules always occupy the output addresses of two channels. Even if some modules (EM235) have only one actual output channel, they still occupy the addresses of two channels. When the programming computer and CPU are actually online, use the Micro/WIN menu command "PLC>Information" to view the actual I/O address allocation of the CPU and expansion modules.