Common problems and solutions in Siemens PLC hardware and programming

36. Can I use all the program memory at any time?

The new features of the 23nd CPU (runtime programming) require a portion of the program memory space. If you want to use the full program memory area, you need to disable the "Run Mode Programming" function for certain CPU models.

37. If I forget my password, how can I access a CPU with a password?

Even if the CPU is password protected, you can use the following features without restrictions:

1) Read and write user data

2) Start, stop the CPU

3) Read and set the real time clock

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

38. How to clear the set password?

If you don't know the CPU password, you must clear the CPU memory before you can reinstall the program. Executing the clear CPU command does not change the original network address, baud rate and real-time clock of the CPU; if there is an external program memory card, its contents will not change. After clearing the password, the original program in the CPU will not exist.

To clear the password, follow the 3 methods below:

1) Select the menu "PLC>Clear" in Micro/WIN to select all three blocks and confirm with "OK".

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

3) In addition, an external memory card containing an unencrypted program can be inserted into the CPU. After power-on, the program is automatically loaded into the CPU and overwrites the original program with a password. Then the CPU is free to access.

39. Can I still use it after POU encryption?

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

The POU can be encrypted separately. The encrypted POU will display a lock tag and cannot open the view program content. The program is downloaded to the CPU and remains encrypted after uploading.

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

40. Can I encrypt the entire project file?

With Step7-Micro/WINV 4.0 or higher, users can encrypt the entire Project file so that people who don't know the password can't open the project.

In the Micro/WIN File menu, the SetPassword command enters 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 the project file created by the old version of Micro/Win?

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

Note: If you find that some networks are displayed as red invalid (Illegal), it may be that the PLC model is too low and the version is too old. You can select a high model or a new version of the CPU. For example, change CPU 222 to CPU224 in PLC>Type of the command menu.

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

After executing PLC>Compile in the command menu in Micro/WIN, the display window (message output window) under Micro/WIN can find the size of the program you are programming, the size of the occupied data block, and so on.

43. What should I do if I compile?

After compiling, if there is an error, you will not be able to download the program to the CPU. You can view the error in the window below Micro/WIN. Double-click the error to enter the location of the error in the program and modify it according to the instructions in the system manual.

44, how to know the scanning time of the program you have programmed?

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

45, how to find the program address space used is reused?

After compiling the program, you can click on the CrossReference button in the View navigation bar to see the detailed cross-reference information of the elements used in the program and the use of bytes and bits. In the cross-reference, you can directly click on the address and enter the address where the address is in the program.

46. ​​When online monitoring, why is the command function block turned red in the block?

If you monitor online in the program editor, you will find a red command function block indicating that an error or problem has occurred. An error causing ENO=0 can be found in the system manual. If it is a "non-fatal" fault, you can check the error type in the menu PLC>Information dialog.

For NetR/NetW (network read/write), XMT/RCV (free port send/receive), PLS, etc., the instructions related to the PLC operating system or hardware settings turn red at runtime, the most likely cause is the command It is still called multiple times during the execution, or the communication port is busy at the time.

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

The high-speed input and output terminals on the S7-200 CPU have the same wiring as the normal digital I/O. However, the high-speed pulse output must use a DC transistor output type CPU (ie, DC/DC/DC type).

48. Can the rotary encoder (and other sensors) of the NPN/PNP output be connected to the S7-200 CPU?

Yes. The digital input on the S7-200 CPU and expansion module can be connected to the source or sink sensor output. When connecting, just change the connection of the common terminal (it is the power supply L+ connected to the input common terminal, or the power supply M is connected to the public). end).

49. Can the S7-200 use a two-wire digital (switching) sensor?

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

50. Does the S7-200 have modules that can be reused for input and output points?

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

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

The new two-way CPU224XP high-speed input supports higher speeds. When used as a single-phase pulse input, it can reach 200KHz; when used as a two-phase 90° quadrature pulse input, the speed can reach 100KHz.

The CPU224XP's two high-speed digital output rates can reach 100KHz.

52, CPU224XP high-speed input (I0.3/4/5) is 5VDC signal, can other input points connect 24VDC signal?

can. Simply connect the common terminals of both signal power supplies to the 1M terminals. Both signals must be either sink or source input signals.

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

No. The same voltage level must be connected in groups.

54. Actually, there is an analog quantity that cannot be filtered?

Since the principle of the analog conversion chip on the body of the CPU 224XP is different from that of the extended analog module, no filtering is required.

55. What is unipolar and bipolar?

Bipolarity means that the signal passes through "zero" during the change process, but the unipolar is zero. Since the analog quantity converted to a digital quantity is a signed integer, the value corresponding to the bipolar signal will have a negative number. In the S7-200, the value range of the unipolar analog input/output signal is 0-32000; the value range of the bipolar analog signal is -32000-+32000.

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

The analog input/output can be converted with the following general conversion formula:

Ov=[(Osh-Osl)*(Iv-Isl)/(Ish-Isl)]+Osl

among them:

Ov: Conversion result

Iv: Conversion object

Osh: the upper limit of the conversion result

Osl: the lower limit of the conversion result

Ish: the upper limit of the conversion object

Isl: the lower limit of the conversion object

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

The proposed input module has two parameters that are easily confused:

1) resolution of analog conversion;

2) accuracy (error) of analog quantity conversion;

The resolution is the conversion accuracy of the A/D analog conversion chip, that is, the number of bits 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 analog change is 1/4096 of full scale.

The accuracy of analog conversion depends on the resolution of the A/D conversion and is also affected by the peripheral circuitry of the conversion chip. In practical applications, the input analog signal will have fluctuations, noise and interference, and the internal analog circuit will also generate 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 a very unstable value that changes a lot?

May be the following reasons:

1) You may have used a self-powered or isolated sensor power supply. 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 produce a high common-mode voltage that vibrates up and down, affecting the analog input value.

2) Another reason may be that the analog input module wiring is too long or the insulation is not good.

Can be solved as follows:

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

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

2) Use an 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 internal software of the module detects an external thermal resistance disconnection, or the input is out of range. Since the above detection is shared by the two input channels, the SF lamp must blink when only one channel is externally connected to the thermal resistor. The solution is to connect a 100 Ohm resistor to the empty channel in the same way 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 forward calibration and negative calibration?

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

61, the technical parameters of the thermal resistance is not very clear, how to set the type on the DIP switch?

The parameters of the thermal resistance should be removed as much as possible. Otherwise you can use the default settings.

62. Can EM235 be used for RTD temperature measurement?

The EM235 is not a module for measuring the temperature connected to a thermal resistor, and it is easy to cause problems if it is used barely. The EM231RTD module is recommended.

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

Without isolation. If isolation is required in the user's system, purchase a separate signal isolation device.

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

The voltage type analog signal, because the internal resistance of the input terminal is very high (the analog module of the S7-200 is 10 megohms), it is easy to introduce interference, so it is meaningless to discuss the transmission distance of the voltage signal. The general voltage signal is used in the setting of the potentiometer in the control equipment cabinet, or in the case where the distance is very close and the electromagnetic environment is good.

Current-type signals are not susceptible to electromagnetic interference along the transmission line and are therefore widely used in industrial fields.

The current signal can transmit a much greater distance than the voltage signal. In theory, the transmission distance of the current signal is subject to the following factors:

1) The load capacity of the signal output is expressed in ohms (eg 700Ω)

2) Internal resistance of the signal input

3) The static resistance value of the transmission line (round trip is double line)

The load capacity of the signal output must be greater than the sum of the internal resistance of the signal input and the resistance of the transmission line. Of course, the actual situation will not completely calculate the ideal result of the symbol. If the transmission distance is too long, the signal will be attenuated and interference will be introduced.

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

Analog input impedance:

Voltage type signal: ≥10MΩ

Current type signal: 250Ω

Analog output impedance:

Voltage type signal: ≥5KΩ

Current type signal: ≤500Ω

66: The power indicator 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 common form, with virtually no analog input signal indicator. Light windows without printed marks are useless.

67: Why does the lowest three digits of the analog value have a non-zero value change?

The conversion accuracy of the analog is 12 bits, but the module shifts the digital-to-analog value to the high position by three bits. If this channel is set to use analog filtering, the current value is the average of several samples, the lowest three are the calculated values; if analog filtering is disabled, the lowest three bits are zero.

68. Does the EM231TC need to compensate the wire?

The EM231TC can be set to implement cold junction compensation by the module, but still requires compensation for the free end compensation of the thermocouple.

69. Why does the SF lamp of the EM231TC module flash?

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

70, M area data is not enough to do?

Answer: Some users are accustomed to using the M area as the intermediate address, but the address space of the M area in the S7-200 CPU is very small, only 32 bytes, which is often not enough. The S7-200 CPU provides a large amount of V area storage space, that is, user data space. The V memory area is relatively large, and its usage is similar to that of the M area, and the V area data can be accessed in bits, bytes, words, or double words. 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?

It is not necessary to configure the I/O address when programming the S7-200.

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

Between modules, the address of a digital signal is always incremented by 8 bits (1 byte). If the physical input point on the CPU does not occupy exactly one byte, the remaining unused bits cannot be assigned to the same type of signals of subsequent modules.

The analog output module always takes up the output address of both channels. Even though some modules (EM235) have only one actual output channel, it takes up the address of both channels. When the programming computer and the CPU are actually connected, the actual I/O address assignment of the CPU and expansion modules can be viewed using the Micro/WIN menu command "PLC>Information".

END