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Recently, a colleague asked me if there are any materials on the S7-200 PLC. Although most people are currently using the S7-200 SMART, I’m here to help you out, so let’s learn about the S7-200 today!
1. In what environment can Siemens Step7 Micro/WIN V4.0 operate normally?
The installation and operating environment for Step7 Micro/WIN V4.0 is:
WINDOWS 2000 SP3 or above
WINDOWS XP Home
WINDOWS XP Professional
Siemens PLC has not been tested on other operating systems and does not guarantee usability.
2. How compatible is Step7 Micro/WIN V4.0 with other versions?
Project files generated by Micro/WIN V4.0 cannot be opened or uploaded by older versions of Micro/WIN.
3. What are the differences in hardware versions of Siemens 200 PLC?
The second generation S7-200 (CPU22x) series is divided into several main hardware versions.
6ES721x-xxx21-xxxx is version 21; 6ES721x-xxx22-xxxx is version 22.
Version 22 has improvements in both hardware and software compared to version 21. Version 22 is backward compatible with the functionalities of version 21.
The main differences between version 22 and version 21 are: www.plcs.cn
The communication rates of the free ports of version 21 at 300 and 600 are replaced by 57600 and 115200 in version 22. Version 22 no longer supports 300 and 600 baud rates, and there are no longer restrictions on the positions of intelligent modules.
4. How should the power supply of Siemens PLC be connected?
When wiring the power supply to the CPU, be especially careful to distinguish which power supply method is used. Connecting 220VAC to a 24VDC powered CPU, or accidentally connecting it to a 24VDC sensor output power supply, can damage the CPU.
5. What is the bit length of the S7-200 PLC processor?
The data length of the central processing chip of the S7-200 CPU is 32 bits. This can also be seen from the data length of CPU accumulators AC0/AC1/AC2/AC3.
6. How to calculate the power requirements for S7-200?
The S7-200 CPU module provides 5VDC and 24VDC power:
When there are expansion modules, the CPU provides 5V power through the I/O bus, and the total 5V power consumption of all expansion modules must not exceed the rated power provided by the CPU. If insufficient, an external 5V power supply cannot be connected.
Each CPU has a 24VDC sensor power supply that provides 24VDC for the local input points and the input points of the expansion modules as well as the relay coils of the expansion modules. If the power requirements exceed the power rating of the CPU module, an external 24VDC power supply can be added to provide power for the expansion modules.
Power calculation refers to using the power capacity that the CPU can provide, minus the power consumption required by each module.
Note:
The EM277 module itself does not require a 24VDC power supply; this power supply is solely for the communication port. The 24VDC power supply requirement depends on the load size on the communication port. The communication port on the CPU can connect to PC/PPI cables and TD200 and power them, and this power consumption does not need to be included in the calculation.
7. Can the 200 PLC work at -20 degrees?
The working environment requirements for the S7-200 are:
0°C – 55°C, horizontal installation
0°C – 45°C, vertical installation
Relative humidity 95%, no condensation
Siemens also provides a wide temperature range product for S7-200 (SIPLUS S7-200):
Operating temperature range: -25°C – +70°C
Relative humidity: 98% at 55°C, 45% at 70°C
Other parameters are the same as those of ordinary S7-200 products.
The wide temperature type products of S7-200 each have their own separate order number, which can be queried on the SIPLUS product homepage. If not found, it means there are currently no corresponding SIPLUS products.
Text and graphical display panels do not have wide temperature type products.
Also, note that there is no stock in the country; if needed, please contact the local Siemens office or distributor.
8. What is the response speed of digital input/output (DI/DO)? Can it be used for high-speed input and output?
The S7-200 has hardware circuits (chips, etc.) on the CPU unit to handle high-speed digital I/O, such as high-speed counters (input) and high-speed pulse output. These hardware circuits work under the control of the user program and can achieve very high frequencies; however, the number of points is limited by hardware resources.
The S7-200 CPU operates in cycles as follows:
Read the status of input points into the input image area
Execute the user program, perform logical operations, and obtain the new status of the output signals
Write the output signals into the output image area
As long as the CPU is in operation, the above steps are executed repeatedly. During the second step, the CPU also performs communication, self-checking, and other tasks.
The above three steps are the software processing process of the S7-200 CPU, which can be considered as the program scan time.
In fact, the processing speed of the S7-200 for digital signals is limited by the following several factors:
Input hardware delay (the time from the moment the input signal status changes to when the CPU refreshes the input image area to recognize its change)
CPU internal processing time, including:
Reading the state of input points into the input image area
Executing the user program, performing logical operations, and obtaining the new status of the output signals
Writing the output signals into the output image area
Output hardware delay (the time from when the output buffer status changes to when the output point’s real level changes)
The above A, B, and C time segments are the main factors limiting the response speed of the Siemens PLC for digital signals.
A practical system may also need to consider the delays of input and output devices, such as the action time of intermediate relays connected to output points, etc.
All the above data is indicated in the ‘S7-200 System Manual’; here is just a comparative list. The delay (filtering) time for some input points on the CPU can be set in the programming software Micro/WIN’s ‘System Block’, with a default filtering time of 6.4ms.
If signals that are easily disturbed are connected to the CPU’s DI points where the filtering time can be changed, adjusting the filtering time may improve the quality of signal detection.
Input points that support high-speed counter functions are not subject to this filtering time constraint when the corresponding function is enabled. The filtering settings are also effective for refreshing the input image area, digital input interrupts, and pulse capture functions.
Some output points may be faster than others because they can be used for high-speed output functions and are specially designed in hardware. If they are not specifically used for high-speed output functions, they are processed just like ordinary points.
The relay output switching frequency is 1Hz.
9. What measures can be taken for S7-200 to handle fast response signals?
Use the built-in high-speed counter and high-speed pulse generator of the CPU to process sequential pulse signals;
Use the hardware interrupt function of some CPU digital input points to handle in the interrupt service program; the delay for entering the interrupt can be ignored;
The S7-200 has ‘Direct Read Input’ and ‘Direct Write Output’ instructions that can bypass the time limitations of the program scanning cycle;
Use the ‘Pulse Capture’ function of some CPU digital input points to capture brief pulses;
Note: The minimum cycle time for timing tasks in the S7-200 system is 1ms.
All measures to achieve fast signal processing must consider the impact of all limiting factors. For example, it is clearly unreasonable to choose hardware with a 500μs output delay for a signal requiring millisecond-level response speed.
10. Is the program scan time related to the program size?
The program scan time is proportional to the size of the user program.
Data on the execution time required for each instruction can be found in the ‘S7-200 System Manual’. In practice, it is very difficult to accurately calculate the program scan time in advance, especially when programming has not yet started.
It can be seen that the conventional PLC processing mode is not suitable for digital signals with high time response requirements. Some special methods may need to be adopted based on specific tasks.
11. What is the maximum speed of high-speed pulse output for CPU224XP?
The high-speed pulse output Q0.0 and Q0.1 of CPU224XP supports frequencies of up to 100KHz.
Q0.0 and Q0.1 support 5-24VDC output. www.plcs.cn However, they must output the same voltage in groups with Q0.2-Q0.4. High-speed output can only be used on the CPU224XP DC/DC/DC model.
12. Is the analog input on the CPU224XP also a high-speed response?
Its response speed is 250ms, which is different from the data from the analog expansion module. The analog I/O chip on the CPU224XP body is different from that used in the analog module, and the application conversion principles are different, hence the precision and speed are not the same.
13. How are the addresses of the analog modules hanging behind CPU224XP allocated?
The analog I/O addresses of S7-200 always increase in increments of 2 channels/module. Therefore, the address of the first analog input channel behind CPU224XP is AIW4; the address of the first output channel is AQW4, and AQW2 cannot be used.
14. What communication protocols are supported by the communication ports on the S7-200 CPU?
1) PPI protocol: A communication protocol developed by Siemens specifically for S7-200;
2) MPI protocol: Not fully supported, can only act as a slave;
3) Free port mode: A user-defined communication protocol for communicating with other serial communication devices (such as serial printers, etc.).
The S7-200 programming software Micro/WIN provides communication functions implemented through free port mode:
1) USS instruction library: Used for S7-200 to communicate with Siemens frequency converters (MM4 series, SINAMICS G110, and older MM3 series);
2) Modbus RTU instruction library: Used to communicate with devices that support Modbus RTU master protocol;
The two communication ports on the S7-200 CPU are basically the same, with no significant differences. They can operate 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 ports on the S7-200 CPU be used for?
1) A programming computer with the Micro/WIN programming software can program the PLC;
2) It can connect to the communication ports of other S7-200 CPUs to form a network;
3) It can communicate with the MPI communication ports of S7-300/400;
4) It can connect to Siemens HMI devices (such as TD200, TP170micro, TP170, TP270, etc.);
5) It can publish data through OPC Server (PCAccess V1.0);
6) It can connect to other serial communication devices;
7) It can communicate with third-party HMIs;
16. Can the communication ports on the S7-200 CPU be expanded?
Communication ports that are exactly the same as the CPU communication ports cannot be expanded.
When the communication ports on the CPU are insufficient, you can consider:
1) Purchasing a CPU with more communication ports;
2) Considering the types of connected devices; if any of them are Siemens HMIs (Human Machine Interfaces), you can consider adding the EM277 module to connect the panel to the EM277;
17. How far can the communication ports on the S7-200 CPU communicate?
The data given in the ‘S7-200 System Manual’ is a network segment of 50m, which can be guaranteed under compliant network conditions. Any distance exceeding 50m should have a repeater added. Adding one repeater can extend the communication network by 50 meters. If a pair of repeaters is added, and there is no S7-200 CPU station between them (an EM277 can be present), the distance between the repeaters can reach 1000 meters. Complying with the above requirements can achieve very reliable communication.
In practice, some users have achieved communication over distances greater 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 ports on the S7-200 CPU are electrically RS-485 ports, which support a distance of 1000m;
2) The communication ports on the S7-200 CPU are not isolated, so it is important to ensure that the potentials of all communication ports on the network are equal;
3) The signal transmission conditions (network hardware such as cables, connectors, and the external electromagnetic environment) have a significant impact on the success of communication;
19. Does the S7-200 have a real-time clock?
The 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 value to start running?
1) Use the menu command PLC>TimeofDayClock… in the programming software (Micro/WIN) to set it through the online connection with the CPU, and the clock will start running after completion;
2) Write a user program using the Set_RTC (set clock) instruction to set it.
21. How are the addresses of intelligent modules allocated?
In the S7-200 system, in addition to digital and analog I/O expansion modules occupying input/output addresses, some intelligent modules (special function modules) also need to occupy addresses in the address range. These data addresses are used by the module for functional control and are generally not directly connected to external signals.
CP243-2 (AS-Interface module) uses IB/QB as status and control bytes, while AI and AQ are used for AS-Interface slave address mapping.
22. How is the compatibility of Step7-Micro/WIN?
Currently, common versions of Micro/WIN include V4.0 and V3.2. Older versions, such as V2.1, are no longer of practical value except for converting old project files.
Project files generated by different versions of Micro/WIN are different. Higher versions of Micro/WIN can be backward compatible with project files generated by lower version software; lower version software cannot open higher version files.
Saved project files: Users are advised to always use the latest version; the current latest version is Step7-Micro/WIN V4.0 SP1.
23. How to set communication port parameters?
By default, the communication port of the S7-200 CPU is in PPI slave mode, with an address of 2 and a communication rate of 9.6K.
To change the address or communication rate of the communication port, it must be set in the system block’s Communication Ports tab, and then the system block must be downloaded to the CPU for the new settings to take effect.
24. How to set communication port parameters to improve network performance?
Assuming there are stations 2 and 10 as master stations in a network, with the highest address of station 10 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 from 11 to the highest station address of 15, including stations 0 and 1.
Master stations in network communication will pass tokens to control communication activities on the entire network separately. All master stations on the network will not join the token passing ring at the same time, so a master station holding the token must periodically check if any new master stations have joined at higher addresses. The refresh factor refers to how many times the token is obtained before checking for new master stations at higher addresses.
If station 2 is set with an address gap factor of 3, it will check one address in the address gap during the third time it obtains the token to see if there are any new master stations.
Setting larger factors will improve network performance (because unnecessary station checks are reduced) but will affect the speed at which new master stations can join. The following settings will improve network performance:
1) Set the highest address closest to the actual highest station address;
2) Arrange all master station addresses consecutively, so that new master station detection in the address gap is no longer necessary.
25. How to set data retention functionality?
Data retention settings define how the CPU handles data retention tasks in various data areas. The selected data areas in the data retention settings are those whose data content is to be ‘retained’. ‘Retained’ means whether the content of the data area remains in the state before power-off when the CPU is powered back on after being powered off. The data retention functionality set here is achieved through the following methods:
The data retention functionality set here relies on the built-in supercapacitor of the CPU. After the supercapacitor discharges, if an external battery (or clock/battery card for CPU221/222) is installed, the battery card will continue to power the data retention until the discharge is complete, and the data will be automatically written into the corresponding EEPROM data area before power-off (if MB0-MB13 is set to retain).
26. What is the relationship between data retention settings and EEPROM?
1) If the storage units in the range of MB0-MB13, a total of 14 bytes, are set to ‘retain’, the CPU will automatically write their contents to the corresponding area of the EEPROM when powered off, and upon re-powering, the contents of the EEPROM will overwrite these storage areas;
2) If the range of other data areas is set to ‘not retain’, the CPU will copy the values from the EEPROM to the corresponding addresses upon re-powering;
3) If the range of data areas is set to ‘retain’, if the built-in supercapacitor (plus battery card) fails to retain the data, the contents of the EEPROM will overwrite the corresponding data areas; otherwise, it will not overwrite.
27. What types of passwords can be set?
In the system block, set the CPU password to restrict user access to the CPU. Passwords can be set in levels, granting different permissions to other personnel.
28. Why can’t I see that the password has taken effect after setting the CPU password?
After setting the CPU password in the system block and downloading it, since you still maintain the communication connection between Micro/WIN and the CPU, the CPU will not protect the Micro/WIN that set the password.
To verify whether the password has taken effect, you can:
1) Stop communication between Micro/WIN and the CPU for more than a minute;
2) Close the Micro/WIN program and reopen it;
3) Power off the CPU and then power it back on;
29. Do digital/analog signals have a freeze function?
The output table for digital/analog signals specifies how the digital output points or analog output channels operate when the CPU is in STOP state.
This function is very important for some devices that must maintain operation, such as brakes or some critical valves that cannot stop during the debugging of the Siemens PLC, and must be set in the output table of the system block.
Digital:
By selecting ‘Freeze output in last state’, the last state is frozen, so when the CPU enters STOP state, the digital output points maintain the state before stopping (1 remains 1, 0 remains 0); if not selected, the selected output points will remain ON (1), and unselected points will be 0.
Analog:
By selecting ‘Freeze output in last state’, the last state is frozen, so when the CPU enters STOP state, the analog output channels maintain the state before stopping, and the table below does not take effect. If not selected, the output values of the analog output channels are specified in the table below when the CPU enters STOP state.
30. What is the function of the digital input filter, and how should it be set?
You can select different input filtering times for the digital input points on the CPU. If the input signal has interference or noise, you can adjust the input filtering time to filter out the interference to avoid malfunctions. The filtering time can be selected in the range of 0.20~12.8ms. If the filtering time is set to 6.40ms, the CPU will ignore digital input signal levels (high or low) that last less than 6.4ms; only those lasting longer than 6.4ms may be recognized.
Additionally: Input points that support high-speed counter functions are not subject to this filtering time constraint when the corresponding function is enabled. The filtering settings are effective for refreshing the input image area, digital input interrupts, and pulse capture functions.
31. What effect does analog filtering have?
Generally, selecting the analog filtering function of the Siemens S7-200 PLC eliminates the need to write a separate user filtering program.
If analog filtering is selected for a certain channel, the CPU will automatically read the analog input value before each program scanning cycle, and this value will be the filtered value, which is the average of the number of samples set. The parameter settings for analog signals (number of samples and dead zone values) are effective for all analog signal input channels.
If filtering is not applied to a certain channel, the CPU will not read the average filtered value at the start of the program scanning cycle but will only read the actual value at the time when the user program accesses this analog channel.
32. How to set the dead zone value for analog filtering?
The dead zone value defines the range of values used to calculate the average of the analog value.
If all sampled values are within this range, the average value set by the number of samples will be calculated; if the current latest sampled value exceeds the upper or lower limit of the dead zone, that value will be immediately adopted as the new current value and will serve as the starting point for future average calculations.
This allows the filter to respond quickly to large changes in the analog value. Setting the dead zone value to 0 disables the dead zone function, meaning all values are averaged regardless of how large the changes are. For fast response requirements, do not set the dead zone value to 0 but set it to the maximum expected disturbance value (320 for full scale 32000 is 1%).
33. What should be noted when setting analog filtering?
1) Use a filter for slowly changing analog inputs to suppress fluctuations;
2) Use a smaller number of samples and dead zone values for rapidly changing analog inputs to speed up response;
3) Do not use filters for rapidly changing analog values;
4) If using analog signals to transmit digital signals or using thermistors (EM231RTD), thermocouples (EM231TC), or AS-Interface (CP243-2) modules, filters should not be used;
34. How to make the monitoring response in Micro/WIN faster?
You can set the background communication time, which specifies the percentage of the entire program scanning cycle that the communication time of Micro/WIN and the CPU used for ‘Run mode programming’ and program/data monitoring occupies. Increasing this time can increase the opportunities for communication during monitoring in Micro/WIN, making the response feel faster, but it will also lengthen the program scanning time.
35. Can the indicator lights on the CPU be customized?
Yes, user-defined indicator lights can be set.
The LED indicator lights (SF/DIAG) of version 23 CPU can display two colors (red/yellow). The red light indicates SF (system fault), while the yellow DIAG indicator light can be customized by the user.
User-defined LED indicator lights 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 mutually exclusive. If both SF and DIAG indicators occur simultaneously, the red and yellow lights will flash alternately.
36. Can I use the entire program storage area at any time?
The new feature (runtime programming) of version 23 CPU requires some program storage space. If you want to utilize the entire program storage area, for certain CPU models, you need to disable the ‘Run mode programming’ feature.
37. What if I forget the password? How can I access a password-protected CPU?
Even if the CPU is password protected, you can use the following functions without restrictions:
1) Read and write user data www.plcs.cn
2) Start and stop the CPU
3) Read and set the real-time clock
If the password is unknown, 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 to reload the program. Executing the clear CPU instruction 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 also not change. After clearing the password, the original program in the CPU will no longer exist.
To clear the password, you can operate in one of the following three ways:
1) In Micro/WIN, select the menu ‘PLC>Clear’, select all three blocks, and press ‘OK’ to confirm.
2) Another method is to use the program ‘wipeout.exe’ to restore the default settings of the CPU. This program can be found on the STEP7-Micro/WIN installation CD.
3) Additionally, you can insert an external storage card containing an unencrypted program into the CPU, and upon powering on, this program will automatically load into the CPU and overwrite the original password-protected program. The CPU can then be accessed freely.
39. Can I still use the POU after encryption?
POU refers to Program Organization Units, including the main program (OB1), subroutines, and interrupt service programs in the S7-200 project file.
POUs can be encrypted individually, and the encrypted POU will display a lock mark, preventing viewing of the program content. Once the program is downloaded to the CPU, it remains encrypted even after being uploaded.
Siemens provides library instructions and subroutines generated by the instruction wizard in the Micro/WIN programming software, all of which are encrypted. Encryption does not hinder their use.
40. Can I encrypt the entire project file?
Using Step7-Micro/WIN V4.0 or higher, users can encrypt the entire project file so that those who do not know the password cannot open it.
In the ‘File’ menu of Micro/WIN, use the ‘Set Password’ command to enter a password of up to 16 characters for the project file.
The password can be a combination of letters and numbers, with case sensitivity.
41. How to open project files created by older versions of Micro/WIN?
You can find the installation software for version V2.1 of Micro/WIN in the ‘Old Releases’ folder on the genuine STEP7 Micro/WIN software CD. This version of Micro/WIN can open project files created by older versions. By using it as a bridge, you can save the older version’s software and then open it in the latest version of STEP7 Micro/WIN software.
Note: If you find that some networks are displayed as red invalid after opening, it may be due to the PLC model being too low or the version being too old. In this case, you can choose a higher model or a newer version of the CPU. For example, in the command menu under PLC>Type, change CPU222 to CPU224.
42. How to know the size of the program I compiled?
After executing PLC>Compile in the command menu of Micro/WIN, you can find the size of the program you compiled and the size of the data blocks occupied in the message output window at the bottom of Micro/WIN.
43. What to do if there is a compilation error?
If there is an error after compilation, the program cannot be downloaded to the CPU. You can check the error in the lower window of Micro/WIN, double-click the error to enter the program at the location of the error, and modify it according to the requirements in the system manual.
44. How to know the scan time of the program I compiled?
After the program has run once, you can view the scan time of the program in the command menu of Micro/WIN under PLC>Information.
45. How to check if the program address space I used is reused?
After compiling the program, you can click the Cross Reference button in the view browser to enter and see detailed cross-reference information about the elements used in the program and the usage of bytes and bits. You can directly click that address in the cross-reference to enter the program at that address.
46. Why is the instruction function block in red during online monitoring in the program block?
If you find that there are red instruction function blocks during online monitoring in the program editor, it indicates that there is an error or problem. You can find the error leading to ENO=0 in the system manual. If it is a ‘non-fatal’ fault, you can check the error type in the PLC>Information dialog in the menu.
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., turning red during runtime is most likely due to multiple calls during the execution of the instruction or the communication port being busy at the time.
47. How to use high-speed input and output on S7-200?
The high-speed input and output terminals on the S7-200 CPU have the same wiring as ordinary digital I/O. However, high-speed pulse outputs must use a DC transistor output type CPU (i.e., DC/DC/DC type).
48. Can NPN/PNP output rotary encoders (and other sensors) be connected to the S7-200 CPU?
Both can be connected. The digital inputs on the S7-200 CPU and expansion modules can connect to source-type or sink-type sensor outputs, provided the common terminal wiring is appropriately changed (whether the power L+ is connected to the input common terminal or the power M is connected to the common terminal).
49. Can S7-200 use two-wire digital (switching) sensors?
Yes, but it must ensure that the static operating current of the sensor (leakage current) is less than 1mA. Siemens has related products, such as proximity switches (BERO) for PLCs.
50. Does S7-200 have modules with reusable input/output points?
The digital and analog input/output points of S7-200 cannot be reused (i.e., they cannot be both input and output).
51. Can the high-speed input and output of CPU224XP reach 100K or 200K?
The new CPU224XP product supports higher speeds for the two high-speed inputs. When used as single-phase pulse inputs, it can reach 200KHz; when used as dual-phase 90° quadrature pulse inputs, the speed can reach 100KHz.
The two high-speed digital output rates of CPU224XP can reach 100KHz.
52. Is the high-speed input (I0.3/4/5) of CPU224XP a 5VDC signal, can other input points accept a 24VDC signal?
Yes. Just connect the common terminal of the two signal power supplies to the 1M terminal. These two signals must be either both sink-type or source-type input signals.
53. If the high-speed output points Q0.0 and Q0.1 connect to a 5V power supply, can other points like Q0.2/3/4 connect to a 24V voltage?
No. They must be connected in groups with the same voltage level.
54. Why are there analog signals that cannot be filtered?
Due to the principle of the analog conversion chip on the CPU224XP body being different from that of the expansion analog module, filtering does not need to be selected.
55. What is unipolarity and bipolarity?
Bipolarity means that the signal changes through ‘zero’, while unipolarity does not. Since the conversion of analog to digital is a signed integer, the values of bipolar signals can be negative. In S7-200, the value range for unipolar analog input/output signals is 0-32000; for bipolar analog signals, it is -32000 to +32000.
56. How should analog signals be converted to the desired engineering value?
Analog input/output can be converted using the following general conversion formula:
Ov=【(Osh-Osl)*(Iv-Isl)/(Ish-Isl)】+Osl
Where: www.plcs.cn
Ov: Conversion result
Iv: Conversion object
Osh: Upper limit of conversion result
Osl: Lower limit of conversion result
Ish: Upper limit of the conversion object
Isl: Lower limit of the conversion object
57. What is the accuracy of the S7-200 analog input signal?
The analog input module has two parameters that are easily confused:
1) The resolution of the analog conversion;
2) The accuracy (error) of the analog conversion;
Resolution is the conversion precision of the A/D analog conversion chip, which refers to how many bits are used to represent the analog value. The conversion resolution of the S7-200 analog module is 12 bits, meaning the smallest unit reflecting changes in the analog value is 1/4096 of full scale.
The accuracy of analog conversion depends not only on the resolution of the A/D conversion but also on the influence of the peripheral circuits of the conversion chip. In practical applications, the input analog signal may fluctuate, have noise, and be disturbed, and the internal analog circuits may also produce noise and drift, all of which can affect the final accuracy of the conversion. These factors can cause errors larger than the conversion errors of the A/D chip.
58. Why are analog values often unstable and fluctuate significantly?
This may be due to the following reasons:
1) You may be using a self-powered or isolated sensor power supply, where the two power supplies are not connected, meaning that the power ground of the analog input module and the signal ground of the sensor are not connected. This would generate a high common mode voltage that fluctuates significantly, affecting the analog input value.
2) Another reason could be that the wiring of the analog input module is too long or poorly insulated.
Solutions include:
1) Connect the negative end of the sensor input to the common M terminal of the module to compensate for this kind of fluctuation. (But be sure to ensure that this is the only connection between the two power supply systems.)
The background is: the internal analog input module is not isolated; the common mode voltage should not exceed 12V; the common mode rejection ratio for 60Hz interference signals is 40dB.
2) Use the analog input filter.
59. Why is the SF red light on the EM231 module flashing?
The SF red light flashes for two reasons: the module’s internal software detects an external thermistor disconnection, or the input exceeds the range. Since the above detection is shared by two input channels, the SF light will inevitably flash when only one channel is connected to the thermistor. The solution is to connect a 100 Ohm resistor in the same wiring manner to the empty channel; or to connect all lead wires of the connected thermistor one by one to the empty channel.
60. What are the forward calibration and reverse calibration?
The forward calibration value is 3276.7 degrees (Fahrenheit or Celsius), while the reverse calibration value is -3276.8 degrees. If a disconnection or input exceeds the range is detected, the corresponding channel’s value is automatically set to the above calibration values.
61. How to set the type on the DIP switch if the technical parameters of the thermistor are unclear?
You should try to clarify the parameters of the thermistor. Otherwise, you can use the default settings.
62. Can EM235 be used for temperature measurement with thermistors?
EM235 is not designed to connect with thermistors for temperature measurement; forcing its use may lead to problems. It is recommended to use the EM231RTD module.
63. Do the S7-200 analog input/output modules have signal isolation?
No isolation. If isolation is needed in the user’s system, a separate signal isolator device must be purchased.
64. How far can analog signal transmission go?
Voltage-type analog signals, due to the high input resistance (10 megaohms for S7-200 analog modules), are easily affected by interference, so discussing the transmission distance of voltage signals is not meaningful. Generally, voltage signals are used for potentiometer settings within the control cabinet or in very close proximity to good electromagnetic environments.
Current-type signals are not easily affected by electromagnetic interference along the transmission line and are widely used in industrial sites.
Current signals can be transmitted over much greater distances than voltage signals. Theoretically, the transmission distance of current signals is limited by the following factors:
1) The load capacity of the signal output end, expressed in ohms (e.g., 700Ω);
2) The input resistance of the signal input end;
3) The static resistance value of the transmission line (for a round trip, it is a double line);
The load capacity of the signal output end must be greater than the sum of the input resistance of the signal input end and the resistance of the transmission line. Of course, the actual situation will not completely match the ideal calculation results; excessive transmission distances will cause signal attenuation and introduce interference.
65. What are the input/output impedance specifications of the S7-200 analog modules?
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, but why is the signal input light not on?
The analog module’s casing is designed and manufactured according to a general format, and there is actually no signal input light for the analog module. Any light window without printed markings is useless and empty.
67. Why do the lowest three digits of the analog value have non-zero changes?
The analog conversion accuracy is 12 bits, but the module shifts the converted digital value up by three digits. If this channel is set to use analog filtering, the current value is the average of several samples; if analog filtering is disabled, the lowest three digits are all zero.
68. Does EM231TC require compensation wires?
EM231TC can be set to implement cold junction compensation by the module, but compensation wires are still needed for the free end compensation of thermocouples.
69. Why is the SF light on the EM231TC module flashing?
If disconnection detection is selected, it may indicate a disconnection. You should short-circuit the unused channel or connect it in parallel to the adjacent actual wiring channel. Alternatively, the input may exceed the range.
70. What to do if the M area data is insufficient?
Some users habitually use the M area as an intermediate address, but the M area address space in the S7-200 CPU is small, only 32 bytes, which is often insufficient. The S7-200 CPU 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, allowing access to V area data by bits, bytes, words, or double words. For example: V10.1, VB20, VW100, VD200, etc.
71. How do I know the addressing of integrated I/O and extended I/O of S7-200 CPU?
When programming the S7-200, there is no need to configure I/O addresses.
The I/O addresses on the S7-200 expansion modules are arranged in increasing order based on their distance from the CPU. The closer to the CPU, the smaller the address number.
Between modules, the addresses of digital signals always increase in increments of 8 bits (1 byte). If the physical input points on the CPU do not fully occupy a byte, the remaining unused bits cannot be allocated to the same type of signal on subsequent modules.
Analog output modules always occupy the output address of two channels. Even if some modules (EM235) only have one actual output channel, they still occupy the addresses of two channels. When connecting the programming computer to the CPU, you can view the actual I/O address allocation of the CPU and expansion modules using the Micro/WIN menu command ‘PLC>Information’.
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