Practical PLC classic questions and answers summary

1. Siemens S7-200 PLC

1. Why use PC/PPI interface? Because S7-200CPU uses RS485, while PC's COM port uses RS232. The electrical specifications of the two are incompatible, so an intermediate circuit is needed to match them. PC/PPI is actually a matching cable of RS485/RS232.

2. What are the advantages of transistor output and relay output? Transistors cannot carry AC220V AC loads, but only low-voltage DC. They have poor resistance to overload and overvoltage. But they can output at high frequency, which is suitable for high-frequency output occasions, such as pulse control. Relays can carry AC220V and DC loads. But due to the characteristics of the relay itself, it cannot output at high frequency. At the same time, the life of the relay is generally about 100,000 times. Therefore, transistors are also suitable for occasions with frequent switching.

3. How far is the 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 distances 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 S7-200 CPU station between them (EM277 can be present), the distance between the repeaters can reach 1000 meters. Meeting the above requirements can achieve very reliable communication.

4. How to set the 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, you must set it in the communication port tab in the system block, and then download the system block to the CPU so that the new settings can take effect.

5. What to do if the M area address is not enough? 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. You can access the V area data by bit, byte, word or double word. For example: V10.1, VB20, VW100, VD200, etc.

6.What are the long-distance communication methods of S7-200?

RS-485 network communication: PPI, MPI, and PROFIBUS-DP protocols can all communicate on the RS-485 network. By adding a relay, the maximum distance can reach 9,600 meters.

Fiber optic communication: In addition to being resistant to interference and having a high rate, fiber optic communication also has the advantage of long communication distance. S7-200 products do not directly support fiber optic communication and require an additional fiber optic conversion module.

Telephone network: S7-200 supports telephone network communication through the EM241 audio modem module. EM241 requires the end of the communication to be a standard audio telephone line, regardless of the communication method between offices. Global communication can be carried out through EM241.

Wireless communication: The communication distance of S7-200 via radio stations depends on the frequency, power, antenna and other factors of the station; the communication distance of S7-200 via GSM network depends on the range of network service; the communication of S7-200 via infrared devices also depends on their specifications.

7.Which communication protocols supported by S7-200 are public and which are private?

PPI agreement: Siemens internal agreement, not public;

MPI agreement: Siemens internal agreement, not public;

S7 protocol: Siemens internal protocol, not public;

PROFIBUS-DP protocol: standard protocol, open;

USS protocol: Universal serial communication protocol for Siemens transmissions. For details, please refer to the manual of the corresponding transmission.

MODBUS-RTU (Slave): Public.

8. 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).

9. 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.

10. How to mix NPN and PNP sensors into S7-200 PLC Everyone knows that Japanese PLCs such as Mitsubishi and OMRON usually use NPN sensors when connecting signals. The common end of European PLCs is generally: Most of them use PNP sensors to connect signals. For example, S7-200/300, etc. When S7-200 PLC is used as a system, the sensors provided are PNP and NPN. How to solve the problem?

Method 1: NPN sensor uses an intermediate relay for switching.

Method 2: When designing, people usually connect the input terminal [M] of 200PLC to 24V-. In fact, 200PLC can also introduce - signal input, connect 1M to 24V, connect I0.0-0.7 to NPN sensor, connect 2M to 24V-, and connect PNP sensor to I1.0-1.7. In this way, the purpose of mixing NPN & PNP sensors into PLC can be achieved. The reason is very simple. 200PLC supports two signal accesses, and the internal bidirectional diode uses photoelectric isolation for signal transmission.

11. How do high-speed counters occupy output points? High-speed counters occupy digital input points on the CPU as needed according to the defined working mode. Each counter occupies fixed input points according to its working mode. Input points that are not used in a certain mode can still be used as ordinary input points; input points occupied by counters (such as external reset) can still be accessed in the user program.

12. Why can't the high-speed counter work properly? In the program, the first scan memory bit SM0.1 should be used to call the HDEF instruction, and it can only be called once. If SM0.0 is used to call or the HDEF instruction is executed for the second time, a running error will occur, and the counter setting when the HDEF instruction is executed for the first time cannot be changed.

13. How to address the high-speed counter? Why can't I read the current count value from SMDx? You can directly use HC0; HC1; HC2; HC3; HC4; HC5 to address different high-speed counters and read the current value, or you can input the above address in the status table to directly monitor the current value of the high-speed counter. SMDx does not store the current value. The count value of the high-speed counter is a 32-bit signed integer.

14. How to reset the high-speed counter to 0? Choose a high-speed counter with external reset mode. When the external reset input point signal is valid, the high-speed counter is reset to 0. You can also use the internal program reset, that is, set the high-speed counter to update the initial value, and set the initial value to 0. After executing the HSC instruction, the high-speed counter is reset to 0.

15. Why does it not work when assigning initial value and preset value to high-speed counter, or the effect is unexpected? High-speed counter can change settings such as initial value and preset value during initialization or operation. The operation steps should be:

Set the update option of the control byte. If you need to update the setting data, set the corresponding control bit in the control byte (set to "1"); if you do not need to change the setting, the corresponding control bit cannot be set;

Then the required values ​​are sent to the initial value and preset value control registers;

Execute HSC instructions.

2. Siemens S7-300PLC 1. How to avoid "communication failure" message when using CPU 315F and ET 200S? When using CPU S7 315F, ET 200S and fail-safe DI/DO modules, you will call the fail-safe program of OB35. Moreover, you have accepted the default setting values ​​for all monitoring times and are willing to receive the "communication failure" message. The default setting of OB 35 is 100 milliseconds. You have set the F monitoring time of the FI/O module to 100 milliseconds, so the I/O module must be addressed at least once every 100 milliseconds. However, since OB 35 is called only once every 100 milliseconds, a communication failure occurs. To ensure that the scan interval of OB35 and the F monitoring time are different, please make sure that the F monitoring time is greater than the scan interval of OB35. This problem occurs with S7 Distributed Safety up to V5.2 SP1 and 6ES7138-4FA00-0AB0, 6ES7138-4FB00-0AB0, 6ES7138-4CF00-0AB0. In the new modules, the F monitoring time is set to 150 ms.

2. What is the monitoring time of the S7-300 CPU on PROFIBUS when a DP slave is not available? When operating a PROFIBUS network with DP slaves on the PROFIBUS interface of the CPU, it is desirable to check during startup whether the expected configuration matches the actual configuration. Two different times are given on the Startup tab in the CPU Properties dialog box.

3. How to determine if there is an error in the power supply or buffer, such as a battery failure? If an error in the power supply (S7-400 only) or in the buffer triggers an event, the CPU operating system accesses OB81. After the error is corrected, access OB81 again. In the case of a battery failure, the S7-400 only accesses OB81 if the BATT.INDIC switch in the battery test is activated. If OB81 is not configured, the CPU does not enter the operating state STOP. If OB81 is not available, the CPU remains running when a power supply error occurs.

4. What should you pay attention to when assigning addresses to I/O modules (centralized or distributed) on an S7 CPU? Please note that the created data area (such as a double word) cannot be configured on the boundary of the process image, because in this data block, only the area below the boundary can be read into the process image, so it is impossible to access the data from the process image. Therefore, these configuration rules do not support this situation: for example, configure an input double word at address 254 of a process image with 256 bytes of input. If such addressing is absolutely necessary, the size of the process image must be adjusted accordingly (in the Properties of the CPU).