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34 classic questions and answers about PLC

Last Updated on 2021-07-20 Hits:6151
Siemens S7-200 PLC

1. Why use PC/PPI interface?

Because the S7-200CPU uses RS485, and the PC's COM port uses RS232, the electrical specifications of the two are not compatible, and an intermediate circuit is required for matching. PC/PPI is actually a matching cable of RS485/RS232.

2.What are the advantages of transistor output and relay output?

Transistor cannot carry AC220V AC load, it can only carry low voltage DC. Poor ability to resist overload and overvoltage. But it can output high frequency, suitable for high frequency output occasions, such as pulse control.

The relay can carry AC220V and DC loads. However, due to the characteristics of the relay itself, it cannot output high frequency. At the same time, the on-off life of the relay is generally about 100,000 times. Therefore, transistors are also suitable for frequent on and off occasions.

3. How far is 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 a guaranteed communication distance under network conditions that meet the specifications. Where the distance exceeds 50m, a repeater should be added. Adding a repeater can extend the communication network by 50 meters. If you add a pair of repeaters, and there is no S7-200 CPU station between them (there can be EM277), the distance between the 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 a repeater. Siemens cannot guarantee the success of such communication.

4. How to set the communication port parameters?

By default, the communication port of S7-200 CPU is in PPI slave mode, the address is 2, and the communication speed is 9.6K. To change the address or communication speed of the communication port, it must be set in the communication port option card in the system block , And then download the system block to the CPU, the new setting can take effect.

5. What should I do if the M area address is not enough?

Some users are accustomed to using M area as an intermediate address, but the address space of M area in S7-200CPU is very small, only 32 bytes, which is often not enough. And 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. The data of the V area can be accessed in bits, bytes, words or double words. Example: V10.1, VB20, VW100, VD200 and so on.

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

7. Which communication protocols supported by S7-200 are open and which are not open?

PPI agreement: Siemens internal agreement, not open

MPI agreement: Siemens internal agreement, not open

S7 agreement: Siemens internal agreement, not open

PROFIBUS-DP protocol: standard protocol, open

USS protocol: the universal serial communication protocol of Siemens transmission devices, please refer to the manual of the corresponding transmission device for public details

MODBUS-RTU (slave station): 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 DC transistor output type CPU (ie, DC/DC/DC type).

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

It's okay. The digital inputs on the S7-200 CPU and the expansion module can be connected to source or sink sensor outputs. When connecting, just change the connection of the common terminals accordingly.

10. The method of mixing NPN and PNP sensors into S7-200 PLC

Everyone knows that the general public terminal of common Japanese PLCs such as Mitsubishi, OMRON, etc. usually uses NPN sensors when signal access. The common terminal of European PLC is generally -, and most of them use PNP sensor to connect the signal. Such as S7-200/300, etc., when S7-200 PLC is used as a system, the sensors provided are PNP and NPN, so how to solve the problem?

Method 1: NPN sensor uses intermediate relay to transfer

Method 2: When designing, you generally connect the input end [M] of 200PLC to 24V-, in fact, 200PLC can also introduce-signal input, connect 1M to 24V, I0.0-0.7 to NPN sensor, and 2M Connect 24V-, connect the PNP sensor to I1.0-1.7 uniformly so that the purpose of mixing NPN&PNP sensor into PLC can be achieved. The reason is very simple. 200PLC supports two kinds of signal access, and the internal bidirectional diode adopts photoelectric isolation for signal transmission.

11. How does the high-speed counter occupy output points?

The high-speed counter occupies the digital input point on the CPU according to the defined working mode. Each counter occupies a fixed input point according to its working mode. Input points not used in a certain mode can still be used as ordinary input points; input points occupied by the counter (such as external reset) can still be accessed in the user program.

12. Why can't the high-speed counter work normally?

In the program, use the first scan memory bit SM0.1 to call the HDEF instruction, and it can only be called once. If it is called with SM0.0 or the HDEF instruction is executed for the second time, it will cause a running error, 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 to read the current value, or enter 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?

Select the 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 to reset, that is, set the high-speed counter to the updateable initial value and set the initial value to 0. After the HSC instruction is executed, the high counter is reset to 0.

15. Why doesn't it work when assigning the initial value and preset value to the high-speed counter, or the effect is unexpected?

The high-speed counter can be changed during initialization or running, such as initial value and preset value. The steps should be:

Set the update option of the control byte. For which setting data needs to be updated, set the corresponding control bit in the control byte (set to "1"); for settings that do not need to be changed, the corresponding control bit cannot be set.

Then send the required value to the initial value and preset value control register.

Execute HSC instructions

Mitsubishi FX series PLC

1. What is the difference between FX3G and FX3GA?

1. FX3GA: The input and output terminal blocks are not removable; FX3G is removable, so there is no need to remove the wires when replacing the PLC;

2. FX3GA: Cannot be equipped with optional batteries; FX3G can be equipped with optional batteries;

3.FX3GA: Only one BD communication board (single-channel expansion) can be expanded at most, and 2 BD boards (dual-channel expansion) can be expanded for FX3G 40 points or more;

4. FX3GA: domestic assembly; FX3G is completely produced abroad.

2. What adjustments and changes need to be made when FX3U replaces FX2N and FX3UC replaces FX2NC?

1. FX3U(C) series correspond to FX2N(C) series, their dimensions are the same, no need to change the electrical cabinet installation;

2. The size and position of the FX3U(C) series input/output terminal blocks are the same. There is no need to remove the wires to replace the PLC. You can directly remove the old terminal block and replace it with the new one;

3. The wiring of FX3UC and FX2NC are exactly the same; FX3U will have one more input signal type selection S/S terminal than FX2N, and this terminal needs to be short-circuited with 24V, while the corresponding position of FX2N was vacant before.

3. What are the FX series PLC download lines and their differences?

FX series download lines include FX-USB-AW (original), USB-SC09-FX, USB-SC09, SC09

1. FX-USB-AW is Mitsubishi's original FX dedicated download cable, the computer side is a USB interface, which is reliable and convenient to use;

2. USB-SC09-FX is a domestic download cable imitating Mitsubishi FX-USB-AW, and the computer side is a USB interface, which is easy to use;

3. USB-SC09 is a domestic download cable, which can be used for FX series PLC and A series PLC. The computer side has a USB interface, which is easy to use;

4. SC09 is mainly for old-fashioned computers (with RS232C 9-pin interface), the computer side is RS232 interface, now the computer needs a USB to RS232 transfer interface to connect and use, such as USB-RS232 2.0

4. After FX2N PLC is replaced with FX3U, do the original modules need to be replaced?

1. After FX2N PLC is replaced with FX3U, the original function expansion board (BD board for communication and special adapters such as FX2N-485BD, FX2NC-232ADP) need to be replaced with the corresponding function expansion board of FX3U series;

2. Expansion equipment (input and output modules, special function modules such as FX2N-16EX, FX2N-2AD) do not need to be replaced and can be used directly.

5. Does the FX0N-65EC extension cable have to be equipped with an FX2N-CNV-BC adapter before it can be connected and used?

When FX0N-65EC is connected to FX2N-32ER/ET, FX2N-48ER/ET, there is no need to add FX2N-CNV-BC

6. What is the difference between FX2N series PLC model suffix ES/UL and -001?

1. The suffix ES/UL is the European version, and -001 is the Chinese version.

2.-ES/UL series input mode can select source type and sink type input mode through S/S terminal. The input mode of -001 series is fixed as sink input.

7. Does FX0N-65EC have to add a FX2N-CNV-BC adapter to connect and use?

When FX0N-65EC is connected to FX2N-32ER/T.FX2N-48ER/T, FX2N-CNV-BC is not needed; other modules need to be added

8. How many axes can be controlled by FX1S, FX1N, FX2N, FX3G, FX3GA, FX3U(C) body with high-speed pulse output points?

1. FX1S, FX1N can control independent 2 axes, the maximum output is 100KHZ pulse train

2. FX1NC can control independent 2 axes, the maximum output is 10KHZ pulse train

3. FX2N can control independent 2 axes, the maximum output is 20KHZ pulse train

4. FX3G, FX3GA 14/24 points can control independent 2 axes, 40/60 points maximum output 100KHZ pulse train

5. FX3U, FX3UC can control independent 3 axis, maximum output 100KHZ pulse train

9. Can FX1N PLC be extended to connect FX2N input and output modules, analog modules, positioning modules, etc.?

The PLC of FX1N can be expanded to connect the expansion module of FX2N. Such as FX2N-16EX, FX2N-16EYR, FX2N-4AD, FX2N-10GM, FX2N-20GM, etc.

10. What should I do if the power LED turns off or flickers when the FX series PLC is running?

Remove the wiring of the [24] (working power supply) terminal on the PLC to confirm whether it is back to normal.

1. If it returns to normal, it may be due to the short-circuit of the load or the excessive load current which caused the overload of the working power supply and the protection function was activated.

2. If it fails to return to normal, please contact professional technicians for repairs.

11. What is the difference between FX3U-ENET-ADP and FX3U-ENET-L, can they replace each other?

FX3U-ENET-L is an Ethernet module, and FX3U-ENET-ADP is an Ethernet port adapter; -L is powerful and can replace ADP; if it can be replaced, it should be judged by referring to whether the function used can be satisfied. .

1. Both ENET-ADP and ENET-L have the MELSOFT connection function, which is connected to the man-machine interface through the Ethernet port, such as Mitsubishi, Weilun touch screen

2. Both ENET-ADP and ENET-L have MC protocol (ie Mitsubishi PLC special protocol). This function is used for upper industrial computer to read, write and control PLC using MC protocol.

3. The ENET-L module has a large number of buffers, with the function of buffer sending and receiving (1024 words/time), which can be used as the master station to communicate with third-party equipment such as instruments, etc., ENET-ADP is just a communication expansion port, without this function

12. Can FX3U-1PG replace FX-1PG-E?

1. FX3U-1PG is an upgraded version of FX2N-1PG/FX-1PG-E; the performance improvement pulse output can reach 200KHZ, and their programs can be universal

2. FX3U-1PG can only be used on the FX3U PLC host. After replacement, determine whether the host is an FX3U PLC

13. How does Mitsubishi FX3G series PLC communicate with the barcode scanner? How to read barcode in Mitsubishi 3G PLC?

You can use the RS command. If you want to read the barcode of the scanner, you must know its communication protocol, whether it is a dedicated or MODBUS protocol, and then you must know the address of the barcode data storage area and the data type and size before you can use the RS command. To read the data of the bar code storage area address of this station address, if it is two words, the feedback data only needs to display the type of these two words.

14. I am going to try to build a control system that controls a stepper motor, four cylinders, and four proximity switch inputs. What type of PLC is cost-effective to choose?

This simple system can be of any model, you can choose FX2N FX1S FX3U, mainly transistor output is just fine.

15. How to control the servo motor with PLC?

Calculate the distance per revolution according to the resolution of the encoder, and then use pulse control.

16. What are the parameters corresponding to the 4-20ma current output of the 4ad module of Mitsubishi FX3U series PLC?


17. Does the PLC positioning module need to be used with servo?

It must cooperate with the servo and output commands to the servo amplifier based on the parameter positioning data.

18. Which is better to choose Mitsubishi FX3GA series PLC or Mitsubishi FX3U series PLC?

Choose according to the specific purpose, the software is the same.

19. What are the differences in programming of these series of Mitsubishi PLCs?

Basically there is no difference, most commands of FX2NC and FX3U are common.

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