Timing program is disturbed after s7-200 communicates with Modscan - Database & Sql Blog Articles
Not long after people contacted plc, they just learned to use the modbus protocol to communicate with the host computer some time ago, and found problems in a few days. The problem is as follows: CPU224CN, doing a MODBUS slave, wrote a small flashing action program. The program is as shown. In the STEP software, the flashing cycle VW2 and the ON time VW4 are set, v1.0 is turned on, and then the PLC is adjusted to the RUN state, and the PLC can work normally, for example, Q0.0 is on for 3 seconds and black is on 3 seconds.
Then use the serial port to connect to the line for mudbus communication, and the computer communicates with the PLC with modscan32 software. The communication is normal and there is no abnormality on Modscan. However, the Q0.0 output lamp found an abnormality. For example, it turns out to be 3 seconds black and 3 seconds, now it is about one second, black 5 seconds or bright 5 seconds black. There are no changes to parameters such as VW2 and VW4, and normal data is read out on Modscan.
I would like to ask if the Modbus communication protocol will occupy or interfere with certain timers or refresh something that causes the subroutine to be interfered. According to my observation, each time modscan32 sends a signal to the PLC, the self-protection circuit of the subroutine in the PLC will be disconnected. Is MBUS_SLAVE other subprograms refreshed when called?
Main program
Subroutines A:
1. Is the Modbus communication protocol occupying or interfering with certain timers or refreshing something that causes the subroutine to be interfered?
There are many versions of the online modscan32 software. This type of software is just a test communication software and cannot be used for precise control. This kind of software is mostly compiled in high-level languages ​​such as VC and VB. Since the operating environment of the operating system of the computer and the background service are different, the operation of the software cannot be required to be real-time.
2. If you use the MIRCOWIN variable monitoring of the S7-200 or observe the indicator status of the output template output point of the S7-200, it is OK to judge the running result of the program.
3. If there are conditions, use WINCC or other official manufacturer's configuration software for monitoring. If the PLC is operating normally, these configuration software will definitely display normal.
4.Modscan software as the communication software for testing Modbus, only need to set the relevant communication parameters. After setting, the Modbus request is automatically issued according to the interval time (default is 1s), and the response of the slave station is accepted by the listening mode, and the software running platform There is no relationship, and it will not affect the operation of the PLC in the listening mode. At the same time, the software has been released to the present and has been tested for a long time, and reliability is unquestionable.
5. The Modscan software is very small in the province. Compared with the configuration software, the real-time performance is higher because only one slave's response is processed. The configuration software may complete the polling scan of multiple serial devices at the same time, and also need to complete the scanning of multiple serial ports or multiple communication cards, and complete the real-time processing of alarms, trends, responses to clients, etc. . Therefore, if it is not handled well in terms of multitasking, etc., real-time performance cannot be guaranteed. Therefore, the test with Modscan is normal, and the configuration software is not necessarily able to test normally.
It is recommended to check the PLC program. If you want to confirm the Modscan effect, it is very simple, disconnect, see if the output of Q0.0 is normal, if it is not normal, the PLC program problem; if it is normal, you need to check the interrupt of the Modbus slave. Program problem.
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