MODBUS communication debugging issues
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I had no prior experience with communication systems. Now, when trying to debug the communication, there are issues; the PLC says it’s fine, and the DCS also claims there’s no problem with it. So I’m seeking advice from experts: how is MODBUS communication usually debugged? What is the typical output voltage in volts? Also, are terminal resistors needed? Our PLC uses a CP341 MODBUS Slave from S7-300, and the DCS is a system from Zhejiang University Control System.
Debugging software can be used to communicate with each of the two manufacturers separately, to determine which manufacturer is at fault
Find a communication testing software, similar to COM Explorer. Siemens S7 communication also allows for the use of dedicated network cards on the operation station
Modbus communication is actually quite simple; as long as the address and data type are set correctly, there should be no problems. Take a look to see if there’s an issue with the connection cables Are the communication settings correct? 2-wire system? 4-wire system? Baud rate? Data bits, stop bits, etc.
Match the settings on both sides to ensure they are the same. Also, pay attention to the polling time of the master station – it shouldn’t be too fast.
From the owner’s tone, it seems like they are a design representative on site who is overseeing the construction; there should be no problems and everything will work out, as that’s the principle. Just let them handle things on their own. Since both are manufacturers of control systems, there’s no need for you to act as an intermediary. Let them communicate with each other, identify the problems themselves, and find solutions on their own. All you need to do is bring them together； As for the specific methods for debugging, it can be done as mentioned on the second floor: find a debugging software that is acceptable to both parties, and try it on both sides. If communication between the two systems can be established in this way, then the connection will work… Check the Modbus protocol – whether it is Modbus RTU or some other variant like Modbus ASCII – and ensure that settings such as device addresses, data addresses, baud rate, start bit, and stop bit are all correct.
The central controller seems to support only MODBUS RTU and not ASCII. Seems so.
I get angry when I see the 6th floor. If you work in such a manner, then it’s entirely reasonable for the PLC or DCS to simply provide you with a test report and that’s it – regardless of whether communication takes place or not. After all, it’s two different systems; so why can you just ignore everything and do nothing about it?
This post was last edited by lyglygly on 2010-5-16 at 22:14. There’s no need to get angry; ah, I’ve encountered this problem as well. When communicating between the S7300 and PKS, we checked all the settings such as the DATA BIT, STOP BIT, FLOW CTRL, and OPERATION MODE of the NPORT5630, and everything seemed fine. In the end, it turned out that there was an issue with one of the ports on the NPORT5630 – replacing that port resolved the problem. . .
9# lyglygly, try using Modbus scan. .
I’ve encountered this situation several times; basically, as long as the settings and wiring on both sides are correct, the issue lies in the conversion of the addresses and data types that are read.