javascript转plc串口modbus rtu问题

Question

I'm trying to establish a communication with a PLC via serial port Modbus RTU using javasccript, I found some libraries online, but I don't know how to manipulate the functions of this libraries in order to write and read data from specific register in the plc, I also guess that I have to do a format changing of data frame from text to hexa. I'm stuck with this for more than a week if you have a lead or explanations plz help me

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Answer 1

As far as I know, because js cannot directly bypass the browser to obtain the communication permission of the serial port, it cannot directly access the PLC through the browser.

But you can try the LECPServer middleware, which is also developed based on Javascript, and can expose the Modbus address for reading and writing through HTTP POST.

BTW, LECPServer is an open source project.

Answer 2

I would recommend get it working between your code and a Modbus emulator, not the PLC. Once you get YOUR code working, then you can work on the PLC.

What's cool about using an emulator initially, you could also use IT to be the Master to talk to the PLC (similar to how you got your code working as the Master to the emulator as the Slave). The emulator can provide good understanding of both sides before getting the 2 ends to talk to themselves (ie the actual Master and the actual Slave(s)). Emulators can also provide good diagnostic details. That's one issue when diagnosing comm - which side is causing the problem (or is it a physical layer issue, eg wired wrong). A good emulator can help diagnose issues before doing "real" comm.