How to get data from RS-485 connected by USB

Question

I need to read data from some Data Aggregator Device over RS-485 interface by USB port (USB-to-RS-485 converter used) for the Desktop App on JavaFX, so I have stacked on checking connection and getting proper values.

Data Aggregator Device collects information about produced current, power etc. from PV-module (Solar Energy). This device has the only RS-485 port. As I read from a documentation, it uses Modbus RTU protocol (with settings: 9600-8N1).

To be honest, It's my first experience with working on COM-ports and Devices. That is why I don't know even am I correctly connect pins. I read a lot of things related with RS-485, Modbus. But, still, I am on the first steps.

So, I use simple UTP cable to connect. And what I have done:

1. This is how I connect USB-to-RS485 converter
2. And this is about Data Aggregator Device

Windows found virtual "COM3" port, after connecting USB to PC. Then I try to check connection with Terminal 1.9 by Bray. I try to send something. And as receiving messages it sent me some data also. However, it doesn't mean that everything connected well. Also terminal shows me some FRAME ERROR. I know, that Modbus protocol based on "master-slave" scheme. Therefore, PC as a client is "Master" and device as a server is "Slave". I have to send some request to the slave to get some expected response. But, how?! Please, check my connections! I don't know what to do next.

Answer 1

RS-485 is differential and requires just two wires, you can safely remove the ground wire (GND). Then, connect the wires to R+ and R- on your USB-to-RS485 converter

Done that, may I ask you how you tried to send something? Modbus requires a final byte, the CRC which is hard to determine by hand. Did you create a valid modbus packet?

A valid modbus packet requires a receiver address, the payload length, the data itself and the final CRC

Answer 2

I don't know about java, but if you do have specification of devices protocol, you can surely use terminal program to send message to modbus and read messages back. That way you will have protocol tested and it shouldn't be to hard to make custom software later. But until you can send message and get back meaningful message using terminal software I recommend you to stick to it.

This youtube video will give basic knowledge needed about modbus RTU packet format: https://www.youtube.com/watch?v=OvRD2UvrHjE

The most difficult part is calculating CRC every time. But luckily you can download Docklight terminal. It actually has an option to calculate CRC automatically for modbus and add it at last positions of the packet. You can download free version at their homepage.

docklight.de

It is very simple to use. Same as Bray terminal you used.. And here is example of how to set up modbus CRC calculation for every package. https://docklight.de/manual/sendingmodbuscommandswithc.htm

I know they also have great support. Once you successfully send message to device and get answer, things will get much easier because you will understand how protocol actually works. Modbus RTU is very mature protocol, but it is still used because many existing systems use it and it is very simple to add it to some device. All device needs is Serial port (UART) to connect to modbus. So it is not that hard to understand it and use it. Just check out explanation video and terminal specifically adopted for modbus that I gave links in this message.