Modbus RS485 data length error


Hi all,

Got a problem with reading out some modbus values through RS485 from a Energy meter (RI-f200)

I'm using a USB to RS485 convertor connected to a raspberry.

below my setup (can only post one picture)

I have tried all the possible combinations (connections & settings) but I can't read something out of it.
is there anyone who has experience with this kind of setup?

thanks in advance,




I'll bite.

I looked at the PDF. Wow.

I'm not sure what the end resistor is doing.

I have had a very similar problem and this is not going to be nice what I am about to say, so please get ready.

1 -
If you can: get another USB-RS485 converter, so you have two of them.

2 -
Connect them to USB ports and open a program like a terminal which you use to talk to remote devices.
You will have two of these running. One on one device and one on the other.
Set them both to the same speed/stop and parity.
Or use two different machines if you can.

3 -
Connect #1's TX to #2's RX and #1's RX to #2's TX at the RS485 side of things. Basically a "null modem".
Type on one and see if you get that on the other program.

Though this is not 100% going to fix the problem, it will take you a long way to knowing what is going on.

Until you can prove that is working, you are really wasting time.
I got stuck with a USB-RS232 converter and wasted about 7 months because of a silly thing.

Good luck.

Oh, just to also check:
You did reverse the TX/RX lines? (You didn't put TX to TX and RX to RX)



It is a pity that you are not allowed to post more pictures. Perhaps you can try to send me additional pictures in a message (by clicking on my avatar and then on the mail button) so as I can share them here in the forum?

It would help to know if the Modbus node managed to connect with the Energy meter. There are a couple of different places in the configuration node where you can force the Modbus node to log errors and status. Can you possibly tick those option, retry to read values from the meter and send the pictures of the results? If yes it would be interesting to have a screenshot of the Node-RED log (as below):



Hi Guys,

thank you both for your prompt replies!
eventually I found the problem using a program "Cas Modbus scanner". I plugged the convertors (I have 2 types and 2 each) and used the discovery mode to find the right adresses, devices & settings. once figured out I switched back to node-red and it work almost immediately.

thanks again for replying, will certainly use this forum again to share the progress.