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I am trying to enable modbus RTU communication between 2 inverters.

Both are set up with unique IDs, 9600Hz, 1 stop bit, No parity bit.

I can communicate with 1 at a time, as shown below in the modscan tool screenshot (in the 1st red box, we have the poll followed by the (valid) response (response bytes highlighted black).

However, once I connect the second device, the response changes. In the second red box, we can see that an empty byte appears to be placed in front of the response bytes (followed by the ID of 10). The response is also shortened.

I opened the inverters up, and have tried placing a jumper pin on the rs485 pcb to see if that helps, but I get the second modscan response immediately when connected to one, as opposed to it happening when i connect the second device

I've tried searching up the pcb number 16-500791-01G(B) but I have not had much success.

Is this a stop bit/parity issue? or something else?

enter image description here

rs485 pcb

asa

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  • \$\begingroup\$ Hello and welcome. My first suspicion is always wiring, so could you please add your wiring to the question ... and also the model of the inverters and whatever device you have on the computer. \$\endgroup\$
    – jonathanjo
    Commented Jul 8 at 9:58
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    \$\begingroup\$ You don't show your wiring and termination. Also RS-485 is not a 2-wire bus, there are only 2 data wires but for proper communication the RS-485 ports must share a common ground reference, and if no other common ground reference exists, there needs to be a ground wire between devices as well. Do all your devices on the bus share a ground reference and are only terminated by two devices on both ends of the bus (no third device terminating, no termination on middle). \$\endgroup\$
    – Justme
    Commented Jul 8 at 10:08
  • \$\begingroup\$ "No parity bit." ... are you quite certain it's set up that way? "Even parity is required, other modes ( odd parity, no parity ) may also be used. In order to ensure a maximum compatibility with other products, it is recommended to support also No parity mode. The default parity mode must be even parity. Remark : the use of no parity requires 2 stop bits" (Standard 1.02, section 2.5.1) \$\endgroup\$
    – jonathanjo
    Commented Jul 8 at 10:42

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