https://forge.codesys.com/forge/talk/Engineering/thread/634cff6b25/2020-02-27T13:56:04.403000ZRecent posts to Modbus RTU Weirdness2020-02-27T13:56:04.403000Z2020-02-27T13:56:04.403000ZdFxhttps://forge.codesys.com/u/dfx/https://forge.codesys.com6849b6777963967ebc6055c6204c037d46437008

<div class="markdown_content"><p>read code 3/4 ?</p></div>

2020-02-27T13:56:04.192000Z2020-02-27T13:56:04.192000ZRoryLDMhttps://forge.codesys.com/u/roryldm/https://forge.codesys.comfd4d6b8471356c21788532bcef24bf8531b6b923

<div class="markdown_content"><p>Hi I was wondering if someone could shed some light on a bit of weirdness I'm seeing. I'll preface this by saying that I DO have experience with Modbus RTU. I use it to control Teco Westinghouse VFD's, Eaton VFD's, Davis Instrument weatherstations, and a few other odd bits here and there.</p> <p>To start off with a new VFD brand I'm trying to just read from a single register. I'm trying to read from a Lenze brand AC Tech SMVector control VFD. According to the documentation drive register 19 should be the VFD Family which will return a value of 72. The drive register is offset by 1, so the modbus register I should be querying is register 20.</p> <p>If I use my PC's Simply Modbus 8.1 modbus master software I can read register 20 and I get back a value of 72. If I use the PLC I get back a value of 0. Register 19 gives me 0, and register 18 gives me back a value of 150. Super confused because if I try to use register 18 on my PC I get back an error that it is an illegal data address, as it should be according to the documentation.</p> <p>Wth is going on.</p></div>