In my case problem solved with your help Mark Hall. Thank you!!! My my environment was: Visual Studio 17.9.0, x64; EasyModbus v5.6 and ModRSsim2. Followed tutorial from YouTube "Create the Modbus Master GUI in 10 Minutes using EasyModbus Library" Adding this line of code solved the problem modbusClient.Parity = System.IO.Ports.Parity.None;