I'm running MODBUS server on the desktop PC via USB-RS485 adapter (baudrate = 115200) and I have request timeouts and stalls. I have activated low level QtSerialBus logs and saw this:
Dropping older ADU fragments due to larger than 3.5 char delay (expected: 2 , max: 15 )
Apparently, m_interFrameDelayMilliseconds is too low for my setup, however there is no direct way to fix it (like QtModbusRtuSerial*Master*::setInterFrameDelay).
Dirty workaround is to open serial port at lowest baudrate (i.e. 1200) and change it via direct call to the QSerialPort::setBaudRate.
So, there are either a bug in m_interFrameDelayMilliseconds or there should be QtModbusRtuSerial*Slave*::setInterFrameDelay call, or both.