Configuration syntax for 'Serial port' in Modbus RTU driver

What is the current format & syntax for configuration of ‘Serial port’ in Modbus RTU driver? For example COM1, COM01,…

There is no help file for that?

It varies with platform. For my systems, it typically will be /dev/ttyS0 or similar. Generally you use what your OS expects. Since the modbus driver is delegating to the serial support module, you might want to look at its documentation, like system.serial.openSerialPort().