Modbus not usable with USB serial

Modbus can be used with USB serial, however, if the USB serial connection is lost, Modbus will hang in an infinite loop.

This is a problem in the handling of select() and read() and could probably resolved by studying the Modbus error handling.

A more USB-friendly solution would be to: (1) Re-connect and (2) re-open the serial drivers. That is what is done is NSH: When the serial USB device is removed, this terminates the session and NSH will then try to re-open the USB device. See the function nsh_waitusbready() in the file apps/nshlib/nsh_usbconsole.c. When the USB serial is reconnected the open() in the function will succeed and a new session will be started.

This is really an enhancement request: Modbus was never designed to work with removable serial devices.