ESXi supports dynamic switching between four different serial port modes.

ESXi supports serial mode dynamic switching to provide maximum platform flexibility, and to allow debugging and supportability in the text box. ESXi examines the input characters for any serial port mode and switches the modes based on the input key sequence. DCUI, Shell, GDB, and Logging modes are supported. If you have two serial ports, only one of the four modes is allowed on each port. Two serial ports cannot be in the same mode. If you attempt a dynamic switch to a mode in use by the other serial port, the request is ignored. Dynamic switching eliminates the need to interrupt the boot process manually or to create a custom image to redirect to a serial port. It also addresses supportability issues regarding headless systems that only have one serial port, by making it possible to switch the serial port between different modes of operation.