ESXi supports dynamically switching between four different serial port modes.

ESXi supports serial mode dynamic switching in order to provide maximum platform flexibility, and to allow debugging and supportability in the field. 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 4 modes is allowed on each port. This means both serial ports cannot be in the same mode. If you attempt to dynamically 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 in order 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.