ESXi supports four serial port modes.
There are four serial port modes in ESXi:
Logging mode – Logging mode is the default mode in a debug build. Logging mode sends the vmkernel.log over the serial port.
GDB mode – Use GDB mode for dedicated debugging.
Shell mode – Shell mode is the shell port access, which is similar to SSH.
DCUI mode – DCUI mode is a Direct Console User Interface. This is the user interface that is displayed when you boot ESXi using a monitor.
Only COM1 and COM2 ports are supported. USB serial or PCI serial cards are not supported.