You connect to the HMC with your laptop via a browser. The serial connection is between the HMC port and the POWER4 serial port.
I never worked with these systems, so I do not know if it is the "normal" com port, of if there is an extra one.
HMC3 is
NOTHING like
HMC7.7.7 today. Also a "waste" of your time, unless you are writing a historical review of the development of HMC code.
Hope this helps!