Can not add serial port to the physical port

the env is like follows:

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1, physical server   HP DL360-G5 

2, VMware Esxi  4

3,Guest  WindowsServer 2000

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when I want  to add  a serial port  to the  Guest OS ,the option of  Physical port  displays gray out,

while I succeeded  on the  ESX3.5

Does the function become restricted from ESXi,or  I have update the VMwareESXi for the lastest drivers?