I assume you are talking about the VMDirect path Configuration.
This is only supported on Nehalem CPU's e.g. an Intel 5500 cpu. I've got a feeling that Serial port passthrough does not work using the standard passthrough in ESX4 (or 3.X versions). You could look at using a moxa or something similar.
This KB article is for ESX(i) 3.5 but you can see that ESX supports use of a physical serial port in a VM and ESXi doesn't. If you'd like to stick with ESXi and need to support a serial port you might consider a serial over IP device for this.
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New book in town - vSphere Quick Start Guide -http://www.yellow-bricks.com/2009/08/12/new-book-in-town-vsphere-quick-start-guide/.
Do you have a system or PCI card working with VMDirectPath? Submit your specs to the Unofficial VMDirectPath HCL - http://www.vm-help.com/forum/viewforum.php?f=21.
We will eventually be moving to ESX, but in our current state I am having to utilise ESXi, I do have a Serial over IP set up in place, but we were just trying to remove this point of failure if it ever was to happen.
Looks like I will have to wait for our ful version to come in.
Thanks also for the additional information.
As some mentioned, Class ESX (console OS version) has support for serial.
Are you using PCI serial port card or standard motherboard ports?If it is former, then you can get a VT-d capable platform and passthru the device to a VM in ESXi (using VMDirectPath). If it is latter,then even VMDirectPath will not help since VMDirectPath deals at PCI device level.