logging serial ports with portmon

I was trying to log all conversations on the first / any com port of a Digi PC/8e board. I used portmon from sysinternals, but it can’t open the port COM4, ntxall3Line1Concentrator1Port1 with following error:
Portmon could not attach to ntxall3Line1Concentrator1Port1 the device is may be in use.
The port is used by an application running as service.

Is there any other way to log the converation? Software only solutions preferred.