No it is not possible for the ISP line to always be low. This is because FM has no control over that signal before the COM port is opened and after it is closed.
If that signal needs to be low all the time then why connect it to your RS232 cable? However in general the ISP pin P2.10 shouldn't need to be low all the time and this indicates to me that there might be a hardware issue.
The fact that the trace is different when using a built-in COM port or a USB to RS232 converter is a concern. From the FM point of view this is a black box. A call is made to Windows to open the COM port and then further calls are made to Windows to set the states of DTR and RTS. What the COM port driver does with those commands is not known.
In general the waveforms I posted match the intention of the software design. This is using one of the low-cost US232R-100 cables:
http://www.ftdichip.com/Products/Cables/USBRS232.htmI would suggest as a next step to try one of these.
Andy
