Sorry, there was obviously something I forgot. Since I last used aspmon version 3.5 some years ago, I also got a new version of the component I'm using for reading the port. Earlier version must have been using an internal buffer since my code is reading 1 byte from that component. I have just changed my code to use a buffer in which to read all bytes ready at once, and then ofcourse aspmon works like I used to see it.
Thanks