Did you know that some Mac models will not recognize the presence of a modem unless the modem is turned on before you boot up your Mac. This is symptom of a more general serial port problem with Macs that use the Tanzania motherboards (including , including StarMax clones and the Apple 4400. The Unofficial StarMax site has a web page on this. No permanent solution is known. (Thanks, Helge Hafstad.)
There is an index of several dozen other StarMax related "issues." It is certainly worth a look if you own one of these clones.
Update: Matt Haffner notes that this problem has plagued Alchemy-based Macs as well and that (as stated on the PowerWatch page) it appears to be fixed in Mac OS 7.6.1.
Of possible relevance, Apple's 7.6.1 documentation states: "Compatibility with serial devices for Performa 5400 and 6400 computers and PowerBook 3400 computers has been improved. There were problems with these computers not recognizing serial devices that were connected at startup. This update fixes this problem."