Command line Hyperterminal for Mac?

I don't believe so. There is an older 3rd-party shareware GUI application called ZTerm that I know used to work on Jaguar but I have not tried it on Panther or Tiger or Leopard. There's a unix-like application called MiniCom that will run in a command-line environment. I have tried that but haven't gotten it working yet -- it may be because I don't know so haven't yet correctly guessed the proper combination of comms port parameters (9600-8-N-1 or .... ???) of the device with which I wish to communicate, or maybe I didn't have the file permissions set up correctly for a non-privileged user...not sure. But it (Minicom) is supposed to be pretty easy and straightforward to deploy and use in a Terminal window.

Keep in mind that serial I/O is really old-school. It seems like there isn't any way to do it but it is actually just that few people do that anymore and it isn't in the front of anyone's mind. MacOS X is built from the ground up to work with serial I/O. Most of the knowledge base of how to use those commands was before the Internet existed.

The reason I point this out is to make you aware that you shouldn't be worrying about whether or not you are using serial I/O. You should be able to write software using any standard input and output ports and test your code using the keyword, disk files, or some other program using pipes. Then you can hook it up to a live serial port after you have debugged the logic.

You might want to look and see if kermit is still available for OS X. It probably can talk to a serial connection.

FWIW, I did get minicom (macports download) working. See this thread.