Thanks, so let me guess that you are using this command line:

mame64.exe rainbow -rs232_a null_modem -bitbngr socket.127.0.0.1:1234 -window

The information in this line that I needed is that you use the 'rainbow' model, not rainbow100a or rainbow190 and that you connect to rs232_a not the rs232_b, and that is why I asked.