If you know what 8N1 means, and have an ECS with a lot of free time, and a Cuttle Cart, you’re in luck. Joe Zbiciak has put together serial port work with a simple termianl emulator. Get it together and call a BBS today! 
Media Links:
youtu.be/dG0nm2Do5Lo
media0101.intvprime.com/ba4ef/intellivision-echo/Software/LTO/ecs-terminal-joe-zbiciak-terminal.zip
Source Links:
atariage.com/forums/topic/168660-intellivision-as-serial-terminal/#comments