I created an arduino project for a tally light. The design plans will be available here.
As will the software.
You can connect to a Blackmagic ATEM switcher by IP address.
You can setup IP addresses for the tally lights. If an IP address is provided, the software will send a http request in the form of:
to the tally light.
It should respond with 'OK' (two bytes) only if it received the GET request correctly.