Arduino library that controls BetaBrite signs over a serial connection.

You will need a simple serial driver circuit.

The venerable MAX232 will do nicely.

Note that the reference manual which led to this work is the Alpha Sign Communications Protocol which can be found by searching that phrase in your preferred search engine. It's at revision F as of this writing (5 June 2019) which hasn't been updated since 2006, so it would appear to be pretty stable.