See the marketing specs that you'd get from sales literature.
Rather than purchasing a cable for your sign, you can make your own cable and it costs approximate $2 to do so.
Once you have a cable, making the sign talk with Linux isn't that complicated with the right commands.
Not much public information exists about the protocol for talking with the sign, however here are notes about the protocol I've been able to collect and derive from experimentation.
It is possible to buy the protocol, but that costs money and takes the fun out of it.