I have a next stop sunrise systems LED bus sign with has the following connections ground, data A, data B, 12v/12v, and aux it uses the J1708 serial connection to communicate.. how can i emulate this serial connection to the arduino?
Haha interesting, I was aboard the Metromover in Downtown Miami this morning and the train lost power and thankfully had enough momentum to roll to a track segment that still had power... derp!
The signs scrolled "DIGITAL RECORDERS 7x96 J1708" then some additional numbers that were probably the address the sign was expecting.
J1708 appears to be based off an RS-485 like physical link, use a MAX485 or similar differential line driver for that and see if the sign gives any clues as to its address, either labelled or on power up. I've seen the Sunshine Systems signs before, usually in combination with Luminator control systems. They usually appear to only be used for text - either static or scrolling.
Now here's where it gets tricky: "Since J1708 protocol only describes the lower layers of the OSI model it is always used with an overlaying application layer. An example of such a layer is J1587 which is used for data exchange between microcontrollers in heavy duty vehicles."
J1708 also only sends 21 byte packets, which means the overlying application layer will have to be figured out.. it MAY be as simple as just send ASCII text, if you're lucky.
I know I've seen the signs on the Metromover have to split up scrolling messages; it'll read out a couple words then blank out and scroll the next few. Can't say I noticed the lengths though.
I also have an old Vultron flip-dot sign that probably uses J1708 but that one's going to be super easy to hang an Arduino on at some point... the CPU can just be pulled out and there are places to easily access sck/srck/data on the shift registers for row and column drive.