If you also read the DMX512 specification you will see that it is essentially a variation on RS485, running at very high speed and with the master constantly repeating commands (or state table). Cables that work for DMX512 should also work for RS485 although perhaps not at the extremely long distances possible with RS485. Before interconnecting grounds you need to measure the difference in potential (i.e. voltage difference). If more than 7V, connect to local ground through a 100-ohm 1/4W resistor. This is detailed in the official RS485 specification. You might also do a web search using "Jan Axelson RS485". You should find a few magazine articles by her, including one which shows using a 555 timer chip to automatically reverse the line after a transmission. She writes very clearly.