Am wiring up an SL70 for a serial interface to a SV1000. My Apollo install manual shows a shielded two wire hook up. An RX from SL 70 Pin 4 to TX input on processor and a GND from SL 70 to a GND on processor. Also shows a shield ground at radio chassis and processor chassis. The SV 1000 install manual for the SL 70 only mentions one wire, the RX to TX input on encoder. It does not mention the need for a GND input nor a shielded cable either. Any advice out there concerning the SL-70 interface.
In a 'metal' aircraft the Ground isn't necessary however I'd use a 2 core shielded wire anyway and use one wire as the gnd & connect the shield at both ends
RS-232 interfaces are really robust, even without shielding. It never hurts to do it though. And as long as your SL70 and SkyView share the same bus, they share ground already.