We just completed a version of our COM extender (4 RS232 ports) where each port is fully isolated. Sadly, it will not go into production as it has a terribly high power consumption.
Anyway, to answer your question - your friend makes a good case and RS232 and other connections have been responsible for much trouble and even damage to systems. Here are the reasons:
a) Ground loops. Multiple ground connections with any sort of area inbetween make good loop antennas that can both radiate as well as receive RF. Isolating the grounds would elliminate any issue here.
b) Ground faults. The typical scenario here is a high power device (like a transmitter) having lost its main ground connection due to a wiring fault.
It will then use any other ground connection (including the one in a typical RS232 hookup) possibly inducing several amps of current into some other device that was never designed to handle this. Smoke is the usual result.
The typical cockpit has several items in close proximity and they are all supplied by the same source and share a common ground.
Thus it is quite possible to NOT wire any RS232 grounds at all. If the devices have true RS232 signals then the signals swing between about -10 to +10 volts and don't need an accurate ground reference.
The same goes for ARINC (differential signals) and CAN that is used on some systems. Use the ground to connect to any shields but only connect on one side.
That is about the best you can do.
CEO MGL Avionics
Originally Posted by akschu
A friend of mine who is very good with electrics (ham, did electronics in the coast guard, does circuit level troubleshooting) was looking at my panel diagram and recommends against running a serial output from one device to several others without an isolator.
For example, the aviation serial output on the GNS430W is connected to the EFIS and Portable GPS.
Far and wide this is minimized by each device having multiple inputs/outputs, but in some cases there isn't enough serial ports and it doesn't seem reasonable to output the same information twice.
So here are the questions:
1. Is it acceptable to split serial outputs?
2. Is there an isolator I can buy?
3. What do certified aircraft use to get around this problem?