Serial output to multiple avionics boxes without isolation.
 

Group,

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?

Thanks,
schu

This is one person at one company's opinion, but I've never heard of an RS-232 isolator, and we've never recommended one in any install. As far as we're concerned, RS-232 is specifically designed to be split a few times without any special work. We probably have thousands of installs with split RS-232 and have never heard of a problem.

Just to make sure, we are discussing splitting one transmit to multiple receivers. You can't combine multiple transmitters on one line.

I've never heard of such an isolator either, and know of hundreds of installations where a single output goes to multiple places, with products other than Dynon as well. We even do this on a certain high-flying aerospace plane I know of (on our internal laptop network), so I think it's been looked at by some pretty smart folks.

Paul

I was told by Garmin support that an RS-232 output on my GNS-480 can feed up to 3 devices. Not sure what (if anything) happens if you go for four...

Interesting subject.
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.

Easy Fix.
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.

Rainier
CEO MGL Avionics

Interesting...then what are the signals referenced to? To ask it a different way, +10 volts is 10 volts above what?

RS-232 is single ended, meaning it's referenced to the ground of the unit that is sending or receiving. Which is also why there must be some sort of ground commonality between two devices.

Not isolation but fault tolerant
 

You could put a 200ohm resistor in series with each output line. It's not isolation but it could save you from having three items lose the signal if one of them gets shorted out.

The Garmin 430 does have 4 separate RS232 output channels. If you're still in the design and wiring phase, and intend to drive two different devices (e.g. EFIS and an autopilot) it makes the most sense to go ahead and use two of the outputs as individual dedicated circuits and program the 430 serial config menu for whatever format is needed for each connected device.

Driving two RS232 inputs with one output shouldn't present any electrical problems however, in case the 430 is already mounted and getting access to the back panel connectors, and wiring a second RS232 connection would be too much of a hassle, and both receiving devices want the same GPS serial data format and baud rate.

Except....you can't really do that. Even though there are 4 outputs, you can only configure one of them to provide GPS serial aviation data out at a time. Don't ask me why, but that's the deal...you can use any one of those ports, just not 2 of them for the same thing. Most all of these devices want the same format, and the 430 only lets you use one of those ports to drive all of those devices who crave the same data!

Cheers,
Stein