Peter,
You can connect the serial altitude out to two devices without any problem provided they speak the same serial protocol. The EFIS can currently output any of 4 serial altitude protocols, and only one can be in use at a time. Generally one would expect devices from the same manufaturer (and of the same hardware generation) to support the same encoding protocols, but this would be something to confirm before wiring up your panel in this fashion. Also note that our optional serial-to-parallel encoder converter module - required only for older "gray code" transponders - requires that you set the EFIS to serial protocol 1. So any other device that you connect in parallel with our converter would also need to be able to understand protocol 1 as well. You can find more info about the protocols we support in the EFIS-D10A install guide at www.dynonavionics.com.
You can use a 1/4" stero jack for the serial connection, but we suggest a 9 pin D-sub connector. With the stereo jack you need a custom cable to go to the PC, but with a 9 pin, you can use a standard serial cable. Also, accidentally plugging your headset into that 1/4" jack could cause some problems. Most of our customers install the serial port under the panel, out of sight, as it is rarley used. Electrically, the 1/4" jack will work fine.
