Kevin writes: <<each time the gear motor runs, the output of the networked sensors (GPS, EMS, and ADAHRS) gets interrupted, resulting in red X’s displayed on screen. When the gear cycle is complete, the EFIS returns to normal. Testing with the EFIS system power isolated from the aircraft's electrical system by operating only on their independent backup batteries resulted in the exact symptom.>

The problem is likely NOT RF interference on the serial communication lines. The problem likely IS poor ground wiring. The hydraulic pump draws a lot of amps. All wire has resistance. Amps times resistance is volts so when the hydraulic pump motor is running the "ground" at the battery and the "ground" at the motor may be different by a several volts. Serial (RS232) 1communication uses a single wire that transitions from one voltage to another voltage to transmit data. The transmitter (sensor) and the receiver (EFIS) compare the signal to the LOCAL ground. If the ground voltage is different between the transmitter and the receiver then the signal may not be received. For example, say the receiver transmits 4 volts as a binary "1" and 1 volt as a binary "0" . The receiver interprets anything less than 2.5 volts as a 0 and anything more than 2.5 volts as a 1. If there is 2 volts of ground shift due to an induced voltage drop (the motor running) then the receiver will "see" the incoming signal changing from 3 volts to 6 volts, which it interprets as all 1s. A 1001101 transmitted message would be received as 1111111. Not good.

Be sure that your EFIS display and all the sensors get their power and ground from the same point on the airframe. Good composite aircraft wiring systems have a single point that is the ground reference with all component grounds coming from this point. You do not want a ground wire run to another part of the airframe and then branching to different devices as the resistance of the wire will allow one device to effect the other. Power ground, signal ground and shield grounds must all be tied to the same point or else you will induce currents (AKA Ground Loops) in the ground wiring that will cause trouble.

Even if you operate the EFIS off of emergency batteries, if a portion of the ground wiring between the sensors and the EFIS share a ground path with the motor then there will be trouble.

As my friend Hamid Wasti says: "You must believe in one true Ground, because if you don't, it will smite you!"

Regards
Brent Regan