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Date: Fri, 08 Apr 2011 15:54:11 -0400
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From: Brent Regan <brent@regandesigns.com>
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Subject: Re Request Electrical System Help
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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