REFLECTOR: RMI Ground

[Brian Michalk]

It may be impossible to say.  Ask RMI what the value should be.  I 
dislike single ended signal acquisition, and prefer differential.  The 
problem comes when a sensor is connected to ground in some way, making 
three signal paths: the two to the sensor, and another unknown current 
to ground.  Take for example gasket CHT thermocouples.  Those are 
necessarily grounded to the engine case, and return two wires to the A/D 
system.

Unfortunately, ground and power are hardly ever clean.  Capturing 
transients with an oscilloscope can be illuminating to say the least.  
Most people don't have this equipment though, it's a little pricey.  
Scoping signal problems greatly simplifies the process, allowing one to 
pinpoint the problem rather than experimenting with solutions.

There are a lot of good tips to apply when wiring for noise immunity.  
I've mentioned a lot of them before, but some seemingly simple decisions 
such as where to mount the alternator power wire to the master bus in 
relation to the battery wire can have a big impact on noise.  The answer 
is that you don't want your bus carrying that high current noisy power.  
Move the alternator wire to the same lug as the battery wire and that 
current is not on the bus.

On 01/02/2011 06:16 PM, aminetech at bluefrog.com wrote:
> With the RMI Monitor removed, I checked the resistance of the ground sockets.  The main and alternate grounds were fine.  The signal ground socket (#14) showed 19 ohms.  Is this because the ground goes through the monitor, or does it indicate a bad ground?
>
> John
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