Hi all,

I've got a client with a light plane with the Vision Microsystems
EPI-800 instrument panel installed

http://www.visionmicrosystems.com/product/epi800specs.html

My understanding is that VM were taken over and old documentation
is not available

There's a minor problem with the tachometer that I'd be interested
to know if others have

The tachometer reading, which is in increments of 20rpm, isn't stable,
even though he can hear that the prop speed is. For example at 2500
rpm it flicks between 2480 and 2520

I've had it apart and find no electrical fault. The sensors are selected
in turn by CMOS 4096 switches, the selected signal is buffered and
scaled by an instrumentation amp, then digitised by an ADC0804. An
8051 sends this data out to the LCD controllers

A quick test with a signal generator shows that the display is stable
if the frequency is a multiple of 20 +/- 2Hz, so my guess is that the
8051's s/w isn't filtering as tightly as it could

My plan, if I have to, is to add a PIC to take the original signal and
buffer its frequency to rpm exactly divisible by 20. Fairly simple to
measure the time between prop signals and output the next highest
/20 frequency

My plan, if I don't have to, is tell him to just ignore it ;-)) I can see
how it would get to bother you though

But I'd be interested in hearing anyone's experience with this now
quite old package

TIA

Joe


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