EMI

[DIRK BROER]

>My question to the group:
>
>Have other encounter similar problems? Has anyone used ALS or AS with a MSD
>ignition module nearby? What have you built and what logic families have you
>used? How much shielding?

Well I have used AS or ASL chips with a cicuit built on a perf board by a 
professional technition.  The chips in quest where 54XX series.  We need a 
quick way to convert 1773 (fiber optic) to 1553 (tri-state broadcast bus).  
Not only did the AS chips prove to be noisy - they actually changed output 
state so fast - due to nose spike etc. - that the fiber optic transmitters 
would latch due to a high pulse and never unlatch do to the negative going 
half of the pulse.  These chips also seemed to be very sensative and as a 
comparator any little ripple was likely to cause a large response.  
Basically they added noise to the circuit.  Since we were only worried 
about signals at about 1M Hz a switch to HC chips was all that was needed.

FYI 54xx series logic is the mil spec version of 74XX logic.

I suspect that rather than a high speed cpu, you might want to consider off
-loading some of the calculations.  For example - frequency counting.

So you get a high accuracy clock.. divide down the output frequencies so 
that you have a 1Hz clock to gather slow data and maybe a 1K Hz clock to 
gather fast data.  At the pulse you latch whatever data is there (and maybe 
reset the counters).  The cpu can then read the latched data at is leasure 
and make adjustments accordingly.

Dirk