JS Safeguard data logging issue

[lagos]

I have a JS safeguard that sends a 0-1.3v analog signal through an 1/8 jack to its display gauge. Using a Y splitter, I decided to input this signal directly to the adc1 analog input on my DIYPNP. There was no problem in doing this, and tunerstudio can display and log this information as adc6. However there was one small side effect to doing this. Whenever I have the signal sent to both the gauge and my MS, the gauge display no longer displays individual cylinders that are being retarded. Instead it will work in whats called "bar mode" where all of the lights will turn on, up to the level of the knock event. This is potentially also retarding all cylinders instead of doing individual cylinder retard or at the very least, making the gauge look like thats what is happening.

I contacted John, the maker of the Safeguard (buy it, its amazing) and he thinks that this is happening due to the signal output not being strong enough to drive the analog input on the ms. Here is the technical information he provided to me....

The driver chip is LM3914. Pin 9 is left open, which selects dot mode. I doubt it's actually switching to bar mode, probably something else going on.

The output that drives the gauge is from a resistor, so it may not be strong enough to drive the analog input on the EMS. Make sure the EMS input is set for high impedance. Disable pull up or pull down resistors on the input.

I have attached a portion of the schematic that shows the output circuit for the gauge. It's a home made 4 bit R1-2-4-8 D/A resistor ladder. Software limits the max count to 10, and R39 loads the output so the max signal is 1.3v to match the bargraph input. R40 and Z4 are to protect the uP from bad things. J1-2 goes to the "ring" terminal of the gauge.

R35=80K
R36=40K
R37=20K
R38=10K
R39=3.65K
R40=1K
Z4=5.1vz

Picture 10.png

Any ideas on how I can verify that my input is running in hi-z mode or doesn't have any pull up/down resistors? Id love to be able to data log this AND have the gauge working correctly!

[Matt Cramer]

Try a 10K resistor in line with the signal to the ADC.

[lagos]

Is the analog input high impedance and are there any pull up/down resistors on the input?

[Matt Cramer]

It is high impedance - but it might not be high enough for this particular application.

[lagos]

I didn't have a 10kohm, so I tried a 33kohm resistor. It completely solved the issue. Thanks Matt!