EMS1 - RDAC XG - Surefly p-lead tach input

[paulfish]

Hi,

I'm configuring the RDAC unit now for tach input and I came across this post.

"Using the p-leads is also possible but yo should take care with this - the p-leads can produce voltages up to several hundred volts and that can damage things. It helps to have a scope to see what this signal looks like but effectively it is one giant pulse followed by a lot of smaller ones. It's the smaller ones that you need to get small enough so they don't get counted by the RPM gauge - the signal also changes with RPM - as RPM goes up the voltages also go up. In such applications you MUST install a dropping resistor in-line to your RPM gauge. For our instruments we usually end up somewhere around 20-30K or so - easiest way to get a good value is to use a 50K trimmer - figure out a setting that works over the RPM range - then measure the resistance of the trimmer using an ohm meter and choose the closest standard resistor value. You want a value that allows the gauge to see the big pulse but not see the smaller ones."

https://www.avcom.co.za/phpBB3/viewtopic.php?t=259637

I'm wondering if this information would be useful in the manual ( I was in the process of installing with the ballast resistor on and no dropping resistor) Given the Surefly is a very common replace and is configured as follows,

"Does the SIM output a tachometer signal?
The SIM outputs a 60Vdc spike with a 10Vdc offset on the p-lead (2 pulses per rev for 4 cylinder models & 3 pulses per rev for 6 cylinder models)."

https://www.surefly.net/faq

Is the dropping resistor required for a digitally produced p-lead signal and is there a know value for the dropping resistor.

Thanks
Paul

[rainier]

60V pulse does not sound like a form of digitally produced signal for RPM. Those kind of pulses tend to be nicely formed signals usually from 0V to 12V or 0V to 5V or just a ground contact (via semiconductor) requiring a pull up resistor to some DC voltage (usually 12V).

The RDAC will handle 60V but it would not be a bad idea to have a resistor in series.

Rainier