Protection for EMS220/221 contact GP input

[mregoan]

I am using the Van's door switches turn on the interior lights when one or both doors are opened.  I also want to use the signals to indicate which door is open on the Skyview display.

One side of the relay coil is connected to +12V.  The other is grounded when the doors are closed, and floating when they are open.  I intend to connect the grounded side of the coil to a general purpose input on the EMS220/221, but want to protect the GP input from voltage spikes from the coil, and also need to prevent the relay from latching closed due to the leakage current through the EMS input. 
Other postings on the forum for normal contact inputs suggest using a 1N4002 diode (anode towards EMS) or a resistor where I have marked a component X on the attached diagram. 
What would best practice be in this case?  If a resistor, what value?
Thanks!

 

[kurtfly]

With the circuit you provided you want to install a diode.  With the door switch open you are basically feeding the EMS input with 12V through the relay coil.  The diode will provide isolation / very high impedance from this. With the diode installed the EMS signal will "Pull-up" to the 5 volts that the EMS provides when door switch open.  The diode will conduct "low impedance" when door switch is closed (grounded). The EMS will see ~.6 Volts in this case. You are correct to place the Anode towards the EMS side, the cathode "bar side" towards the relay side.  This will allow the EMS to sense when door switch is "closed" or grounded.