Overdriving EMS Type A inputs.

[vlittle]

I have a situation where I am overdriving type A inputs on the EMS.  In this situation (diagram A below), current is being sunk by the EMS input, causing the LED to glow faintly.

Diagram B is my proposed fix, but is it also possible to just progam the input pull-up to 200K?  I'm worried about the current flow into the EMS, even with the network cable unplugged.

Thanks, Vern

 

[dynonsupport]

We only have 10K, 1K, and 200 ohm pull up resistors inside. The C type inputs can have the pull up turned off.

Because the all the resistors are turned on/off via a FET, if you backpower them, the current can go through the body diode in the FET to our 5V rail. This does mean that if you put 12V on the input, you have a 7V difference across the 200 ohm resistor, and you can get 35mA to flow. The EMS is designed to handle this, but the current is flowing so if you have something like an LED you will see it.

So, your diode solution is really the correct one. I do have to ask though, what is Diode-1 getting you in your schematic?

 

[vlittle]

We only have 10K, 1K, and 200 ohm pull up resistors inside. The C type inputs can have the pull up turned off.

Because the all the resistors are turned on/off via a FET, if you backpower them, the current can go through the body diode in the FET to our 5V rail. This does mean that if you put 12V on the input, you have a 7V difference across the 200 ohm resistor, and you can get 35mA to flow. The EMS is designed to handle this, but the current is flowing so if you have something like an LED you will see it.

So, your diode solution is really the correct one. I do have to ask though, what is Diode-1 getting you in your schematic?

... I knew you'd ask.  The diagram is idealized.  The actual circuit is here: IL-4A.

I've put the diodes in, everything is good now.  I had this problem years ago with another engine monitor and had the same fix.  Too bad you can't make every input type C.  The reason I have both annunciators and the EMS is that I have a single Skyview screen and there is not enough room on the EMS page to clearly show all of the status signals.  Additionally, the annunciators are more legible and will function even with the SV turned off or failed. 

Vern