How to Power Up with Serial Connected

Other Hardware General Electronics
1

I will start with a simplified schematic of my circuit.
Serial Problem

My problem with that when serial is connected (pin 2 or 3 from a prolific USB-Serial port and pin 5) before 12 volts is applied to the main power circuit, the DC-DC converter (R-78E5.0-0.5) will not output any voltage. As soon as either ground or data is disconnected, it is fine and also if it is reconnected after.

Is there a simple cost effective way to isolate the incoming lines? I have heard people using picofarad caps, but does anyone have experience with this?

2

A resistor (1k) in the data line may be a solution.
Or a diode in the RS-232 Vcc line that prevents current out of the Vcc pin.

There may exist other obscure problems, I had not expected an LM78xxx not working in your circuit. Does it mean that Vcc with 12V disconnected is above 5V?.

3

An LM7805 does work in the circuit, but not the DC-DC converter.

4

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