Often when you have a problem like you are describing you are much better off going back to the source and adjusting the thing that causes the problem instead of trying to cover it over with a leaky bandage.

In the case of your microcontroller project I would suggest that you think about changing your logic about the status dumping logic. You could do many different things but here are three ideas.

1) Add a serial command that can be sent from the PC to the MCU to turn on the status data stream.

2) Add some big divider resistors to an unused A/D input on the MCU and then connect to some outside pin on your project board. Only send the serial status out the serial port when you detect 24V (or whatever you choose) to this outside pin and the A/D gives a reading over some threshold.

3) Add some combination of user control panel switches that when simultaneously pressed (or pressed in some special sequence) will turn on the serial status flow.

In any of these cases it is probably sufficient to just have a turn on command and then let the MCU logic persist in the serial output mode until the next power cycle of the product.

Note that if you have an active data flow on your serial port all the time that may have an impact on your EMI testing requirements at agency approval time.

Michael Karas