Tried to read from Tx while using Serial. Damage possible?

I'm reading some pins from a module. The pins have a high voltage of 4.97V and a low voltage of 0.03V. They can sink 0.8 mA and source 2.5 mA. The module is powered from Arduino's +5V/GND.

I unadvertedly wired one of those pins to the arduino digital pin 1 (Tx) and attempted to read from it while at the same time using Serial.print in my program. This was my setup code:

        void setup() {
		Serial.begin(9600);  
	  
		pinMode(1, INPUT);
	}

The pins in the module have an attached LED so I know when they are HIGH or LOW. The LED for the pin wired to Arduino's Tx was never lit up despite the module was trying to set it to HIGH. The variable I was using to hold the reading for Tx was always 0. The serial log was working fine, no odd characters displayed.

AFAIK Arduino would have been switching Tx from HIGH to LOW while transmitting, which is bad because if the module sets the pin to HIGH and Arduino sets it to LOW then the Arduino pin would have been damaged or destroyed.

However this has not been the case.

What could have saved the Arduino pin? And why did I always read LOW from it?
Is there some protection circuitry involved?

The module is spec'ed at 0.8mA sink 2.5mA source you say?

ATmega is spec'ed at like 20mA either way, up to 40mA max. I would expect the Arduino to completely overwhelm that module.

I don't think digitalRead/Write work on the serial pins while serial is in use. I'm not sure how they behave in this case, though.

You can actually get away with being a lot rougher with the IO pins than you're supposed to be, as long as you don't leave it like that for long. You shouldn't do this intentionally, of course, but you can get away with it happening. I've certainly briefly shorted output pins to ground when they're trying to go high, without damaging the chip. I think I've only trashed one AVR, and that was when I put it down on a piece of metal while it was powered, and the +30v voltage sense line from the battery (no current limiting) got connected to something other than the high side of a voltage divider.

You can actually get away with being a lot rougher with the IO pins than you're supposed to be, as long as you don't leave it like that for long.

This is not good advice. You can expect damage, if you EVER exceed absolute maximum ratings. The damage may not be fatal, but it is there.

Did you read the next sentence, where I said you shouldn't do that intentionally?

Yes. Doesn't change the fact that the former sentence is BAD ADVICE.