I don't have an arduino to hand, else I'd try, but
I need an extra pin in the design ...
so I have the serial links, I'm using all the serial receives , but two of the serial links, I never transmit anythign, I just listen.
so is it OK to define the tx pins as digital IO and read / write to them , even though they are explicitly defined when I declare the serial 2 and 3 ?
The '2560 data sheet implies that one can be used without the other:
"22.4 USART Initialization
The USART has to be initialized before any communication can take place. The initialization process
normally consists of setting the baud rate, setting frame format
and enabling the Transmitter or the Receiver depending on the usage.
For interrupt driven USART operation, the Global Interrupt Flag should be cleared (and interrupts globally disabled) when doing the
Whether the Serial Library (Serial.begin(speed), Serial1.begin (speed), etc. for 2 & 3) supports that is another question.
You can always try it & see what happens. Try writing first, as the TX pin will be set as an output already. Then try pullig the pin low via switch and series 1K resistor, see if you can read a low.