user53178: How can I program my Pro Mini using the native RS232 Serial port on my Mainboard (aka com1). 
Google-ing did not really help and a quick search here for "pro mini programming com1" did not deliver any results.
I already have a MAX232 module connected between the Pro Mini and the PC.
I have already tested the com1 port for function by sending stuff to an Arduino mega via its usb-to-serial port and echo-ing that to com1. 
Which succeeded (at least sort-of [receiving some random clutter in between characters or missing some characters at all]) 
I did try the obvious (i.e. selecting pro mini from the board menu and then selecting the com1 port and hit upload).
But that did just produce a timeout-error… 
An Option to set the programmer to simple Serial Connection or something similar does not exist to my knowledge (well it does but only if we're talking about STM32 based boards like the blue pill).

user37622: To upload a hex file to Arduino over Serial interface, the bootloader must be active. The bootloader is active after reset. It waits a short time if a new application is uploaded. If not, it jumps to existing application.
To reset the MCU on Arduino boards with independent USB chip a circuit connected to DTR line of serial is used.
On a board like the Pro Mini there is no chip for USB neither an auto reset circuit. You must reset the board with the reset button right before avrdude sends the hex file. avrdude tries ten times, so you have 10 chances to time it right after you hit the Upload button. 
Some USB-to-TTL-Serial adapters (or RS232 adapters) have the DTR line and it can be used to reset the board.
Or use ISP flashing over the ICSP header with a programmer or other board as programmer.

53178: <p>How can I program my Pro Mini using the native RS232 Serial port on my Mainboard (aka com1). </p>

<p>Google-ing did not really help and a quick search here for "pro mini programming com1" did not deliver <em>any</em> results.</p>

<p>I already have a MAX232 module connected between the Pro Mini and the PC.</p>

<p>I have already tested the com1 port for function by sending stuff to an Arduino mega via its usb-to-serial port and echo-ing that to com1. </p>

<p>Which succeeded (at least sort-of [receiving some random clutter in between characters or missing some characters at all]) </p>

<p>I did try the obvious (i.e. selecting pro mini from the board menu and then selecting the com1 port and hit upload).</p>

<p>But that did just produce a timeout-error… </p>

<p>An Option to set the programmer to simple Serial Connection or something similar does not exist to my knowledge (well it does but only if we're talking about STM32 based boards like the blue pill).</p>
 37622: <p>To upload a hex file to Arduino over Serial interface, the bootloader must be active. The bootloader is active after reset. It waits a short time if a new application is uploaded. If not, it jumps to existing application.</p>

<p>To reset the MCU on Arduino boards with independent USB chip a circuit connected to DTR line of serial is used.</p>

<p>On a board like the Pro Mini there is no chip for USB neither an auto reset circuit. You must reset the board with the reset button right before avrdude sends the hex file. avrdude tries ten times, so you have 10 chances to time it right after you hit the Upload button. </p>

<p>Some USB-to-TTL-Serial adapters (or RS232 adapters) have the DTR line and it can be used to reset the board.</p>

<p>Or use ISP flashing over the ICSP header with a programmer or other board as programmer.</p>
