Since my PC has a COM1 serial port, I wanted to make an RS232-to-TTL adapter using a MAX233 chip bought during the mid-90's to program the Arduino. Functionally it is similar to the popular Sparkfun FTDI Basic board shown below.



A feature of this board is the DTR pin which is pulsed low by the IDE to reset the ATMEGA328P processor when a sketch is uploaded. On reset, the bootloader begins to read the executable code from the serial port and writes it to flash memory. On completion, the chip resets and execution begins at the address of the loaded user program.

There are many USB to TTL serial adapters available on ebay but some only bring out the CTS signal which cannot be used for Arduino programming. A typical clone of the Sparkfun adapter(with fake FTDI chips) looks like:

Adapter Schematic

Since the COM1 port is a PC hardware port, no USB interfacing is required. Instead, a MAX233 chip is used to convert the 12V RS232 signals to TTL(5V) and vice versa. It uses no external components and is recommended for applications where PCB space is tight. DTR is available and the schematic also shows a separate 3.3V interface module.

Adapter pictures

Programming the Arduino

The Arduino Pro Mini's programming header is press-fitted onto the output pins of the adapter. An external power supply provides 5V to the adapter and Arduino. Finally the Blink sketch is uploaded via COM1 and runs on the Arduino board.