That sounds like a too common mistake. Don't connect a Tx to talk into another transmitter, Tx. Tx to Rx and Rx to Tx is the way to do it.
Please save the photographing and use schematics! A criminal detective having no clues might take on those pictures. You spend the time needed, not we.
Anyway I repeat my question, is it possible to upload firmware to other units through the Arduino without usb/ttl converter ? is it possible ?
I don't think it is necessary to know a whole schematic, when what is really necessary is to know where the TX, RX and GND pins of the unit to be flashed are.
An FTDI interface board has all the signals you need. It's often used to upload to boards that have no USB interface, like the Pro Mini.
That is, assuming that your Arduino has a normal bootloader installed. If not, you would have to use the ICSP pins with a USBASP or another Arduino running the bootloader install software.
Confused. You said you were using the Arduino as a USB-serial converter. On most of them, the USB-serial is connected to the AVR RX/TX pins, so to use the Arduino as a pure USB-serial, you would tie the reset line down, to disable the processor. Otherwise, the processor will utilize those pins and conflict with the serial device that you are talking to (i.e. SIM900).
Also see:
If you only want to use the Arduino as a passive serial device, i.e. as an USB-serial interface, all you need to do is disable the processor by grounding the reset input.
If you only want to use the Arduino as a passive serial device, i.e. as an USB-serial interface, all you need to do is disable the processor by grounding the reset input.
It is exactly what I need, I have the PIN reset of the arduino to GND, as I had indicated, the problem is that it does not work.
