Using a Arduino uno can I upload a program to a Arduino pro mini? The program compiles and uploads to the uno. I connected the TXO to RXI, VCC to 5v, GND to GND, DTR to RST. I jist hangs and after a while it gives an error message.
Not like that.
If you remove the chip from the Uno (without removing the chip, it's not possible, because you need to pulse reset on the pro mini; on a serial adapter you have the DTR pin connected to the corresponding pin of the pro mini to do that, but that line isn't exposed on the Uno - all you have is the reset line after the cap, but if the chip is on the uno, when it's not in reset, will interfere with communication with the pro mini), and connect reset to reset, and TX to TX, RX to RX, then it might work (though the reset pulse will be shortened since there's an extra 10k to Vcc there).
The TX and RX pins on Uno are marked by which pin of the '328p on the uno they correspond to. You're replacing that with a pro mini, so you don't cross over (the "TX" pin on uno is connected to the RX pin of the on-board serial adapter - so you want that connected to the TX pin of your pro mini).
You know that USB-serial adapters that don't require this sort of setup are like $2 shipped, right?
Yes, you can: https://www.arduino.cc/en/Tutorial/ArduinoISP
You can even program a ProMini with another ProMini. I do it all the time, and don't even need a terminal to do it.
Regards, Ray L.
DrAzzy: If you remove the chip from the Uno (without removing the chip, it's not possible, because you need to pulse reset on the pro mini
There is no need to remove the chip. all that is required is a sketch not using 'Serial' Either you can connect the reset on the Uno and the Pro-mini and it should work just fine (does for me), or you can do the reset 'manually' releasing the rest button on the Pro-mini just after the IDE is done with 'compiling' Just make sure you have the TX-RX pins connected correctly. On my Uno the TX/RX that are printed on the PCB refer to the pins of the USB input. Pro-minis have a different layout then most other Arduinos