Role of capacitor for Arduino Mini coupled with Mini USB adapter

[Cross]

From here:
http://arduino.cc/en/Guide/ArduinoMini

The 0.1uF capacitor from the reset pin is connected to the RTS pin on the mini USB adaptor. This enables auto-reset when the serial port is opened, meaning you don't have to press the reset button every time you upload new code. If it gives you problems, you can remove it, and press reset every time.

I do not understand how/why the capacitor works in that case.
In AC circuits it makes sense to have capacitors where the charge moves back and forth, but here in a DC circuit I would think of it as "unconnected" / open. Assuming that the RTS pin stays high once a connection has been made I do not understand why the capacitor apparently acts like a connection instead of being unconnected/open.
If I was sending more data than the FTDI chip can relay it would pull RTS low. Wouldn't that reset the Arduino ?

[Grumpy_Mike]

What a capacitor does is to turn a change in DC level into a pulse. So when the DTR pin changes ( not the RTS ) there is a quick pulse on the reset line. Without the capacitor the reset line would be held permanent in a reset state and would not run.

[Cross]

Thanks. So the capacitor differentiates the signal from the USB connector. That makes sense!
The schematic does call the output pin RTS but maybe it's just poorly labelled just like the TX/RX pins.  :|

[Grumpy_Mike]

No not the USB connector but the output from the USB / serial device one of the control handshaking lines.

[TomGeorge]

The 0.1uF capacitor from the reset pin is connected to the RTS pin on the mini USB adaptor. This enables auto-reset when the serial port is opened, meaning you don't have to press the reset button every time you upload new code. If it gives you problems, you can remove it, and press reset every time.

This means that without the cap the controller is NOT in RESET mode, so it will try to run, for it to run program that you have downloaded you will have to press RESET.

What a capacitor does is to turn a change in DC level into a pulse. So when the DTR pin changes ( not the RTS ) there is a quick pulse on the reset line. Without the capacitor the reset line would be held permanent in a reset state and would not run.

This true until it says "Without the capacitor the reset line would be held permanent in a reset state and would not run."
Without the capacitor the reset line would be held off and the program will try to run.

Tom

[Grumpy_Mike]

Without the capacitor the reset line would be held off and the program will try to run.

I think you missunderstand. What I was saying is that if the capacitor were not in circuit, that is you connected a handshaking line directly to the reset pin. It would be held in reset. Ofcourse if you removed any connecting between a handshaking line and the reset the processor would continue to run.

[liuzengqiang]

I purchased a couple of ftdi basic USB UART adapters from ebay but couldn't get them to upload sketch to my standalone arduino projects. Turns out they (adapter) don't have the cap. The SPARKFUN version doesn't have the cap either, which I suspect is the design the ebay adapter based on. But SPARKFUN arduino pro has the cap so it works with the adapter. I also only used moderndevice usb bub ii that has the cap. So I didn't learn any of the above GM wrote until I got into trouble. Reading schematic helped. Yeah, caps do derivative on voltage. i=C dV/dt