It'd be useful (to me) to be able to attach callbacks to events that get fired off when the state of the DCD and DSR pins change (with the argument supplied to the callback being the current state of the pin in question.) Being able to raise and lower DTR would also be helpful.
These pages put it better than I likely could (not much need to read past the first paragraph):
In general DCD is used by a peripheral to send a status change to the computer it's connected to. In the case of an external modem, for example, a change from low to high on this pin means that a connection has been made, and from high to low means that the call has ended.
DSR and DTR are used for flow control purposes. When DSR is low, the peripheral is not ready to receive data; when DSR is high it is ready. DTR serves the same purpose, but in the opposite direction - the computer signalling its readiness to the peripheral. (It's possible that DTR/DSR flow control is already being handled at a lower level by this module - I do see that a toggle was added some time ago. Being able to toggle DTR would still be a bonus, if for some reason I wanted to tell the peripheral not to send data to me for a while.)
Ability to manually toggle DTR would be of great use for Arduino applications given Arduino has DTR wired up to the reset circuitry. Being able to force reset Arduino over the the serial connection would be great. Especially in the older revisions which have a bit weak reset circuitry that often ends up in a reset-loop .
The manual set_dtr method was there in commit 642070a but was dropped in the massive f44e7e6 commit for some reason.
Do we know the capabilities of the FTDI (and the chips arduino's other than the UNO use). DTR is probably pretty standard, but I know not all interfaces support all capabilities.
Uno, Mega, Nano: DTR (http://arduino.cc/en/Main/arduinoBoardMega2560)
Leonardo, Micro, Yun, Lilypad USB: The reset is triggered when the Leonardo's virtual (CDC) serial / COM port is opened at 1200 baud and then closed. (http://arduino.cc/en/Main/arduinoBoardLeonardo)
Due: Similar mechanism to Leonardo and Micro but instead of reset performs Reset + Erase which is less useful (http://arduino.cc/en/Main/ArduinoBoardDue)
Looking at Lilypad FTDI schematic the FTDI pins are: DTR, RXI, TXO, VCC, CTS, GND. The DTR is wired to the reset pin through a capacitor. So it seems atleast on the FTDI Lilypad the DTR is used to trigger the reset. Assuming the DTR of the FTDI is wired straight to the DTR of the USB bit in between.
Actually looking at the description on the Arduino Pro:
One of the pins on the six-pin header is connected to the reset line of the ATmega168 or ATmega328 via a 100 nanofarad capacitor. This pin connects to one of the hardware flow control lines of the USB-to-serial convertor connected to the header: RTS when using an FTDI cable, DTR when using the Sparkfun breakout board.
Voodoo It would be great to have DTR control.
For me it is important to manual control DTR
Consolidating to ticket #384