Beginning with OS X Mavericks (10.9), Apple began shipping the AppleUSBFTDI driver preinstalled. While this is great in theory, their driver does not properly implement several important features. Currently, it lacks support for the full range of baud rates and doesn't allow for control of the RTS and DTR lines.
If you need to access these additional features, you will need to disable the AppleUSBFTDI driver and use Serial's built-in driver to access the device. The following method temporarily disables the driver until you restart your computer next.
Open Terminal.app and run the following command:
sudo kextunload -b com.apple.driver.AppleUSBFTDI
To re-enable the driver, run the following command:
sudo kextload -b com.apple.driver.AppleUSBFTDI