The RS232 serial standard has lots of variants and includes six additional control signals.
The Raspberry Pi GPIO serial drivers (and the Bluetooth TTL module used in the following example) only support RX and TX signals. If you require support for other signals, such as DTR, which is often used for a reset prior to the programming of AVR/Arduino devices, then alternative GPIO serial drivers may be needed to set these signals via other GPIO pins. Most RS232-to-USB adapters support the standard signals; however, ensure that anything you connect is able to handle standard RS232 voltages:
The RS232 9-Way D connector ...
