I want to communicate using a serial cable to a device that uses RTS/CTS flow control and 115200 baud speed. I want to use this Sparkfun TTL to rs232 adapter, but it has no RTS or CTS pins. What can I do to get hardware flow control or is there a software solution?
The software solution is called XON/XOFF flow control. It basically consists of sending ^S/DC3 (0x13) to suspend transmission and ^Q/DC1 (0x11) to resume it. Naturally this requires the other side to support it, so if only hardware flow control is supported then it won't be a viable solution.
If both ends are always fast enough to receive all the other end sends, there is no need for flow control. This is often the case with UART speeds.
If your device requires hardware flow control, one hack is to connect its RTS to its own CTS pin. This way, it's telling itself that it's OK to send. The hack is that if you do overrun its input buffer, it will suppress its sending instead. If sending data is required for consuming its own input (e. g. input echoing or command/response sequences), you can end up in a deadlock so you need to be careful not to send stuff too fast.