Hi.
Here is a good place to read about serial programming in linux using C language.
I personally use another rs232-usb converter -- PL2303 (ordinary old Nokia data cable actually

) to control my 3D printer. But many people use FTDI's absolutely the same way as I am, so there are probably no big differences.
It can be a pain to implement and debug serial communication in C so I suggest you to debug your device using higher-level language, like python, first.
For example:
1) Install `
python-serial' package
Code:
sudo apt-get install python-serial
2) To check my device's current position I would type (to file test.py)
Code:
#test.py
import serial
s = serial.Serial('/dev/ttyUSB0', baudrate=19200)
s.write('M114\n') # M114 -- get current position
print s.readline()
Code:
$ python test.py
ok X:0.000,Y:0.000,Z:0.000,E:0.000,F:50
$
'/dev/ttyUSB0' is a device file associated with my usb-serial converter.
read/write operations are buffered so you do not need any delay:
Code:
while 1:
print s.readline()
should work (if your packets end with '\n').
This way you can assure yourself that sensor works properly.
Hope this helps.