It's a FT2232D chip, and the LED is connected to BDBUS6.
The library is less documented than I might like (better than FTDI's own library though, which doesn't even work on modern kernels), the only example code I can find that does this uses a deprecated function (I tried, it doesn't seem to work), and I'm absolutely stumped.
The harder I try with this thing, the more difficult it seems. I'm not looking for someone to do my homework for me so much as I just need a nudge in the right direction. Any help appreciated (even speculative).
Update: I've been trying this, though ftdi_enable_bitbang() is deprecated. The following code compiles, it runs without barfing, but no blinkenlighten. Schematics of the device in question are available at http://www.semtech.com/images/datasheet/sx1211ska_v1_std.pdf , page 23. BDBUS6 and BDBUS7 are hooked up to the LEDs.
#include <stdio.h>
#include <stdint.h>
#include <string.h>
#include <stdlib.h>
#include <ctype.h>
#include <ftdi.h>
#define FTDI_VID 0x0403
#define FTDI_PID 0x6010
static struct ftdi_context ftdic_context;
int main(void) {
int ret;
unsigned int i;
unsigned char c = 0;
// Initialize bitbang.
// ret = ft2232_bb_init();
ftdi_usb_open(&ftdic_context, FTDI_VID, FTDI_PID);
ftdi_set_interface(&ftdic_context, INTERFACE_B);
ftdi_enable_bitbang(&ftdic_context, 0xb0);
// Trying to blink some lights.
printf("\nNow let's try to blinkenlights...\n");
for (i = 0; i < 20; i++) {
c ^= 0x80;
ftdi_write_data(&ftdic_context, &c, 1);
sleep(1);
}
return EXIT_SUCCESS;
}