Originally, the AT90S2313 microcontroller in the C2N232 was meant
to be programmed via a separate interface.  In the cost reduced
design "smdcr.brd", the firmware can be downloaded via the RS-232
interface when the RESET line of the microcontroller is tied to
ground.

It is easy to convert the older designs "smd.brd" and "smdl.brd"
to allow the firmware to be downloaded via the RS-232 interface.
You can permanently add three wires, connecting the pins 9 and 18,
pins 8 and 19, and pins 2 and 17 of the AT90S2313 with each other.
Remove the pull-up resistor from the RESET line (if it was
installed) and add a jumper wire from RESET to ground that is only
inserted while programming the device.

The RS-232 programming interface is supported by the cisp software,
available from the "../firmware" directory.  Try something like
"cisp -c c2n232 /dev/ttyS0" or "cisp -c c2n232 COM1".