Hi,
I confirm you need a RS232-TTL converter (you can easly buy it on internet, I suggest one with a MAX3232 controller) and have to plug it into the box.
I recently installed this way ffp-busybox on my DIR-685. But finally, it doesn't seem to be able to launch any fun_plug script (I didn't find any chk_fun_plug.sh script in the box's native Linux). I suppose this is why fonz doenst provide a fun_plug script for this box but only a ffp-busybox package. However, the ffp-busybox package is still interesting since you can get some advantages from the new commands included.
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GG