FL_PREV_OK
req_ans = !(sc->state & FL_PREV_OK);
sc->state |= FL_PREV_OK;
sc->state &= ~FL_PREV_OK;
len_field |= (sc->state & FL_PREV_OK) ? FRAME_SENT_OK : FRAME_SENT_BAD;
sc->state |= FL_PREV_OK;