UMCR_RTS
SET(combits, UMCR_RTS);
CLR(sc->sc_mcr, UMCR_DTR | UMCR_RTS);
if (how == TIOCMSET || ISSET(combits, UMCR_RTS))
ucom_rts(sc, (sc->sc_mcr & UMCR_RTS) != 0);
if (ISSET(combits, UMCR_RTS))
sc->sc_mcr = UMCR_DTR | UMCR_RTS;