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