SHM-B ASYNC
Because DTR, DSR, RTS, and CTS are tied together internally, dropping any one of these four signals to a negative state will also drop DCD on the remote side to a negative state. DCD will also go LOW if the remote SHM-B is powered down, or if the twisted-pair wire is broken.
X-ON/X-OFF MODE
(RTS/DTR CONTROL JUMPER IN DIS POSITION)
If X-ON/X-OFF characters are used for handshaking control, rather than hardware logic levels, move the RTS/DTR control jumper to the DIS position.
Typically, units that use software X-ON/X-OFF flow control do not want RTS to affect DCD on the remote SHM-B. Moving the jumper to the DIS position disables the RTS/DCD relationship. The only thing that will force DCD low with the jumper in the DIS position is the absence of power on the remote SHM-B or a broken twisted-pair wire.
