15
SHM-B PLUS ASYNC
3.4 Flow Control Switch
The SHM-B uses a clever, yet simple way to allow this
unit to work with a broad line of equipment.
S
TANDARD
EIA-232C H
ANDSHAKING
(RTS/DTR C
ONTROL
S
WITCH
S3
IN
EN P
OSITION
)
The individual SHM-B modems force their own DTR,
RTS, CTS, and DSR control lines HIGH (>+3V) when:
• The twisted-pair wires are installed correctly.
• Power is applied to the modems.
• No computer devices are attached to the modems.
This forces DCD HIGH on the remote modem; thus, the
remote modem knows to expect data. In this state, “all
systems are go.” The only way this state changes is if your
equipment overrides this condition. For example, if your
equipment drops DTR to a NEGATIVE signal, your
equipment’s DTR being low will override the SHM-B’s
DTR, and DCD on the remote modem will go LOW.
Because DTR, DSR, RTS, and CTS are tied together
internally, dropping any one of these four signals to a
