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4–12
This section provides guidelines for selecting the modems you use with the
1785-KE in multidrop applications using half-duplex protocol.
The multidrop link may either be a two-circuit system (master sends and
slaves receive on one circuit, slaves send and master receives on the other),
or a one-circuit system (master and slaves send and receive on the same
circuit).
The half-duplex protocol is a polled protocol that only allows one node to
transmit at a time. Because of this, the modems must operate in switched
carrier mode (they only switch their carriers on when they transmit data).
The RS-232-C handshaking lines are used to control the switching of the
carrier for a message transmission as follows (figure
4.6
):
Request to Send (RTS) -- sent from the Allen-Bradley module to the
modem to indicate the module has a message to send and wants a carrier
to be established.
Clear to Send (CTS) -- sent from the modem to the Allen-Bradley
module to indicate that the carrier is established and that the module can
send its message.
Data Carrier Detect (DCD) -- sent from the modem to the Allen-Bradley
module to indicate that a carrier has been detected and that a message is
on the way. The module uses this signal to enable its receiver.
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