Rockwell Automation 1775-KA PLC-3 Communication Adapter Module User Manual - Manual (Page 30)

Installation
Chapter 2
2Ć18
DTR, DSR, and DCD signals. It incorporates timeouts and tests to
properly operate these types of modems.
Auto–answer: these modems have self–contained timeouts and tests,
and can answer and hangup the phone automatically.
The modem port has no means to control an auto–dial modem, although it
is possible that it can be used in conjunction with a separate auto–dialer.
Answering
The module continually asserts DTR when it is waiting for a call. Under
this condition the modem will answer a call and assert DSR as soon as
ringing is detected. The module does not monitor the RING indicator in
the RS–232 interface. Once DSR is detected the module starts a timer
(around 10 seconds) and waits for the DCD signal. When DCD is
detected communication can start.
If DCD is not detected within the timeout, the module turns DTR off. This
causes the modem to hangup and break the connection. When the hangup
is complete the modem drops the DSR line. This causes the module to
reassert the DTR line and wait for another call. This feature protects
access to the phone if someone calling a wrong number reaches this
station.
Once DCD is detected the module continues to monitor the DCD line. If
DCD goes false the timeout is restarted. If DCD is not restored within the
timeout, the hangup sequence is initiated. This feature allows the remote
station to re–dial in the event the connection is lost by the phone network.
Note that this handshaking is necessary to guarantee access to the phone
line. If this handshaking protocol is defeated by improper selection of
modem options, or jumpers at the connectors, the modem may answer a
call, but if the connection is lost the modem will not hangup. It will be
impossible for the remote station to reestablish the connection because it
will get a busy signal.
Character Transmission
Data is sent serially over the RS–232 interface, one eight–bit byte at a
time. The transmission format conforms to ANSI X3.16, CCITT V.4, and
ISO 1177, with the exception that the parity bit is retained while
extending the data length to eight bits.