!"##$%&'#()*"+,)-./010/!
2$3'%$)&")&4$)5678/9:
!"#$%&'()
4–11
The 1785-KE module continually asserts DTR when it is waiting for a
call. Under this condition, a modem attached to the 1785-KE module will
answer a call and assert DSR as soon as it detects ringing. The 1785-KE
module does not monitor the RING indicator signal in the RS-232-C
interface. Once it detects DSR, the module starts a timer (approximately
10 seconds) and waits for the DCD signal. When the module detects DCD,
communication can start.
If the module does not detect DCD within the timeout, the module turns
DTR off. This causes the modem to hang up and break the connection.
When the hang-up is complete, the modem turns off DSR. 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
the module.
After detecting DCD, the module continues to monitor the DCD line. If
DCD goes off, the module restarts the timeout. If DCD is not restored
within the timeout, the module initiates the hangup sequence. This feature
allows the remote station to re-dial in the event that the connection is lost
through a fault in the phone network.
This handshaking is necessary to guarantee access to the phone line. If you
defeat this handshaking protocol by improper selection of your modem
options or by jumpers at the connectors, the modem may still answer a call,
but, if the connection is lost, the modem will not hang up. It will then be
impossible for the remote station to re-establish the connection because it
will get a busy signal.
You can connect the module to standard American dialup modems and
some European modems.
Other European standards specify that the DTR signal will make the
modem answer the phone, whether it is ringing or not. This causes the
phone to always be “busy”. Since the 1785-KE module asserts the DTR
signal while waiting for a call, you cannot use the 1785-KE with such
modems.
TE-+.(%"&(89:;3<=(-+(#+
Q0%*3Q+EU&'(Q$$M-,#%-*+