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MODEM 325 AND MODEM 325 CARD
5.12 Dial Backup
Dial backup allows the modem to switch to a dial-backup mode if the data connection on the leased line
becomes unacceptable for communications. This can be accomplished in two ways:
• Automatic—the modem can be set to switch to the backup line automatically after an extended loss of
carrier or 4 unsuccessful retrains in 3 minutes
• Manual—you can switch to dial backup from the front panel or with the *LB or *LD command.
* Not supported in V .22 mode.
In 4-wire leased-line operation, if both units have autodial backup enabled, one must be
configured for forced answer. This prevents both units from dialing if the leased line fails.
Both methods will cause the modem to dial the prestored autodial number. The originate modem will
go to idle mode for 5 seconds and then initiate the call. The answer modem will remain idle while
looking for a ring. The modems then connect and begin communicating over the dial-up line. If the dial
connection is unsuccessful after three attempts, a retrain on the leased line will be initiated.
In manual mode, the return to leased line is only done when commanded from the front panel or AT
command. In automatic mode the return to leased line is initiated after the lookback time in register S28
has elapsed. To prevent unnecessary termination of the dial-line connection, a leased-line lookback test is
performed. If the leased line is not acceptable, the dial connection is resumed with a retrain. If the
leased line is acceptable, the dial connection is dropped and normal leased-line mode is resumed.
When the unit attempts to return to leased line, the LCD displays LEASE LOOKBACK. If the leased
line has been restored to service, data can be passed approximately 10 seconds after LEASE LOOKBACK
was initiated. The LCD will continue to display LEASE LOOKBACK for slightly more than a minute.
During this time the dial-line connection is maintained in case a return to dial-line operation is required.
When the LCD displays ON LINE again the dial line is disconnected.
A diagnostic test initiated during dial-backup mode will end when the modem performs a
leased-line lookback.
5.13 Placing a Call
5.13.1 D
C
IRECT
ONNECT WITH A
• Lift the telephone receiver. Set the LCD to with a TALK. Wait for the dial tone.
• Dial the number of the remote site.
• When the answer tone is heard, immediately press the TALK/DATA button. The originating modem
goes off hook and normal operation will begin within 30 seconds. If not, hang up and return to the
first step.
• After the link is established, hang up the telephone.
4 8
CAUTION
NOTE
S
T
TANDARD
ELEPHONE
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