Multi-Tech Systems BL-Series - Manual (Page 37)

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Chapter 4 - Manual Dial and Automatic Answer
4.3 Automatic Leased Line Restoral Operation
When the modem is in Dial Backup mode, it periodically checks the leased line to see if it's
operational and tries to restore the leased line if possible. S-Register S18 determines how often
restoral attempts occur. The default for S18 is 30 minutes, and can be set in one minute increments
from 10 to 255 minutes. Setting the restoral attempts under 10 minutes causes excessive breaks in
the dial-up operation.
Note: both local and remote modems must have S-Register S18 set identically. Refer to Chapter 6
for more information on S-Register S18.
4.4 Manual Dial Backup Call Termination
With your modem in leased line mode (DIP-Switch #10 in the DOWN position) and with dial backup
operation in process, there are two ways to manually terminate the dial backup call (other than
automatic leased line restoral). In each case, you will try to establish the leased line connection
because it is back in operating condition. The methods of dial backup call termination are:
1) Manual Control. The "Voice/Data" switch can be used to change from a dial back line to leased
line by toggling down once. When that is done, the modem tries the leased line connection, and,
if it is good, the modem disconnects the dial back call and establishes a leased line connection.
2) DTR Control. If DTR (Data Terminal Ready) is turned off for 50 milliseconds or more, a
disconnect occurs. This is probably the most common method used by computer systems at the
automatic answer end of the line to cause the answering modem to disconnect after toggling off
procedures.
4.5 Dial Backup and Leased Line Restoral
The dialing associated with the MultiModem, when in leased line with dial-back mode, involves
placing a call from the originating modem due to a leased line failure.
After a preset period of time (determined by S-Register S18), the modem automatically tries to
restore the leased line.
The parameters used to determine if a leased line is down (so automatic dial back can occur), is
based on the modem doing a “retrain” on the leased line due to an error condition in the transmission.
An error condition is defined as a "hit" on the line (the Carrier gets interrupted).
The retrain is a "handshake" procedure between the modems to establish the Carrier again. If the
retrain fails, both modems (originate and answer modems) start their Dial-back timers. The time is
determined by S-Register S19 settings. The S19 default setting is one minute. During that minute, the
originate modem tries to establish the leased line link. If the leased line is established during that
time, the timer is cleared and everything is back to normal. If the timer expires, the modems goes to
dial-back mode.
The purpose of the timer for the Answer modem is to determine when it accepts a dial-up call. When
both timers have expired and the leased line has not been established, the dial-back procedure
starts. The number dialed is the one stored in location N9 of the originate modem's phone number
memory. In the preparation for proper dial-back operation, enter the proper number in the N9 location
using the commands detailed in
Chapter 5 . Keep in mind that the number also can be dialed in
V.25bis Command mode.