Auto. Leased Line Restoral Operation (336+ Only). Black Box MD1641A, MD1640A

CHAPTER 4: Manual Dial and Automatic Answer

4.3 Automatic Leased Line Restoral Operation

(336+ Only)

When the Series II Modem 336+ (MD1640A) 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 (336+ Only)

With your Series II Modem 336+ (MD1640A) 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 (336+ Only)

The dialing associated with the Series II Modem 336+ (MD1640A) when it's in "leased line with dial-back" mode, involves placing a call from the originating modem due to a leased line failure.

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SERIES II INTELLIGENT DATA/FAX MODEMS

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 Series II 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.

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