Black Box MD1641A, MD1640A manual Series II Intelligent DATA/FAX Modems

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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