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Tech Notes

STL backup with the ISU128 Terminal adapter and Comrex Digital Audio codecs

It is possible to design an effective STL backup system using the Adtran ISU128 and Comrex DXR.1 or DXP.1 codecs. This system will automatically dial the stored ISDN phone number when told to do so by a remote control, silence detector, or other sensing device. The link will remain established until notified that it is no longer required. Should the ISDN connection fail during backup, it will be automatically redialed by the system.

The Adtran ISU128 has a special mode called “Dial #0 if DTR on”. This allows an external handshaking line to control the placement of a call as well as disconnection. The DTR signal is sent from the codec to the terminal adapter. In Comrex codecs, this pin is always tied valid, but it can be switched through an external contact closure before it feeds the Adtran ISU128. Here’s how it’s done:

  1. Configure the ISU128 according to your parameters (see Setting up the Adtran ISU128 with Comrex Codecs)
  2. Set the ISU128 for using the V.35 connector.
  3. Use the standard Comrex/V.35 pinning shown in the codec manual, except for the connection between pin 24 on the Comrex DB25 and pin H on the V.35 connector.
  4. Run the above signal through a type A contact closure which will close when the STL backup is required, and open when it is not.
  5. Enter the ISU128 “Dial” menu and select option 6, “store/review #”. Change selection “0” here to the number of the far end of the backup link.
  6. Back out with the “cancel” key and enter the “config” menu. Select 2, “DTE options”, and then 2, “sychnronous”.
  7. Select 6, “DTR options”, and option 5, “Dial #0 if on”. Cancel back to the main menu.

The system will now dial on the contact closure, and disconnect on the open.