Microsoft KB Archive/101878

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Article ID: 101878

Article Last Modified on 10/31/2006


  • Microsoft Windows NT Advanced Server 3.1
  • Microsoft Windows NT Server 4.0 Standard Edition
  • Microsoft Windows NT Workstation 4.0 Developer Edition

This article was previously published under Q101878


Windows NT Remote Access Server attempts to reset its serial ports when it detects that the phone line has been hung up. The server detects a disconnect when either Data Carrier Detect (DCD) or Data Set Ready (DSR) signals are dropped by the server's modem. After a few seconds, the server sends an initialization string to the modem, putting the modem in auto-answer mode, and configures the modem options. In Windows NT 4.0, the initialization string is pulled from the particular manufacturer's Mdm*.inf file. In Windows NT versions prior to NT 4.0, the initialization string is pulled from the Modem.inf file for the particular modem type.


Data Carrier Detect (DCD)

Pin 8 on a DB 25 serial connector. This line is set high by the server modem when it is receiving a carrier signal from the remote modem. The carrier signal is a tone at a specific frequency that, by itself, does not carry any information. Information is actually conveyed by changes in the carrier signal's amplitude, frequency, phase, or some combination. When a phone line is disconnected, the carrier tone is interrupted. This causes the DCD signal to be dropped.

Data Set Ready (DSR)

Pin 6 on a DB 25 serial connector. When this signal is high, it indicates that the modem is powered on and is ready to receive commands.


If you hang up your Remote Access Server (RAS) client and your RAS server modem does not reset to receive a new call, you may be having troubles with DSR or DCD. On the server, look at the port status in the RAS Admin application. If the port status says "Connected, user authenticated" even though the modem is disconnected, this indicates a problem. Check your serial cable wiring with the RAS requirements found in the online help files. If the modem is not wired properly, the DCD or DSR signals may not be received by the server.

Some modems allow you to override DSR and DCD with dip switches and modem commands. Make sure these signals are allowed to function normally. These problems sometimes show themselves as the server not resetting the port until the AUTODISCONNECT timer expires. The line disconnects but the server does not detect it. The server leaves the line open, waiting for more data. However, more data never arrives. After the server waits for AUTODISCONNECT time (20 minutes is the default), it drops the line and sends the initialization strings that will reset the modem.

Additional query words: prodnt NTRAS

Keywords: kbnetwork KB101878