end of the cable.
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5.10 Disconnection
When the IBM 7171 terminates a session, it disconnects or drops the line. There are two classes of disconnection. In the first, the host computer has initiated the disconnection. The user may have given the host the command to logoff for example. In the second, the IBM 7171 has recognized a problem and has discon-nected the line itself. For example one of the RS-232-C signal lines may have gone inactive for a period exceeding the timeout limit. The first is called "host initiated"
and the second is called "unsolicited." While either class is acceptable for discon-nection, there are differences in the way in which the IBM 7171 handles them. If the disconnection is unsolicited, one cause could be trouble in the telephone
system. If a new connection is then established, the IBM 7171 must make sure that some new user does not dial into the middle of the previous user's session.
5.10.1 Method of Disconnection
The IBM 7171 disconnects a port by driving DTR and R TS inactive on that port.
The modem should respond by driving its lines inactive. Normally these signals are held inactive for about four seconds, but in certain situations the disconnection sequence may be repeated.
5.10.2 Types of Disconnection
Unsolicited
• Loss of DSR beyond limits.
The IBM 7171 monitors the DSR signal from the modem. If and when DSR is driven inactive, a timer starts on that port. If DSR is not yet active when the timer runs out, the IBM 7171 will disconnect that port. That timer is set to 20 milliseconds during the initialization phase and defaults to 100 milliseconds in the operational phase. The operator has the ability to change operational phase value if desired. Use the Maintenance Facility command:
sw DCOO:0290 XXXX
where XXXX is the new delay value expressed in hexadecimal (e.g. for a new delay value of 600 milliseconds XXXX would be 0258).
Some modems drive DSR with a mechanical relay. Since relays give erratic signals as they switch, the IBM 7171 will allow 50 milliseconds during initial-ization for the bouncing to stop. A typical relay will bounce for a couple of milliseconds.
If the modem is powered off, or if the cable becomes loose, the loss of.DSR will cause the unsolicited disconnection.
Loss of RLSD beyond limits.
The IBM 7171 monitors the RLSD signal from the modem. This signal is sometimes called DCD (Data Carrier Detect) and is active when the modem attached to the IBM 7171 port is detecting the carrier signal from the other
Chapter 5. IBM 7171 I/O Interface to Terminals
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modem. If the carrier is lost, RLSD indicates that the other modem has been turned off, the cable is disconnected, or some similar problem. If the IBM 7171 detects that the modem has driven RLSD inactive, it will start a timer. If RLSD is not driven active again before the timer runs out, the IBM 7171 will disconnect that port. The IBM 7171 is delivered with the RLSD timer set to 500 milliseconds for all ports, but the operator can change it if desired. Use the Maintenance Facility command:
sw DCOO:0292 XXXX
where XXXX is the new delay value expressed in hexadecimal (e.g. for a new delay value of 600 milliseconds XXXX would be 0258).
• Loss of CTS beyond limits.
The IBM 7171 monitors the CTS signal from the modem. During the line initialization of some of the higher speed modems, several seconds may elapse while the modems condition the transmission line. CTS can also be used by the modem (or null modem attached device) to pace the data being sent to it.
When the IBM 7171 detects that CTS is being driven inactive at a port, it starts a ten second timer. If CTS is not driven active again before that timer expires, the IBM 7171 will disconnect that port.
• Invalid interface control lead sequences.
Certain unexpected control signal sequences will also cause the IBM 7171 to disconnect a port. If, for example, a port is in the operational phase with data flowing back and forth between the IBM 7171 and a terminal, and the Ring Indicator should become active indicating a new incoming call is being received, the IBM 7171 will recognize an inconsistent state on that port and will disconnect it.
Terminal Powered Off
If a modem or directly attached terminal is powered off, the RS-232-C signals on that device will drift to their inactive states. The IBM 7171 port will detect them and will start the timers. When the first of the timers runs out, the IBM 7171 will disconnect that port.
• Abort Timer. (Switched Network or Autoconfigure)
The line will be disconnected if the RLSD signal is not activated within thirty (30) seconds of answering an incoming call in the Switched Network (over-ride) or Autoconfigure mode. The Abort timer is started whenever the line is configured switched network, or is percieved as switched network when in Autoconfigure mode.
There are two (2) ways of starting the timer, depending on the configuration mode defined:
Switched Network (override) configuration.
DSR activated by attached device.
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Host Initiated
Autoconfiguration (default)
Ring Indicator cycled on then off during initialization handshaking. (One complete ring cycle.)