Chapter 5. IBM 7171 I/O Interface to Terminals
S.2.4 Interface Leads Monitored by the IBM 7171
The following four (4) leads are monitored at each of the IBM 7171 ports.
DSR Data Set Ready (Pin #6) CTS Clear To Send (Pin #5)
RLSD Received Line Signal Detector (Pin #8) RI Ring/Calling Indicator (Pin #22)
These signals driven by the modem reflect the states of the modem's operation and are used by the IBM 7171 to establish the connections, send and receive data and disconnect the modem.
The IBM 7171 can use the Ring Indicator to distinguish a switched network config-uration from the leased line and locally attached configuratio~s.
5.3 Transmission Mode Supported
Devices attached to the IBM 7171 ports must be Full Duplex. They must assert the Request to Send (RTS) lead when ready to establish a connection with the IBM 7171, and this signal must be held in the active state for the duration of the session.
Chapter 5. IBM 7171 I/O Interface to Terminals
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5.4 Baud Rates Supported
The following baud rates are supported by the IBM 7171.
• 50
• 75
• 110
• 134.5
• 150
• 300
• 600
• 1200
• 1800
• 2000
• 2400
• 3600
• 4800
• 7200
• 9600
• 19200
The highlighted baud rates are supported for "autobaud detection" described below. Any port can be assigned any of the above rates by making an entry into Non-Volatile RAM (NV-RAM). Section 9.8, "C - Configure IBM 7171 Ports"
on page 9-7 describes how to configure a port including setting the baud rate.
5.4.1 Autobaud Detection Option
The IBM 7171 is able to detect the baud rate of a terminal when the connection is established as long as the rate is one of the highlighted rates listed above. If a port is configured for auto baud detection, the IBM 7171 will monitor the first character transmitted by the attached terminal. The terminal must transmit a carriage return, and from that the IBM 7171 can determine the baud rate which will be used for the remainder of the session.
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IBM 7171 Reference Manualand Programming Guide5.5 Autoanswer Mode
The IBM 7171 operates in an auto answer mode. When it is available and online, it drives the DTR output control lead active at the RS-232-C interfaces. When DTR is held active, a modem with the auto answer feature will automatically answer incoming calls to the IBM 7171.
At certain times (when the IBM 7171 is powered off for example) DTR is not driven active. Switched network modems should not answer incoming calls when DTR is inactive.
5.6 Port Initialization
5.6.1 Monitoring of Control Leads for Initializing a Port
When the IBM 7171 is powered on and running, it monitors the input signals of each port.
During the initialization phase of each port, the IBM 7171 drives DTR and R TS active. When it is ready to begin a session, the attached device must bring the proper input signals active as described in the EIA RS-232-C standard. This attached device must be either a modem or another device which appears to be a modem. When the signal states satisfy the requirements of both the IBM 7171 and the attached device, the initialization phase is complete, and the IBM 7171 will enter the operational phase.
Initialization Control Leads Sequence
By driving DTR and R TS active, the IBM 7171 notifies the attached device that it is available for use. The attached modem must use DTR to enable auto answer mode, while R TS is used to enable the drivers in the modem.
The IBM 7171 supports three types of configurations: switched network, leased lines, and direct connection. During its initial power on sequence, the IBM 7171 references NV -RAM, to determine the type of configuration for each port.
NV-RAM contains a table of information for each port, and one entry in that table defines the type of attachment. The default entry is "auto-configuration," but that can be overridden as described in section 9.8, "C - Configure IBM 7171 Ports" on page 9-7. In the auto-configuration mode or switched network (dedicated) mode, the IBM 7171 monitors and reacts to the Ring Indicator signal sequencing. The Ring Indicator lead (pin 22) must be driven to a valid state. If the attached device does not drive the signal to a defined active/inactive level, it must be jumpered to Signal Ground (pin 7), or the configuration should be specified as described in section 9.8, "C - Configure IBM 7171 Ports" on page 9-7.
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5.7 Operational Phase
The operational phase is the normal running state of each of the IBM 7171 ports.
It is entered when the RS-232-C control lead handshaking of the initialization phase has been completed. The operational phase is evident when the terminal keystrokes are echoed back to the display, or messages are displayed. It is possible for the port to be in the operational phase, but for various reasons the IBM 7171 is unable to communicate with the host operating system. The user will normally see one of several indications when his port enters the operational phase.
• On a port for which the baud rate and terminal type have been defined, the host operating system logo will appear if the system is available.
• If the baud rate is defined, but the terminal type is not, the message "ENTER TERMINAL TYPE" will appear.
• If the terminal type is defined, but the baud rate is not, no indication will be seen. The user will have to type the CARRIAGE RETURN key, and then the host operating system logo will appear if the host connection is up and running.
• If neither the baud rate nor the terminal type is defined no indication will be seen. The user will have to type the CARRIAGE RETURN key, and then the message "ENTER TERMINAL TYPE" will appear.
Once the IBM 7171 enters the operational phase for any port, it will begin trans-ferring the characters and commands to and from the attached device. The RS-232-C signals are monitored and will affect the data transfer. For example, a printer may accept a line of characters at a high rate, but then will not accept any more until that line is printed. The printer will drive the CTS signal inactive when it cannot accept any more data, and will drive it active when it is ready for more characters. The IBM 7171 will react properly to those control signals.
5.7.1 Control Leads During Operational Phase
5.8 Pacing
During the operational phase, the control signals are monitored. There are two normal conditions which will cause these signals to be driven inactive. The first is caused by the attached device wanting to stop the data transfer for a moment.
That stopping of the data transfer is called "pacing" and is covered in the following section. The second is caused by the attached device signalling the end of a session. When DSR, CTS or RLSD is driven inactive, the IBM 7171 starts a timer for that port. If the timer runs out before that signal becomes active again, the IBM 7171 will disconnect that port. More details on disconnection are found in the section 5.10, "Disconnection" on page 5-13.
Some attached devices, such as printers, may not be able to handle characters as fast as they can be received. Some of those devices have the ability to send a char-acter back to the transmitting unit asking it to stop momentarily while the receiving unit catches up. When it has caught up, the receiving unit can then send another character signalling the transmitting unit to continue. This form of stopping and