Chapter 5. IBM 7171 I/O Interface to Terminals
S. 1 XON/XOFF
Pacing Enable
starting the transmitting unit on demand is called "pacing," and such devices can be used with the IBM 7171.
In a similar sense, the IBM 7171 has the ability to send pacing characters to an attached device when managing its respective input buffer resources.
Although the IBM 7171 will receive any pacing characters sent by the attached device, it will not respond to them, or send them, unless pacing recognition has been enabled. Pacing character recognition is enabled with the "Restore Pacing"
setup function, and it is disabled with the "Suppress Pacing" set up function.
These two functions are described in section 3.1.6, "Setup Functions" on page 3-3.
XON & XOFF Characters Received From a Device.
When pacing is enabled for a port, all characters received by the IBM 7171 on that port are compared to the defined XON and XOFF characters for that port. Dif-ferent ports may have difDif-ferent characters for XON and XOFF.
When the attached device sends the XOFF character, and pacing is enabled, the IBM 7171 will stop sending characters to that attached device. If a second XOFF character is received, it will just be ignored.
When the attached device sends the XON character, and pacing is enabled, the IBM 7171 will resume sending any buffered characters to that attached device. If a second XON character is received by the IBM 7171 on that port, it will be ignored, and a BEL character will be sent to that attached device.
XON & XOFF Characters Sent to the Device.
When pacing is enabled for a port, the IBM 7171 may send the attached device Pacing characters. The IBM 7171 will send the device an XOFF character when the input buffer accumulates three hundred (300) characters. If the input buffer continues to fill, another XOFF will be sent when the count reaches 308, 316,324, and 332 characters. The input buffer will overrun at three hundred forty (340) characters.
Note: The buffer count is not necessarily related to block lengths, since internal proc-essing of data commences immediately.
If an XOFF character is sent to the attached device because one of the input buffer thresholds has been reached, then an XON character will be sent to that device when the input buffer has been processed within forty (40) characters of being emptied.
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5.8.2 Control Signal Pacing
As mentioned above, the attached device can drive CTS, DSR, or RLSD inactive which stops any data transmission. Normally the CTS signal is used as a pacing control signal, and will cause the IBM 7171 port to stop transmitting momentarily.
There are time limits associated with each of these control lines, and exceeding those limits will cause the IBM 7171 to disconnect that port. CTS can be inactive for as long as ten seconds, but DSR is limited to 100 milliseconds and RLSD is limited to 500 milliseconds. The CTS value is fixed, but the DSR and RLSD values are default values in the port table in NV -RAM, and are alterable as described in section 5.10, "Disconnection" on page 5-13.
5.9 Attachment Configurations Supported.
The IBM 7171 supports three types of attachment configurations:
1. Switched Network 2. Leased Line 3. Locally Attached.
The configuration for each port is stored in NV-RAM and can be modified as shown in section 9.8, "C - Configure IBM 7171 Ports" on page 9-7. A fourth configuration is called auto-configuration. In that mode the IBM 7171 will attempt to determine which of the above three modes to use for that port.
For the auto-configuration mode to work properly, the Ring Indicator lead (pin 22) at the IBM 7171 RS-232-C port must be properly conditioned. It must not be left floating. If the modem or terminal does not drive the Ring Indicator to the marking condition (except to indicate a ringing state at which time it is driven to the spacing condition), then that signal must be jumpered to Signal Ground (pin 7) at the IBM 7171 end of the cable. Because the receiver input impedance is high, noise can be easily coupled into a floating input which can cause the IBM 7171 to disconnect that port as described in the paragraph on invalid interface lead sequences on page 5-14.
5.9.1 Switched Network
General Description of Switched Network Configuration
The switched network is the basic dial-up telephone network used with normal telephone calls. A port attached to the switched network through a modem would have a telephone number. A user at a terminal prepares the terminal and then dials the telephone number of the modem on the IBM 7171 port. The signals on the IBM 7171 port allow the modem to answer the call and signal the IBM 7171 when the line is ready.
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IBM 7171 Reference Manual and Programming Guide~,
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Cabling Requirements
To attach a modem (DCE) to the IBM 7171 port (DTE), a common one-to-one RS-232-C cable with a male connector on the DCE end and a female connector on the DTE end is used. That cable can be built from components or can be pur-chased from many vendors. A description of the cable is included in Appendix B of IBM 7171 ASCII Device Attachment Control Unit Description and Planning Guide.
General Switched Modem Specifications
5.9.2 Leased Line
Modems interfacing between the IBM 7171 port and the switched telephone network should be configured to automatically answer incoming calls. They should also be configured (if possible) to answer incoming calls only when the DTR signal is driven active by the IBM 7171 port. When DTR is not active, the IBM 7171 port is not ready to receive any calls.
Modems should be configured so the RLSD signal reflects the true state of the tele-phone carrier signal. If the remote modem is powered off for any reason, the IBM 7171 port should be able to detect it and react accordingly.
The Ring Indicator signal is expected to be driven by the modem even if it is con-figured for the auto answer mode. The IBM 7171 expects that the Ring Indicator signal will be driven active and then inactive at least once before the line is answered.
The Data Set Ready signal is expected to be driven inactive for a period of at least 200 milliseconds between sessions. In the switched network configuration (over-ride), or autoconfiguration perceived as switched network environments, the line will not be restarted until the DSR drop has occurred.
General Description of the Leased Line Configuration
Cabling Requirements
A leased line is a direct connection between two modems. The lines are not routed through the telephone company switching equipment, but rather bypass the
switches. A leased line modem sequences its RS-232-C interface signals differently from a modem attached to the switched network. The Ring Indicator, for example, is never driven active by a modem on a leased line. Those differences are impor-tant to the IBM 7171 as it initializes a session and as it disconnects a modem. The modems connected to leased lines are frequently very different from those used in the switched network. The IBM 7171 uses the differences between switched and leased line modems to determine how to sequence the RS-232-C signal lines.
To attach a modem (DCE) to the IBM 7171 port (DTE), a common one-to-one RS-232-C cable with a male connector on the DCE end and a female connector on the DTE end. That cable can be built from components or can be purchased from many vendors. A description of the cable is included in Appendix B of IBM 7171 ASCII Device Attachment Control Unit Description and Planning Guide.
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Warning: For auto-configuration to work properly, the Ring Indi-cator lead (pin 22) must be properly conditioned. If the modem does not drive Ring Indicator to the marking condition, then that signal must be jumpered to Signal Ground (pin 7) at the IBM 7171 end of the RS-232-C cable.
5.9.3 Locally Attached (Direct Connection)
General Description of Local Attach Configuration
Cabling Requirements
The RS-232-C interface was designed to allow connection between DCE's (modems) and DTE's (terminals and computers). The RS-232-C standard allows up to 2500pf of capacitance in the signal lines which means that the cable could be up to about 50 feet long. Many users have terminals within 50 feet of their processors and do not see a need for buying two modems just to go that distance.
The problem then is that they want to attach a DTE (terminal) to another DTE (computer), and that is not included in the RS-232-C standard. It has been deter-mined that by wiring the RS-232-C cable between the DTE's in a special manner, the cable can successfully emulate a DCE. In that cable, some signals are just wrapped back to other pins on their connectors, while other signals are crossed from one end of the cable to the other. Such a cable is commonly referred to as a
"null modem cable."
Users have also realized that 50 feet is a somewhat arbitrary distance. While they are in violation of the RS-232-C standard, they have been able to use cables which are much longer than the standard allows and have been able to run successfully. It is not uncommon to find cables several hundred feet long, especially when running at lower baud rates. The long cables degrade the shape of the data signals which will potentially cause more transmission errors.
The null modem cable will be specific to each computer and terminal pair although many are the same. Each cable will have to be assembled by the user since they are not available from vendors.
A formal description of these cables is found in Appendix B of IBM 7171 ASCII Device Attachment Control Unit Description and Planning Guide.