ZYPCOM 8/16/96

    1 Dial-up Access to DEC VAX Computers using Zypcom Modems

    This Technical Note provides instructions for connecting Zypcom Z-Series modems to DEC VAX computers running under the VMS Operating System.

    Important: In all configurations, the cables used to connect the modem to the DTE (whether the PC or the VAX port) must support specific signalling requirements, with fully-shielded connectors. For a description of each pin and its use in the various communication modes, see section 3, Modem-to-DTE Cable Requirements, at the end of this technical note.

    1.1 Remote System Configuration

    This section explains the procedures for configuring the modem attached to the remote terminal, and special software considerations when using a PC as a VT terminal.

    1.1.1 Terminal/PC Modem Setup

    A VT terminal can access the DEC VAX over dialup lines. Because terminals cannot use software, the modem must be configured directly using the Standard AT Command Set. Set the terminal port speed to 19200 bps for Zypcom Z-Series. Because VT terminals support only XON/XOFF flow control, configure the modem to match this setting. Connect the modem to the terminal via an RS-232 cable and issue the following commands.

    &F Restores factory settings
    &C1 DCD tracks actual carrier status
    &D2 Modem hangs up on loss of DTR
    &K4 Enables XON/XOFF flow control
    &W0 Stores settings in User Profile 0

    When accessing a VAX from a remote PC, communications software performs
    the modem configuration and the above commands are unnecessary.

    1.1.2 Terminal/PC Setup

    Because a DEC VAX normally provides asynchronous terminal ports, dial-up access from a PC requires no special hardware other than a modem and serial port. The communication software must also provide VT terminal emulation and should be configured for Z-Series.

    To benefit from Z-series system product adaptive compression, the computer must send data to the modem faster than the modem transmits it over the telephone line connection. Set the computer or terminal port speed to 19200 bps for the Z-series. The difference in speeds over the communication link and the need to retransmit incorrectly received data make it necessary to regulate the flow of data between the computer and modem. The Z-series system products support three local flow control methods: RTS/CTS, XON/XOFF, and Transparent XON/XOFF. One of these flow control options should be used in conjunction with the method offered in the communication software. For a complete discussion of local flow control, see Zypcom manual.

    1.2 Host Configuration

    This section explains the procedures for configuring the modem attached to the host and the port to which the modem will be connected.

    1.2.1 VAX Modem Setup

    The Z-series system product connected to the VAX port must be
    configured for XON/XOFF flow control to match the method supported by
    the VAX. To benefit from Z-series system product compression, configure
    the VAX port to a fixed speed higher than the speed of the telephone
    connection, as detailed in section 1.2.2. In order to avoid speed
    mismatches when other modems access the VAX, enable the
    automatic speed buffering feature.

    Issue the following commands to configure the modem.

    &F Restores factory settings
    M0 Turns off modem's speaker
    &D2 Modem hangs up on loss of DTR
    &S1 DSR operates according to RS-232 specifications
    &C1 DCD tracks actual carrier status
    Q1 Disables return of result codes
    E0 Disables command state character echo
    &K4 Enables XON/XOFF flow control
    &W0 Stores settings in User Profile 0

    These commands can be issued using an asynchronous terminal, or a personal computer running an asynchronous communications package such as Smartcom II( or Smartcom III*. The modem will determine the transmission rate to the VAX port by examining the speed at which the last AT command was received. Be sure to issue the commands at a speed that will match the VAX port speed setting. This should be 19200 bps for the Z-series products.

    Because XON/XOFF flow control is not compatible with XMODEM file transfer protocols, use Kermit when uploading and downloading files. If your VAX system supports RTS/CTS flow conrol, replace the &K4 command with &K3. The above command string assumes the VAX will keep the DTR signal on. If this is not the case, replace the &D2 command with &D0.

    1.2.2 VAX Port Setup

    To access the VAX, the port to which the modem will be attached must be configured. Under the VMS Operating System, include the following Set Terminal command in the system manager's directory:


    PERM Sets the parameters permanently
    SCOPE Establishes the terminal as a video terminal
    HOSTSYNC Implements XON/XOFF flow control
    MODEM Indicates whether terminal is attached via cable or modem
    DIALUP Specifies terminal as dialup terminal
    HANGUP Instructs modem to hang up after user logs out
    SETSPEED Allows /SPEED qualifier to be used to change terminal speed
    /SPEED=rate Specifies rate at which port sends and receives data. Should be set to 19200 for Z-series.
    /NOAUTOBAUD Disables automatic baud rate detection
    dd Device type
    c Controller
    n Port number

    1.3 Modem-to-DTE Cable Requirements

    Even though modem connections are made with a DB25 connector, some cables may actually use as few as three pins: Tx data, Rv data, and ground. Z-series error-control communications require RS-232 signals not used in normal asynchronous communication. To ensure that all necessary signals are transmitted, verify that all cables used in the set ups described in this technical note support the pin configuration outlined below:

    Pin No. Circuit Direction Description
    1 AA N/A Protective Ground
    2 BA To Modem Transmit Data
    3 BB From Modem Receive Data
    4 CA To Modem Request To Send
    5 CB From Modem Clear to Send
    6 CC From Modem Data Set Ready
    7 AB N/A Signal Ground
    8 CF From Modem Data Carrier Detect
    12 CI * From Modem Select Alternate Rate
    15 DB ** From Modem Transmit Clock
    17 DD ** From Modem Receive Clock
    20 CD To Modem Data Terminal Ready
    22 CE From Modem Ring Indicator
    23 CI * From Modem Select Alternate Rate
    24 DA To Modem Transmit Clock (DTE Source)

    * CI is output on both pin 12 (Bell 212A assignment) and pin 23 (EIA RS-232C assignment).

    ** These signals are used for synchronous communication.

    ZYPCOM 8/16/96


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