Configuring Virtual COM ports

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Configuring Virtual COM ports

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Modifying Application Settings


For a Windows application to use the COM/IP Redirector, it must be configured to use COM/IP COM ports.  The general procedure is to find the settings, preferences, or options setting in the application that will specify the COM port or modem device to be used by the program.  Select a COM/IP COM port, or a COM/IP modem device.  Some older Windows applications will not recognize COM ports higher than COM4.  If using such an application, then create COM/IP COM ports in the COM1-4 range.


Any baud rate may be specified in the communications software, but be aware that the COM/IP Redirector will inform the remote Telnet server of the baud rate that is specified.  The recommended approach is to choose either a very high baud rate (e.g., 38400 or 57600) for TCP/IP connections through a network, or the actual baud rate that will be used.  The throughput to the COM/IP port will solely depend on the bandwidth of the TCP/IP connection, not the baud rate setting, unless the S1021 register is modified.  View Appendix C. COM/IP S-Registers for more information.


Transferring binary files through a Telnet connection may be difficult because the interpretation of binary mode and carriage-return (CR) padding varies widely among Telnet implementations.  To get binary file transfers to work properly, experiment with the S1002 and S1005 registers to find the best settings.  On one UNIX-based Telnet server, S1002=1 and S1005=0 worked best with binary files, although many files will transfer fine with S1002=0 and S1005=3.  Also, switching to the YMODEM protocol, rather than the ZMODEM protocol, may have better results.  View Appendix C. COM/IP S-Registers for more information.



Making Outbound Connections


The COM/IP software must be correctly installed and configured.  The Windows application must be configured to use a COM/IP COM port.  The destination of the connection must be accessible on the network.  For most communications software, the IP address and TCP port number of the destination must be known.


Although COM/IP can process the DNS name of the remote system, many Windows applications may fail to properly provide a name in an ATD command.  Therefore, the most reliable way to specify the connection destination is with the IP address, and to do so in a form that will not be altered by the communications software.


COM/IP will accept a simple 12-digit format for the IP address, which should correctly pass through Windows applications to COM/IP.  To translate an IP address into this format, follow this procedure: For each of the four numbers in the IP address, add leading zeros to make each number three digits long (if necessary), and remove the dots between the numbers in the IP address.  This will leave a single 12-digit number. Examples:


The IP address translates to 010182050003.
The IP address translates to 010182002130.


IP addresses that are specified in this format will appear as a phone number to the communications software, but will be transformed into the correct IP format by the COM/IP software.


By default, COM/IP uses port 23 (the Telnet port) on outgoing connections.  This can be overridden with the COM/IP S-register S1009 register.  If connecting to a port that is different from the S1009 setting, then specify the port number in the phone number of the remote host.


This can be done by adding an additional five digits containing the TCP port number using leading zeros, if necessary.  For example, port 6000 would translate into the following 17-digit equivalent:  01018205000306000.



Receiving Inbound Connections


By default, COM/IP is not configured to listen for incoming calls.  To enable incoming calls, it is recommended to add S1008=23 to the Init String field, so that COM/IP will listen for incoming connections on port 23, which is the standard port for Telnet connections.  Most PCs do not have software that will accept incoming Telnet connections, so this port should not conflict with the existing software.  In the case that port 23 may be an issue, it is recommended to use a port number higher than 4096.


If there is a firewall between the PC and the computer initiating the connection, then it must be configured to permit inbound connections on the TCP port number(s) that will be used.  The Windows application must be set to answer incoming data calls.


COM/IP can support several ports that are waiting for incoming connections on a single TCP/IP port number.  For example, specify S1008=23 on multiple COM/IP COM ports, such as COM5, COM6, COM7 and COM8.


When more than one COM/IP COM port listens on a single TCP/IP port number, and a connection comes in on that port number, COM/IP will route the connection to the lowest-numbered COM port that is available.  Continuing with the above example, if COM5 and COM7 are currently active on other connections, then an incoming connection on port 23 would be routed to COM6, which would then emit the RING message until the application answers.




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