Using Virtual COM Ports

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Using Virtual COM Ports

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Access by Windows Modem Devices


Most modem applications do not use COM ports directly, but instead access Windows modem devices, that in turn use virtual COM ports.


When installing the DialOut/EZ Redirector, create a Windows modem device for each virtual COM port which will access the modems.


In the dial-out application, find the settings, preferences, or options that control the selection of the Windows modem device that will be used.  Select the modem devices that are installed on virtual COM ports.  The application can now use TAPI (the Windows Telephony API) to initialize, dial, and communicate with the Windows modem device.



Direct Access as COM Ports


To use virtual COM ports directly from the application, find the settings, preferences, or options in the application that will allow configuration of the COM ports that are going to be used.  Virtual COM ports will appear, along with any local hardware COM ports.


Select the virtual COM ports to access the modem on the modem server.  The application will now be able to use this COM port in the same way that it would use a local COM port.


Some older Windows applications will not recognize COM ports higher than COM4.  If using such an application, then create virtual COM ports in the COM1-4 range.


A common programming error may cause the application to fail when attempting to open COM10 and above.  These applications will be limited to using virtual COM ports COM1-COM9.



Hunt Groups


The term hunt group (sometimes called a pool or a rotary) refers to the ability of a modem server to respond to a connection by choosing an unused modem.


A modem server typically has only one hunt group, and makes it available on a single TCP port number.  Each time a redirector connects to that TCP port, the server will find an unused modem from the hunt group, if one is available.


When using a hunt group, configure each virtual COM port to use the TCP port number of the hunt group.


If it is possible that the hunt group will be overloaded, then test how the application and modem server will respond if no modem is available in the hunt group, when one is requested by the redirector.


Remember that all devices in the hunt group must have similar characteristics and capabilities, so that applications using the hunt group will function properly, no matter which modem is selected.  It is strongly recommended that the same modem hardware be used for every member of the hunt group.


Various modem servers will differ in how they allocate unused modems to redirectors.


By default, TacServe will share its modems out on TCP port 7000 and Cisco routers will use TCP port 7001.




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