Tactical Software Tech Note


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The COM Port Control Protocol RFC 2217


Tactical Software redirectors for modem servers or serial servers support a public protocol known as "COM Port Control," specified by IETF RFC 2217.  COM Port Control overcomes certain limitations of networked modem servers that can otherwise cause compatibility issues with some PC applications. 

Many PC applications can use modem/serial servers that don't support the COM Port Control protocol.  For those applications that need the extra features provided by the protocol, Tactical Software redirectors do include support for it.   Naturally, for the protocol to benefit the application, the server must support the protocol as well.

The User Guides for Tactical Software redirectors include suggestions on how to identify PC applications that need to use servers that support the COM Port Control protocol.

The Additional Capability of Servers Supporting the COM Port Control Protocol

COM Port Control is an extension of the Telnet protocol, so it will only be available if the Tactical Software redirector is configured to use Telnet.  When the redirector is so configured, it will always request that the connection use COM Port Control.  If the server agrees to use COM Port Control, only then will it become active on the current connection to the server.  The redirector indicates a successful negotiation of COM Port Control by displaying three extra "lights" next to the active COM port name in the Port Monitor window.

On a connection without COM Port Control, the client application is able to interact with a server only by sending and receiving data.  Although being able to send and receive data is fundamental, it is not the only way to interact with the server.  Serial ports on servers have control signals that can be manipulated by the client software (such as DTR), and report status signals that can be monitored by the client software (such as DSR, DCD and RI).  In addition, there are several parameters that control how the server's serial port communicates with the redirector, including baud rate, data length, number of stop bits and parity type.  Without COM Port Control, the client software is unable to affect any of these parameters or signals, and the server cannot report the status signals to the client software.

Alternatives to Servers Supporting the COM Port Control Protocol

If a PC application requires special COM port settings, it may still be able to work with a server that does not support the COM Port Control protocol.  Often, an effective workaround is to configure some server ports specifically for the use of such an application. For example, some modems on a server might need to be configured for 1200 E-7-1 in order to work with an alphanumeric paging application; but those modems from then on would likely work only with that one application, given the specialized nature of those settings.

COM Port Control Protocol Features Available in Tactical Software Redirectors

The COM Port Control protocol includes a number of features.  Some are relevant to Tactical Software redirectors, others are not.  The set of protocol features that are supported have proven sufficient for nearly all applications with a need for some degree of port control.

Tactical Software redirectors use the COM Port protocol to provide the following port controls:

  • Baud rate (e.g., 9600)
  • Data Length (e.g., 8 bits)
  • Parity bit (e.g., none)
  • Stop bits (e.g., 1 stop bit)
  • DTR signal (often used to disconnect a call)

Tactical Software redirectors use the COM Port protocol to report the following port conditions:

  • DCD (signals that a call succeeded)
  • DSR (signals that the modem is ready)
  • RI (signals that the phone is ringing)

Tactical Software redirectors may support additional elements of the COM Port Control protocol, as documented in Tactical product release notes.