Tactical Software Tech Note

 


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Application Compatibility Options

Introduction

The Application Compatibility checkbox options in the DialOut/EZ Control Panel can help certain types of modem applications work with modem servers that do not support the COM Port Control protocol.  (When COM Port Control is available on the modem server, these options have no effect.)

DTR Is Modem Escape

The DTR Is Modem Escape option controls how the DialOut/EZ Redirector responds to the manipulation of the DTR line.  Without COM Port Control, there is no way for the redirector to relay this information to the modem server.  The most common situation with DTR is when the client software drops the line from high to low, which in most cases will cause the modem to return to the AT Command mode.  This returns an "OK" response and awaits new commands.

Windows Unimodem uses the DTR line in the course of the hangup sequence. This sequence is as follows:

  1. Drop DTR to low
  2. Wait for an "OK" which signals that the modem is awaiting a command
  3. Send "ATH" to instruct the modem to hang up

Without COM Port Control, the DialOut/EZ Redirector cannot instruct the server to drop the DTR line, leaving Unimodem without a way to initiate this sequence.  Fortunately, nearly all modems respond to an escape sequence ("+++" followed by a pause) which performs the same action as dropping the DTR line.  Therefore, when the DTR Is Modem Escape option is enabled and the DTR line drops from high to low, the redirector responds with the following sequence:

  1. "+++" is sent to the modem
  2. Data transmission is blocked for one second to enforce the pause after the "+++"

With DTR Is Modem Escape enabled, the DialOut/EZ Redirector is able to emulate the DTR dropping with the escape sequence, allowing Unimodem to successfully perform the hangup sequence.

Remember that this is the only behavioral change caused by this option.  The correct value for the DTR Is Modem Escape option is enabled when there is no COM Port Control, and disabled when there is COM Port Control.  These are the settings that the Configuration Wizard will use when it determines the capabilities of the modem-sharing server.

It is almost always counterproductive to experiment with this setting during a troubleshooting session.  The value chosen by the Configuration Wizard is usually the correct one.  There should be good reason to believe that the DTR emulation is causing application compatibility problems before attempting a fix by configuring this option manually.

Assert DCD/DSR After COM Port Open

This option controls how the DialOut/EZ Redirector presents the value of the DCD (Data Carrier Detect) and DSR (Data Set Ready) lines.  Similar to the DTR line, without COM Port Control it is not possible for DialOut/EZ to directly know the actual status of the lines. Most applications work correctly when DialOut/EZ reports both lines as low. Because many Windows modem devices will not dial properly unless these lines are reported as low, this is the default behavior of DialOut/EZ.

However, certain applications will not dial properly when the DCD/DSR lines are always held low. The dialing sequence should be as follows:

  1. Dial the number by typing the ATDT command
  2. Wait for the modem to respond with "CONNECT"
  3. Check that the DCD line is high

Note the last step. If the DCD line is low, the application considers the dialing to have failed. Note, too, that it is not necessary for the application to see a low-to-high transition during the course of dialing (which is how most modems will actually behave), but it is necessary for the DCD line to be high when pcAnywhere checks its status. Therefore, when the Assert DCD/DSR option is enabled, DialOut/EZ always reports the DCD and DSR lines as high. This enables the application's dialing sequence to complete successfully.

It can be helpful to try changing the value of this option when troubleshooting a DialOut/EZ Redirector installation. For example, if HyperTerminal dials successfully but another application does not, enabling this option is a useful experiment.

Note: There are applications that simply will not work without COM Port Control. These are beyond the help of the Assert DCD/DSR option, as some applications not only require DCD to be high, but also require that DCD start out as low and then transition to high during the dialing phase. This is behavior that the DialOut/EZ Redirector cannot emulate. Therefore, COM Port Control is required so that the redirector can report the true status of the DCD line.