Problems with RS232 COM callback events in LabWindows 4.01p2

[Telephonics]

I am using CVI/LabWindows 4.01p2 to create a program that handles
calling another modem, and receiving calls from other modems. In order
to know whether or not a connection has been made, I use the
event via a COM Callback. This works perfectly fine with an external
modem or PCI modem. However, with a PCMCIA modem or built-in modem in a
laptop, it isn't very consistent. Calling a station and having their
modems negotiate to connect seems to generate the event.
However, upon receiving a call, the modems can be heard negotiating and
they do establish a connection, but the event never gets triggered (the
callback never occurs).

What can be the problem? Is there a patch
for this? Does a newer
version of LabWindows correct this problem? Or how else can I determine
that there has been a change in state of connection? Or how can I look
at any given time to see if there is a data connection esablished or not?

[Jeremiah Cox]

Hello Telephonics,
LabWindows/CVI should be using the same calls to the operating system regardless of the type of modem or serial port it is using. Possibly the built in or PCMCIA modems are not triggering the RLSD event. There is generally a setting on a modem to have either the RLSD follow the carrier detect OR always stay high or low. If these modems are configured to always have RLSD stay high or low, they would not be triggering the RLSD event upon carrier detect. I would recommend consulting the manufacturers documentation for further information on this option. If you would like to further troubleshoot this, I would recommend the Portmon software available from http://www.sysinternals.com. It is like NI-Spy for serial ports (it allows you to watch all of the
serial related Operating System calls/events). As a note, CVI 5.5 and higher now has the GetComLineStatus() function which allows you to probe the status of the CTS, DSR, RI, and RLSD lines. If you are interested in upgrading, please see http://www.ni.com/upgrade.

Jeremiah Cox
Applications Engineer
National Instruments
http://www.ni.com/ask