FIX: COMM.DRV Locks Up with Full Duplex Communications

The information in this article applies to:

• Microsoft Windows Device Driver Kit (DDK) for Windows version 3.0

SYMPTOMS

When it is subjected to a heavy load during full-duplex communications, the transmit portion of the Microsoft Windows communications driver (COMM.DRV) stops working (no characters are sent). This problem occurs in real, standard, and enhanced modes. The receive portion of the communications driver is unaffected.

CAUSE

COMM.DRV encounters a "deadlock" situation during full-duplex operation.

RESOLUTION

To work around this problem, an application must detect whether deadlock has occurred. This can be done by setting a timer in Windows that sends a message to the application every one or two seconds. When the application receives timer message, check the txQueue count in COMM.DRV. If the count has not changed since the last message (indicating that no characters have been sent), the application calls the FlushComm function and resends the data that was not transmitted. The alternative, which is not recommended, is to close the COM port and then reopen it. No ill side effects, other than some performance degradation, have been observed from using these techniques.

For applications that are very transmission intensive -- for example, sending data through a full-duplex communications line for an extended period of time -- the 1 to 2 second wait might be considered too long. For these applications, shorten the checking interval as desired. However, if the interval is too short, the application may detect deadlock when it has not actually occurred. Unnecessary calls to FlushComm and resending data would degrades the performance of the application.

STATUS

Microsoft has confirmed this to be a problem in Windows version 3.0. This problem was fixed in the Windows for Workgroups 3.11 COMM.DRV and above.