Accessing the Modem Status Register (MSR) in Windows (101417)

SUMMARY

In the Microsoft Windows graphical environment, versions 3.0 and 3.1 of the communications driver (COMM.DRV) do not provide the absolute states of the following control lines:

   CTS     Clear To Send
   DSR     Data Set Ready
   RI      Ring Indicator
   RLSD    Receive Line Signal Detect; Carrier Detect (CD)

The RLSD signal is probably the most important (for modem software), and is often necessary. This article provides a method to obtain this information. Although this method is undocumented, it will be supported in future versions of 16-bit Windows.

MORE INFORMATION

The Windows Software Development Kit (SDK) documentation indicates that SetCommEventMask() can be used to determine the absolute state of these signals, using the EV_CTSS, EV_DSRS, EV_RING, and EV_RLSDS events. However, in all cases, the EV_* bits are OR'd into the EvtWord without clearing the previous value of the EvtWord. If a modem status register (MSR) bit has become set, it will not be cleared until GetCommEventMask() is called. Therefore, if a bit is set, but then changes state, the EvtWord will still indicate that it is set until GetcommEventMask() is called. (Note, however, that EV_RLSDS will never be set. This is a bug in the Windows versions 3.0 and 3.1 COMM.DRV.)

One way to obtain the state information is to read the Modem Status Register directly from the Universal Asynchronous Receiver Transmitter (UART). However, this method clears the delta bits of the MSR making them unavailable to COMM.DRV. Therefore, DO NOT DO THIS!

This information can also be obtained from COMM.DRV. COMM.DRV maintains an extended data control Block (DCB) called the DEB. Within this structure is a field called the MSRShadow. This is a shadow (copy) of the MSR on the UART.

The following code will provide direct access to the MSRShadow byte. DO NOT MODIFY THIS LOCATION!

   #define COMM_MSRSHADOW 35
   #define MSR_CTS              0x10  /* Absolute CTS state in MSR  */ 
   #define MSR_DSR              0x20  /* Absolute DSR state in MSR  */ 
   #define MSR_RI               0x40  /* Absolute RI state in MSR   */ 
   #define MSR_RLSD             0x80  /* Absolute RLSD state in MSR */ 

   LPBYTE GetCommMSRShadow(short nCid)
   {
      /* Return far pointer to MSR shadow byte */ 

      return (((LPBYTE)SetCommEventMask(nCid, 0)) + COMM_MSRSHADOW);

   } /* GetCommMSRShadow */

Once this pointer is available, it can be used as follows:

      lpMSRShadow = GetCommMSRShadow(nCid);
      bRLSD = (*lpMSRShadow) & MSR_RLSD;

      if (bRLSD)
          /** RLSD is high **/ 
      else
          /** RLSD is low **/

Althought this method is supported in a Win32-based application, you should use GetCommModemStatus() instead.