Q140525: HOWTO: Receive from the Serial Port by Using MScomm32.ocx

Article: Q140525
Product(s): Microsoft FoxPro
Version(s): 
Operating System(s): 
Keyword(s): kbcode kbnokeyword kbvfp300 kbvfp500 kbvfp600
Last Modified: 29-JUL-1999

-------------------------------------------------------------------------------
The information in this article applies to:

- Microsoft Visual FoxPro for Windows, versions 3.0, 5.0, 6.0 
-------------------------------------------------------------------------------

SUMMARY
=======

This article gives you two techniques you can use to receive data from the
serial port using the Mscomm32.ocx control. The first uses an event-driven
method and does not require you to poll the serial port to check for the
presence of received characters. This technique allows the most flexibility and
does not require extensive coding to prevent buffer over-runs. The second
technique requires you to poll the input buffer periodically to check for the
presence of received characters. This article describes these two techniques and
provides examples for each.

MORE INFORMATION
================

You need to set the following properties regardless of which technique you use:

- CommPort property: Set this numeric property to the desired communications
  port. Valid values are 1, 2, 3, or 4 depending on the serial ports available
  and the configuration of the individual computer. These values correspond to
  Com1, Com2, Com3, and Com4 respectively.

- Settings property: Set this character property to the baud rate, Parity, Data
  Bits, and Stop Bits required by the device connected to the serial port. This
  property is a character, comma-delimited list. For example, to set the serial
  port to 14,400 baud, Even Parity, 7 Data Bits, and 1 Stop Bit, set the string
  to:

  

     14400,E,7,1

- PortOpen property: Set this logical property to true to open communications
  to the serial port. You can also check this property to determine if the port
  opened correctly.

Technique One: Event-Driven Receive
-----------------------------------

The event Driven technique generates an OnComm event when there are characters
waiting in the input buffer. Also, the CommEvent property will contain a numeric
2. For the OnComm event to be triggered, you must set the Rthreshold property to
a value other than zero (its default). The most common setting for the
rthreshold property is 1, meaning that the OnComm event is triggered if a
minimum of one character is waiting in the input buffer.

For example, you can place the following code in the OnComm event to append
received data to a property of a form called mybuffer:

     Procedure MyCom.OnComm
       IF This.CommEvent = 2
         ThisForm.mybuffer = ThisForm.mybuffer + This.Input
       ENDIF
     ENDPROC

Technique Two: Polling the Input Buffer
---------------------------------------

Polling the input buffer requires that the program periodically stop what it is
doing and check to see if there are characters waiting in the input buffer. When
using this technique, leave the Rthreshold property at 0 (its default value),
and check the InBufferCount property to see if it is greater than zero, which
indicates that there are characters waiting in the buffer.

NOTE: Using a technique such as checking the length of the Input property results
in lost characters because as soon as the Input property is accessed, the Input
buffer is emptied. Use the InBufferCount property instead.

Assuming the Mscomm control is on the form and has the name MyCom and that there
is a form property named mybuffer, the following code illustrates how to poll
for waiting characters:

     Procedure myform.myproc
       IF Thisform.MyCom.InBufferCount > 0
         Thisform.mybuffer = Thisform.mybuffer + Thisform.MyCom.Input
       ENDIF
     ENDPROC

The procedure code could be called in a timer method to facilitate checking for
characters at semi-regular intervals. However, if large amounts of data are
expected to be received from the serial port, Technique One will reduce the
chance of over-running the input buffer.

Additional query words:

======================================================================
Keywords          : kbcode kbnokeyword kbvfp300 kbvfp500 kbvfp600 
Technology        : kbVFPsearch kbAudDeveloper kbVFP300 kbVFP500 kbVFP600
Issue type        : kbhowto

=============================================================================

THE INFORMATION PROVIDED IN THE MICROSOFT KNOWLEDGE BASE IS PROVIDED "AS IS" WITHOUT WARRANTY OF ANY KIND. MICROSOFT DISCLAIMS ALL WARRANTIES, EITHER EXPRESS OR IMPLIED, INCLUDING THE WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE. IN NO EVENT SHALL MICROSOFT CORPORATION OR ITS SUPPLIERS BE LIABLE FOR ANY DAMAGES WHATSOEVER INCLUDING DIRECT, INDIRECT, INCIDENTAL, CONSEQUENTIAL, LOSS OF BUSINESS PROFITS OR SPECIAL DAMAGES, EVEN IF MICROSOFT CORPORATION OR ITS SUPPLIERS HAVE BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES. SOME STATES DO NOT ALLOW THE EXCLUSION OR LIMITATION OF LIABILITY FOR CONSEQUENTIAL OR INCIDENTAL DAMAGES SO THE FOREGOING LIMITATION MAY NOT APPLY.

Copyright Microsoft Corporation 1986-2002.