Visual Basic Communications Demonstration Program: COMMDEMO

The Visual Basic COMMDEMO program was designed to demonstrate how to call
the communications related Windows API functions to establish basic serial
communications. The COMMDEMO sample program is an example of a simple
terminal emulation program. This program was designed mainly for
educational purposes and therefore is not an example of a full featured
terminal emulation program. The concepts presented in this sample program
should provide a solid foundation from which you can implement serial
communications into your own Visual Basic application.

The sample program contains comments where needed indicating other possible
values for variable settings to allow you to easily modify the sample
program. For example you could modify the sample program to open any
communications port; or change the line and event settings to your own
custom settings.

The COMMDEMO sample program demonstrates the following:

o How to open and connect to the communications port
o How to read from the communications port
o How to write to the communications port
o How to change the port settings such as the baud rate
o How to change the event settings such as detecting when a character
is received.
o How to trap and process errors after calling the communications
related API functions.

Cross-Reference of the Main Features
---------------------------------------------------------------------------
The following list provides references to the main functional areas of the
program. The following reference should make it easier for you to find a
desired feature related to communications. All of the procedures listed
below are contained in the main form called COMMDEMO.FRM.

o How to open and connect to the communications port:

The code to initialize the communications port line settings is located
in the Form_Load event procedure. The communications port line settings
are initialized by calling the Windows API function BuildCommDCB.

The code to open the communications port is located in function
CommConnect. Function CommConnect calls the functions SetLineSettings
and SetEventSettings which demonstrate how to set the communications
line (e.g., baud rate, stop bits, hardware handshaking) and event (e.g.,
ring detect) settings respectively. The function CommConnect calls the
Windows API function OpenComm to open COM1. The function CommConnect
contains a call to the Windows API function SetCommState to set the
communications line settings. The function SetEventSettings contains a
call to the windows API function SetCommEventMask to enable
communications event trapping.

Function CommConnect is called from the MNU_Connect_Click event
procedure for the Connect! menu.

o How to read from the communications port

The code that reads in all characters waiting in the communications
receive buffer is located in the Timer_CheckInBuf_Timer event procedure.
If a successful connection is made, the timer is initiated from the
CommConnect function . The timer is disabled in the CommDisconnect
function. The Timer_CheckInBuf_Timer event procedure contains a call to
the Windows API function ReadComm to read the characters waiting in the
receive buffer.

o How to write to the communications port

The Text_TransmitWin_KeyPress event procedure contains the code which
writes characters to the communications port as they are entered in the
transmit window. This event procedure contains a call to the Windows
API function WriteComm to write characters to the transmit buffer.

o How to change the port settings such as the baud rate.

The MNU_BaudRate_Click event procedure for the Baud menu demonstrates
how to change the baud rate. The baud rate can be changed while the
communications port is opened or closed. COMMDEMO demonstrates how to
change between baud rates of 1200, 4800 and 9600. The
MNU_BaudRate_Click event procedure demonstrates how to call the Windows
API function SetCommState to change the baud rate line setting.

o How to change the event settings such as detecting when a character is
received.

The MNU_Events_CharDetect_Click event procedure for the Events menu
demonstrates how to enable or disable character received events. When
character detect event trapping is enabled, a message is displayed in
the Communications Status window each time a character is received. The
character detect event trapping can only be enabled or disabled when the
communications port is opened. The MNU_Events_CharDetect_Click event
procedure contains a call to the Windows API function SetCommEventMask
to change the character detect event setting.

o How to trap and process errors after calling the communications related
API functions.

The function GetOpenCommError demonstrates how to process errors related
to opening the communications port. This function simply interprets the
return code from the Windows API function OpenComm. The
GetOpenCommError function is called from the CommConnect function.

The function ProcessCommError demonstrates how to call the Windows API
function GetCommError to process general communications errors. This
function is called from the procedures Timer_CheckInBuf_Timer and
Text_TransmitWin_KeyPress when reading from and writing to the
communications port respectively.

Cross-Reference to the Windows API functions called by COMMDEMO
---------------------------------------------------------------------------
Below is a reference to the location(s) in the code where the
communications related Windows API functions are called. All calls to the
Windows API functions are located in the main form called COMMDEMO.FRM.
The declaration for the Windows API functions and related Type ... End Type
structures is located in the global module called COMMDEMO.GLB.

Windows API function Location in COMMDEMO.FRM
-------------------- --------------------------------------------
BuildCommDCB...............Form_Load event procedure
OpenComm...................CommConnect
CloseComm..................CommConnect
..................CommDisconnect
FlushComm..................CommDisconnect
ReadComm...................Timer_CheckInBuf_Timer event procedure
WriteComm..................Text_TransmitWin_KeyPress event procedure
SetCommState...............SetLineSettings
...............MNU_BaudRate_Click event procedure
SetCommEventMask...........SetEventSettings
...........MNU_Events_CharDetect_Click event procedure
GetCommError...............ProcessCommError
...............Timer_CheckInBuf_Timer event procedure