Under DOS, it was possible to directly access these ports. However, Windows 95 and above forbid direct hardware access. Therefore, this page exists to help get around windows.
Generic References
Basic Commands
Borland provides the following Delphi FAQ's (delphi-faq.zip 915K).
Writing a Port
PhoneNumber := 'ATDT 1-555-555-1212' + #13 + #10;
CommPort := 'COM2';
hCommFile := CreateFile(PChar(CommPort),
GENERIC_WRITE,
0,
nil,
OPEN_EXISTING,
FILE_ATTRIBUTE_NORMAL,
0);
WriteFile(hCommFile,
PChar(PhoneNumber)^,
Length(PhoneNumber),
NumberWritten,
nil)
This code works for comports 1 thru 9, but not for ports 10 and higher. Instead, use code similar to the following
CommPort := '\\.\COM33';
Reading a Port
You must configure the TimeOutBuffer. I don't know what good values should be, so I used 300.
var //hCommFile : THandle; // Global variable set elsewhere TimeoutBuffer: PCOMMTIMEOUTS; begin GetMem(TimeoutBuffer, sizeof(COMMTIMEOUTS)); GetCommTimeouts (hCommFile, TimeoutBuffer^); TimeoutBuffer.ReadIntervalTimeout := 300; TimeoutBuffer.ReadTotalTimeoutMultiplier := 300; TimeoutBuffer.ReadTotalTimeoutConstant := 300; SetCommTimeouts (hCommFile, TimeoutBuffer^); FreeMem(TimeoutBuffer, sizeof(COMMTIMEOUTS));Having done that, ReadFile waits until I scan a barcode. Eventually, I will use ReadFileEx because it will call a routine when a barcode is scanned.
Once the port is open and configured, here are 2 routines that will read the port. For test purposes, each was connected to its own button. Uh, both routines work when run from the IDE ... the second refuses to work if you double click on the exe file. I have no idea why.
procedure TForm1.Read_ComPort_UIButtonClick(Sender: TObject);
var
InputBuffer : string;
NumberOfBytesRead : dword;
Buffer : array[0..255] of char;
i: Integer;
begin
if hCommFile=INVALID_HANDLE_VALUE then exit;
for i := 0 to 255 do // This is just for test
Buffer[I] := #42; // It demonstrates why InputBuffer
// is built inside a "for" loop
if ReadFile(hCommFile, Buffer, sizeof(Buffer),
NumberOfBytesRead, nil) = false then
begin
ShowMessage('Unable to read from comport');
exit;
end;
InputBuffer := '';
for i := 0 to NumberOfBytesRead - 1 do
InputBuffer := InputBuffer + Buffer[i];
Test_UIEdit.Text := InputBuffer;
end;
This next routine works from the IDE, but not if you run the app
by double clicking the exe file.
procedure TForm1.ReadToString_UIButtonClick(Sender: TObject);
var
InputBuffer : string;
NumberOfBytesRead : dword;
MaxBytesToRead : dword;
begin
if hCommFile=INVALID_HANDLE_VALUE then exit;
InputBuffer := '11111111111111'; // the string must be longer than
// the number of bytes you expect to read
//InputBuffer :=''; // This fails, the string must have a length
MaxBytesToRead := 14; // could use sizeof(InputBuffer) instead
if ReadFile(hCommFile, PChar(InputBuffer)^, MaxBytesToRead,
NumberOfBytesRead, nil) = false then
begin
ShowMessage('Unable to read from comport');
exit;
end;
InputBuffer := copy(InputBuffer, 1, NumberOfBytesRead);
Test_UIEdit.Text := InputBuffer;
end;
When CreateFile, ReadFile, and WriteFile are used, Delphi does not use MSComm. Instead, it uses the following Kernel32 commands (found using Depends.exe)
SetCommTimeouts, SetCommConfig GetCommTimeouts, GetCommConfig CommConfigDialogA
If you try to search the Delphi 5 help for ReadFile, there are no hits. This is because ReadFile is a Windows API command ... not a Delphi command. Instead, click on it in your code and press F1.
Here is a reference that helped me develop this section.
TCommPort
I have been successful using TCommPort by Dirk Claessens (freeware with full source code).
I strongly suggest using someone else's component rather than trying to build one yourself. Why? Because they have already been tested on numerous systems and various undocumented "features" have been taken into account.
Additional References