Article - Using the Serial Ports in Delphi 2.0

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Introduction

This article was written by Peter Johnson. Peter has written a number of articles, components and applications, unfortunately the web address we have is no longer valid, if you know where it is now please let us know.

When writing Delphi 2.0 communications applications it is necessary to use the Win32 interface to the serial ports. This varies considerably from 16-bit code as access to the communications ports is achieved through file handles. This document gives a brief introduction to serial communications under Delphi 2.0 - it does not cover event-driven applications.

Opening the serial port

First step is to open the communications device for read/write. This is achieved using the Win32 'CreateFile' function.

Var
   DeviceName: Array[0..80] of Char;
   ComFile: THandle;

   StrPCopy(DeviceName, 'COM1:');

   ComFile := CreateFile(DeviceName, 
                         GENERIC_READ or GENERIC_WRITE, 
                         0, Nil,
                         OPEN_EXISTING,
                         FILE_ATTRIBUTE_NORMAL, 0);

   if ComFile = INVALID_HANDLE_VALUE then
         { Raise an exception }

Setting up the serial port

Setup is performed using the 'SetupComm', 'GetCommState', 'BuildCommDCB', 'SetCommState' and 'SetCommTimeouts' Win32 functions. The following code demonstrates. See the Win32 API documentation for more information on the parameters that may be passed to each call.

Const
   RxBufferSize = 256;
   TxBufferSize = 256;

Var
   DCB: TDCB;
   Config : String;
   CommTimeouts : TCommTimeouts; 

if not SetupComm(ComFile, RxBufferSize, TxBufferSize) then
   { Raise an exception }

if not GetCommState(ComFile, DCB) then
   { Raise an exception }

Config := 'baud=19200 parity=n data=8 stop=1' + NUL;

if not BuildCommDCB(@Config[1], DCB) then
   { Raise an exception }

if not SetCommState(ComFile, DCB) then
   { Raise an exception }

with CommTimeouts do
begin
   ReadIntervalTimeout := 0;
   ReadTotalTimeoutMultiplier := 0;
   ReadTotalTimeoutConstant := 1000;
   WriteTotalTimeoutMultiplier := 0;
   WriteTotalTimeoutConstant := 1000;
end;

if not SetCommTimeouts(ComFile, CommTimeouts) then
   { Raise an exception }

Writing a string to the serial port

The Win32 'WriteFile' function is used to write data to the serial port. Note that we pass an offset of 1 when referencing the string to send to avoid sending the length information that is held by Pascal. Also note that we send a CR (13) / LF (10) sequence to simulate the operation of a WriteLn statement. If the string cannot be sent within the time limit specified by the 'SetCommTimeouts' function because of flow control an error will occur. We have set the timeout in the example above to 1000 milliseconds, or one second.

Var
   s: String;
   BytesWritten: Integer;

 s := 'Test string' + #13 + #10;

if not WriteFile (ComFile, s[1], Length(s), BytesWritten, Nil) then
   { Raise an exception }

Reading a string from the serial port

The following is an example of using the 'ReadFile' function to read data from the serial port. Note that some conversion is required to get the data back to a Pascal style string because we pass a raw buffer to the function. If no data is available within the time limits set by 'SetCommTimeouts' the call will return zero bytes read and hence we will have a zero length string. We have set the timeout in the example above to 1000 milliseconds, or one second.


Var 
   d: array[1..80] of Char;
   s: String;
   BytesRead, i: Integer;

 if not ReadFile (ComFile, d, sizeof(d), BytesRead, Nil) then
   { Raise an exception }

s := '';
for i := 1 to BytesRead do s := s + d[I];

Closing the serial port

The following code closes the serial port. Your application should be coded with proper exception handling so that once the serial port has been opened it is properly closed when an error occurs.

    CloseHandle(ComFile); 

Closing Comments

This document is only intended to be a starting point to programmers new to the Win32 communications calls. More detailed information on the communications parameters that are available and the meaning of the timeout values can be found in the Win32 API documentation. This document is copyright 1997 by Peter Johnson and may not be reproduced in any form without the express permission of the author except solely for the purpose of personal education.


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